1 U. S. ENVIRONMENTAL PROTECTION AGENCY Washington, D. C. 20460 OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES Date: September 15, 2005 Chemical: Thidiazuron PC Code: 120301 DP Barcodes: D289980 MEMORANDUM SUBJECT: Environmental Fate and Effects Division Revised Risk Assessment for the Reregistration Eligibility Decision of Thidiazuron for Use on Cotton TO: John Pates, Chemical Review Manager Reregistration Branch I Special Review and Reregistration Division ( 7508C)

FROM: William Evans, Biologist James Hetrick, Ph D., Senior Physical Scientist Edward Odenkirchen, Ph. D., Senior Biologist Environmental Risk Branch I Environmental Fate and Effects Division ( 7507C)

APPROVED BY: Kevin Costello, Acting Branch Chief Environmental Risk Branch I Environmental Fate and Effects Division ( 7507C)

This memorandum summarizes the attached Environmental Fate and Effects Division's ( EFED) Thidiazuron Environmental Risk Assessment for a proposed Reregistration Eligibility Decision ( RED) for use on cotton. All environmental fate and ecological toxicity Data Evaluation Records ( DERs) which were submitted with this action are attached.

Use of Thidiazuron. Thidiazuron is currently registered for use as a defoliant ( herbicide) to aid in the harvest of cotton. The maximum single application rate is 0.2 lb ai/ A. A second application is recommended under low temperature conditions when defoliation does not reach the expected level, but the total of the two applications must not exceed 0.3 lb ai/ A. The intervals between applications range from 5 7 days in some of the labels, while the other labels make no recommendations in this regard. The Quantitative Use Assessment prepared by BEAD suggests 2 that average application rates may range from 0.05  0.07 lb ai/ A.

Environmental Fate Summary In soil, thidiazuron is persistent, as evidenced by laboratory and field half lives and rotational crop intervals on the order of one year. It has intermediate soil sorption coefficients ( sorption coefficients ranged from 4.4 to 19 Freundlich units). Such persistence and intermediate mobility would allow some year to year accumulation over time and, therefore, more opportunity for favorable conditions for runoff from application sites to occur. For example, with a one year soil half life and without redistribution or dispersal from the site of application, theoretical build up in soil would asymptotically approach twice that of annually applied amounts. Of course, given time and variable climatic conditions, redistribution or partitioning of thidiazuron into other environmental compartments does occur ( via runoff, leaching, plant uptake, etc.); therefore, maximum build up in surface soil averages less than the theoretical factor of two.

When thidiazuron reaches surface water, photolysis is expected to be the major route of transformation; other degradative processes are essentially negligible by comparison. Aqueous photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of the photodegradates is a structural isomer of parent, whereas the other has a substantially altered chemical structure.

Based on its solubility and vapor pressure ( Henry's law), and other laboratory evidence, thidiazuron is non volatile. Based on its relatively low n octanol/ water partitioning coefficient, thidiazuron should not bioconcentrate.

No foliar dissipation data are currently available, and since thidiazuron is used to defoliate cotton prior to harvest, dislodgement from or transformation in/ on leaves may be an important fate pathway. The effects of this uncertainty and alternative assumptions are considered in the risk assessment.

Ecological Risk Summary This assessment is based on mainly on toxicity testing with typical end use products ( formulations) containing thidiazuron as the only active ingredient. In addition to parent thidiazuron, we have also factored into our assessment limited data for three of its transformation products ( degradates/ metabolites).

On the basis of testing only with thidiazuron and its degradates, apparent risks may be underestimated because: 1) in four of eleven registered products thidiazuron is co formulated with either of two other registered herbicides, and there are no test data for any of these coformulated products; 2) there are no data for mixtures with adjuvants ( tank mix data), which product labels state improve performance; and 3) there are no test data with mixtures that include organophosphate pesticides, which labels state may increase phytotoxicity. 3 Highlights of our ecological risk concerns, based on toxicity testing with typical end use products containing thidiazuron as the only active ingredient and the modicum of data on three transformation products, are the following:

Non target Plants: Non target terrestrial and semi aquatic dicots, but not monocots, are at potential direct and indirect ecological risks. Aquatic plants do not appear to be at risk.

Mammals: Mammals are at potential chronic risk, but not acute.

Birds: Potential chronic risk to birds cannot be determined because of the absence of chronic data. Potential acute risks to birds are below all levels of concern.

A principal ecological risk concern for the proposed thidiazuron use on cotton is the potential risks to non target plants. If non target plants come into contact with thidiazuron, these plants may be killed or damaged enough to prevent the plant from reproducing or successfully competing with other plants for resources. The use of thidiazuron could apply selective pressure to dicots along field edges, resulting in changes in species composition.

Even though thidiazuron is only applied at cotton harvest, application timing is a consequential factor to consider. Reproduction abnormalities are some of the plant injuries that can occur due to thidiazuron exposure during particular developmental stages. Although the plant may survive, sterile florets or non viable seed production can occur. This will have effects on the non target plant populations in future years. Also, plant material serves as a primary food source for many species of animals. If the available plant material ( including seeds) is reduced due to the effects of thidiazuron, this may have negative effects throughout the food chain.

The risk assessment for terrestrial plants was based on risk quotients ( RQs) calculated from toxicity studies using a typically formulated end use product ( TEP). However, it appears that no surfactants or other adjuvants were added to the product during the studies, and all labels clearly state the addition of adjuvants has been shown to improve the performance. Therefore, the risk to terrestrial non target plants may be greater than thidiazuron RQs indicate. Hence, the production of actual tank mix data would reduce some of the uncertainties associated with risk to plants.

Additionally, in four products thidiazuron is co formulated either with the herbicide diuron or the herbicide dimethipin ( see table in Use section of this document). In three of these products, diuron comprises 33% and thidiazuron 67% of the total active ingredients. In the other formulation, dimethipin comprises 80% of the total active ingredients, while thidiazuron comprises only 20%. Potential effects of the addition of these other two chemicals are not factored into the risk assessment. Combining ( adding) known effects for these other two chemicals with those for thidiazuron, or, alternatively, producing specific test data for these mixed products would reduce the risk uncertainties associated with their co application.

All thidiazuron labels list precautions when applying the products adjacent to lettuce, citrus, or 4 cantaloupe. The labels further state that mixing with organophosphates may increase non target crop phytotoxicity. Special restrictions to reduce drift to lettuce and citrus appear on all the labels. In addition, an incident was reported to have caused an estimated $ 350,000 of crop loss to 50 acres of lettuce when thidiazuron allegedly drifted onto 150 acres. Thidiazuron products can not be applied by air within ½ mile of lettuce or 5 miles downwind of the point of application when citrus is in flush ( burst of new growth, as in springtime) in the Rio Grande valley of Texas. Ground applications are restricted to 100 feet away from lettuce and ½ mile downwind from the point of application for citrus. Labels do not specify a distance for cantaloupe.

Acute risk quotients for mammals did not exceed levels of concern ( LOCs). However, chronic risk quotients for mammals did exceed LOCs for mammals feeding on short grass or broadleaf forage, with RQs ranging to a maximum of 1.8. To further investigate the chronic risk to mammals, in addition to the standard default foliar dissipation half life of 35 days, EFED evaluated the impact of a hypothetical half life as low as one day on thidiazuron residue concentrations on reducing risk. We also evaluated potential effects at predicted maximum and mean residues for single applications. No RQ exceeds the chronic level of concern when considering mean predicted residues.

The 2 generation reproduction study with rats indicates a possible concern for endocrine disruption, based on delayed sexual maturation and disruption of the estrous cycle. When appropriate screening and/ or testing protocols have been developed, thidiazuron and its degradates may be subject to additional screening and/ or testing to better characterize effects related to endocrine disruption in mammals or other organisms.

Acute risk quotients for birds did not exceed LOCs. Chronic risk to birds cannot be evaluated because chronic data were not submitted.

Data Gaps Environmental Fate: Status of Environmental Fate Data. Although some of the submitted fate studies had deficiencies, taken as a whole and within the constraints stipulated in the fate assessment, the data enable the Agency to adequately assess the environmental fate of thidiazuron. Although no foliar dissipation studies are currently available to the Agency ( these are not generally required), having such data would make any fate assessment more precise, especially in the case of a defoliant such as thidiazuron. However, as discussed in the fate assessment, if dislodgement from or transformation in/ on treated plants were to be an important fate pathway, then EFED's inferences from existing fate studies and assumptions for exposure modeling for parent and potential degradates should reasonably and conservatively cover the possibility. If the registrant wishes to demonstrate that definitive data on foliar dissipation would substantially affect any of our inferences, assumptions, or environmental risk conclusions, then the registrant should submit appropriate data. Otherwise, except for spray drift requirements, the Agency needs no additional environmental fate studies at this time. 5 Ecological Effects: The ecological toxicity data base is fairly complete, however, there are some additional key studies that could assist in reducing the uncertainties in this risk assessment. The details are summarized below.

! Non target Terrestrial Plant  Definitive tier II EC25s and NOAECs are lacking for the vegetative vigor of all the monocots tested and for the seedling emergence of two monocot species tested; this is because test concentrations were not low enough to establish these endpoints. However, risk quotients from the tests for which definitive endpoints were established were below levels of concern. Furthermore, concentrations in those tests for which endpoints were not determined were sufficiently low to establish that, when compared to estimated field exposure concentrations that correspond to the currently labeled application rates, risk quotients would not exceed any of the terrestrial plant LOCs. Therefore, since projected risk quotients for monocots are below levels of concern, additional tier II testing with a typical end use product that contains thidiazuron as the only active ingredient is not required at this time for these specific crops at currently labeled application rates.

However, there are other considerations. Even though all labels clearly state that the addition of adjuvants has been shown to improve performance, it appears that none were added the during the tests. Therefore, apparent risk to terrestrial non target plants may be greater than present RQs indicate. The production of actual tank mix data would reduce this uncertainty.

Additionally, as previously mentioned, three thidiazuron products are mixtures of thidiazuron ( 67% of total active ingredients) and diuron ( 33% of total active ingredients), and another is a formulation of thidiazuron ( 20% ) and preponderantly dimethipin ( 80%). Potential effects of the mixtures with these other two chemicals are not factored into the risk assessment. Combining ( adding) the known effects for these other two chemicals with those for thidiazuron, or, alternatively, producing specific test data for these mixed products would reduce the uncertainties associated with their co application.

Concerning the potential for additional, undetermined risk, the current labels contain warnings and specific instructions about the use of thidiazuron in the vicinity of lettuce, cantaloupe, and citrus. The language from one of the labels is the following:

" Particular care should be taken when applying Dropp ® UltraTM adjacent to lettuce, citrus, or cantaloupe."

" In addition, for citrus crops, do not apply Dropp ® UltraTM by air when citrus in flush is within five ( 5) miles downwind of the point of application. Do not apply Dropp ® UltraTM by ground when citrus in flush is within one half ( 1/ 2) mile."

Labels also state that " do not apply" distances from lettuce are one half ( 1/ 2) mile by air 6 and 100 feet by ground.

! Birds  Chronic risks to birds can not be quantitatively evaluated at this time because chronic data were not submitted. In order to assess this potential risk, we recommend chronic testing with an upland game bird species ( bobwhite quail) and a waterfowl ( mallard duck).

Recommended Label Language If thidiazuron is reregistered, the following statements should be included in the " ENVIRONMENTAL HAZARDS" labeling:

For Manufacturing Use Products " Do not discharge effluent containing this active ingredient into lakes, streams, ponds, estuaries, oceans, or other public waters unless this product is specifically identified and addressed in an NPDES permit. Do not discharge effluent containing this product into sewer systems without previously notifying the sewage treatment plant authority. For guidance, contact your State Water Board or Regional Office of EPA."

For End Use Products " Do not apply directly to water or to areas where surface water is present or to intertidal areas below the mean high water mark. Do not contaminate water when disposing of equipment washwater or rinsate."

Surface Water Advisory " This product may contaminate water through drift of spray in wind. This product has a high potential for runoff for several months or more after application. Poorly draining soils and soils with shallow water tables are more prone to produce runoff that contains this product. A level, well maintained vegetative buffer strip between areas to which this product is applied and surface water features such as ponds, streams, and springs will reduce the potential for contamination of water from runoff. Runoff of this product will be reduced by avoiding applications when rainfall is forecasted to occur within 48 hours. Sound erosion control practices will reduce this product's contribution to surface water contamination."

Groundwater Advisory " This chemical has properties and characteristics associated with chemicals detected in ground water. Use of this chemical in areas where soils are permeable, particularly where the water table is shallow, may result in groundwater contamination." 7 Spray Drift Management Drift is transport of pesticides through air away from the target site, and includes, for example, spray drift and volatilization. Spray drift, the movement of pesticide droplets off target during or shortly after application, has been well studied, and is not dependent on the properties of the active ingredient. Short range spray drift and resulting exposures to non target organisms is quantified in our risk assessments. Currently, when specific label instructions are not available, we account for spray drift in both drinking water and ecological exposure assessments by the use of default assumptions for the percentage of deposition based on application method. We have established default spray drift percentages for pesticides for aerial application, ground application, and orchard airblast.

Default values for drinking water exposure scenarios are 16%, 6.4%, and 6.3% of the application rate for aerial, ground boom and airblast applications, respectively. The default values for the drinking water exposure were generated using AgDRIFT version 1.03 which uses a semimechanistic model ( AGDISP) for aerial applications and is based an empirical data set ( Spray Drift Task Force data) for ground boom and airblast applications. The derivation of the spray drift loading values is described in Appendix B of the Guidance for Use of the Index Reservoir in Drinking Water Exposure Assessments and is based on the following application and wind speed assumptions:

For ground boom applications ° nozzle height no more than 4 feet above the ground or crop canopy ° wind speed is 10 mph or less at the application site and, ° medium or coarser spray according to ASAE 572 definition for standard nozzles.

For aerial applications ° boom width less than 75% of the wingspan or 90% of the rotary blade, ° use of upwind swath displacement, ° wind speed 3  10 mph, and ° use medium or coarser spray according to ASAE 572 definition for standard nozzles.

For orchard/ vineyard airblast applications ° spray is not directed above trees/ vines, ° outward pointing nozzles are shut off at row ends and outer rows, and ° wind speed is 3  10 mph at the application site as measured by an anemometer outside of the orchard/ vineyard on the upwind side.

Default values for ecological exposure scenarios are 5% for aerial applications and 1% for ground applications, with no set value for orchard airblast. For the ecological assessment, the default values of 5% for aerial applications and 1% for ground applications were established based on open literature and research prior to establishment of the Spray Drift Task Force, and were therefore not generated using AgDRIFT. Testing of the default spray drift values for ecological 8 assessments using AgDRIFT provides context for the default assumptions. Analysis of the 1% ground application drift percentage using AgDRIFT ( with the same assumptions as for drinking water outlined above) indicates that the default value likely underestimates drift from single events under relatively high drift conditions. A similar underestimation likely occurs for the 5% aerial drift value.

EECs predicted for drinking water and ecological assessments are based on the assumptions of drift outlined above for ground and aerial applications. Variance from these assumptions will result in differences in predicted concentrations. For the ecological assessment, as with the drinking water assessment, variance from the assumptions behind these default values could result in increased risk. In general, a decrease in droplet size or increase in wind speed at the time of application will result in higher predicted EECs and risk to non target organisms. Alternatively, if droplet size is coarser or wind speeds are lower, exposures due to drift would be lower.

Transport resulting from volatilization is highly dependent on the properties of the active ingredient ( e. g., Henry's Law constant) as well as a number of environmental parameters. Based on its physicochemical properties, thidiazuron is not expected to volatilize. 1 Environmental Fate and Effects Division's Risk Assessment for the Reregistration Eligibility Document For the Use of Thidiazuron on Cotton Page 2 of 129 Table of Contents I. Environmental Risk Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 4 of 129 Environmental Fate Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 4 of 129 Terrestrial Plant Risks of Concern . . . . . . . . . . . . . . . . . . . . . . . . Page 4 of 129 Terrestrial Vertebrate Risks of Concern . . . . . . . . . . . . . . . . . . . . Page 5 of 129 II. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6 of 129 Pesticide Mode of Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6 of 129 Uses and Use Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6 of 129 Risk Assessment Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 8 of 129 Assessment Endpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 8 of 129 Measures of Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 8 of 129 Measures of Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 10 of 129 Aquatic Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 10 of 129 Active Ingredient and Degradate Aquatic Organism Exposure Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 10 of 129 Terrestrial Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 11 of 129 Terrestrial Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12 of 129 Risk Characterization Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12 of 129 Problem Formulation Analysis Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12 of 129 III. Integrated Environmental Risk Characterization . . . . . . . . . . . . . . . . . . . . . . Page 17 of 129 Risks to Aquatic Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 17 of 129 Risk to Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 17 of 129 Acute Risks  Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 17 of 129 Chronic Risk  Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 18 of 129 Risks to Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 18 of 129 Acute Risks  Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 18 of 129 Chronic Risk  Mammals . . . . . . . . . . . . . . . . . . . . . . . . . Page 18 of 129 Risks to Non Target Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 20 of 129 Risks to Terrestrial Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 20 of 129 Endocrine Disruption Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 41 of 129 Endangered Species Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 42 of 129 IV. Environmental Fate Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 43 of 129 Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 43 of 129 Fate Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 44 of 129 Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 44 of 129 Physical/ Chemical Properties . . . . . . . . . . . . . . . . . . . . . . Page 45 of 129 Transformation Products ( degradates/ metabolites) . . . . . Page 49 of 129 V. Drinking Water Assessment Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 52 of 129 VI. Aquatic Hazard, Exposure, and Risk Quotient Calculation . . . . . . . . . . . . . . Page 53 of 129 Toxicity to Fish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 53 of 129 Page 3 of 129 Toxicity to Aquatic Invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 54 of 129 Toxicity to Aquatic Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 55 of 129 Aquatic Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 55 of 129 Risk Quotients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 55 of 129 VII. Terrestrial Hazard, Exposure, and Risk Quotient Calculation . . . . . . . . . . . Page 56 of 129 Toxicity to Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 56 of 129 Toxicity to Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 56 of 129 Toxicity to Non target Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 57 of 129 Toxicity to Earthworms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 58 of 129 Toxicity to Non Target Plants . . . . . . . . . . . . . . . . . . . . . . . . . . Page 59 of 129 Terrestrial Plant Exposure and Risk Quotient Calculations . . . . . Page 60 of 129 Terrestrial Exposure for Birds and Mammals . . . . . . . . . . . . . . . Page 62 of 129 Terrestrial Risk Quotients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 63 of 129 APPENDIX C: Model Input Parameters Derived from Environmental Fate Studies for Thidiazuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 79 of 129 APPENDIX LIST APPENDIX A: Detailed Drinking Water Assessment Memo . . . . . . . . . . . . . . . Page 64 of 129 APPENDIX B: Chemical Structural Formulas, Names . . . . . . . . . . . . . . . . . . . Page 76 of 129 APPENDIX D: GENEEC Input & Output Files for Ecological Assessment . . . . Page 82 of 129 APPENDIX E: Ecological Hazard Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 87 of 129 APPENDIX F: The Risk Quotient Method . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 105 of 129 APPENDIX G: Detailed Risk Quotients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 108 of 129 APPENDIX H: Data Requirements for Thidiazuron . . . . . . . . . . . . . . . . . . . . Page 124 of 129 Page 4 of 129 I. Environmental Risk Conclusions Environmental Fate Abstract In soil, thidiazuron is persistent, as evidenced by laboratory and field half lives and rotational crop intervals on the order of one year. It has intermediate soil sorption coefficients ( sorption coefficients ranged from 4.4 to 19 Freundlich units). Such persistence and intermediate mobility would allow some year to year accumulation over time and, therefore, more opportunity for favorable conditions for runoff from application sites to occur.

When thidiazuron reaches surface water, photolysis is expected to be the major route of transformation; other degradative processes are essentially negligible by comparison. Aqueous photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of the photodegradates is a structural isomer of parent, whereas the other has a substantially altered chemical structure. The photolysis process requires special exposure and modeling considerations.

Based on its solubility and vapor pressure ( Henry's law), and other laboratory evidence, thidiazuron is non volatile. Based on its relatively low octanol/ water partitioning coefficient, thidiazuron should not bioconcentrate.

No data on foliar dissipation are currently available to EFED. Since thidiazuron is used to defoliate cotton prior to harvest, dislodgement from or transformation in/ on leaves may be an important fate pathway. The effects of this uncertainty and alternative assumptions on the risk assessment are considered.

Terrestrial Plant Risks of Concern Thidiazuron poses potential risks to terrestrial plants based on the results of a screening level assessment. The risk quotients exceed the levels of concern for non endangered and endangered plants adjacent to treated sites as well as plants inhabiting semi   aquatic sites. This is especially true for dicots for both single and multiple applications. The RQs resulting from single applications at the maximum label rates range from 0.39 to 9.09 for non endangered plants and up to as high as 52.63 for endangered plants. The RQs for multiple applications range from 0.59 to 9.09 for non endangered species and up to 52.63 for endangered species.

In addition to the RQ calculations from the maximum label rates, EFED completed a characterization of potential risks by assessing the effect of RQ calculations using EECs predicted from modeling with average application rates ( 0.05  0.07 lb ai/ A) as reported in the BEAD QUA report. When the average application rates are compared to the maximum allowable application rates, RQs are reduced by at least one half, but levels of concern are still exceeded, except for the single ground application for non endangered species scenario.

It should be noted that RQs for this risk assessment were based on toxicity studies using the typical end use product " Thidiazuron SC42" ( presumably DROPP SC ( Reg. #: 264 700)). All labels clearly state the addition of adjuvants has been shown to improve the performance, and yet Page 5 of 129 no surfactants or adjuvants were added to the product during the studies. Therefore, the risk to terrestrial non target plants may be greater than RQs indicate. The production of core data using actual tank mixes may assist in reducing some of the uncertainties associated with risk.

Additionally, as previously noted, three thidiazuron products are mixtures ( co formulations) of thidiazuron ( 67% of total active ingredients) and diuron ( 33% of total active ingredients), and still another is a combination of thidiazuron ( 20% ) and preponderantly dimethipin ( 80%) ( see table in section on Use and Use Characterization). Adding the known effects of these other two chemicals to those for thidiazuron, or, alternatively, having test data specifically for these products would reduce the risk uncertainties associated with their co application.

Since the sole use of thidiazuron is as a cotton defoliant, the primary concern is for non target plants which inhabit areas near cotton fields. If such plants are sensitive to leaf drop and other symptoms which may result from thidiazuron exposure, death, severe damage, or reproductive impairment could occur. This damage could apply selective pressure to dicot plants along field edges and changes in species composition could occur.

There is another reason to believe that our assessment may underestimate risk to terrestrial plants. Thidiazuron labels state that mixing with organophosphates may increase non target crop phytotoxicity. Data have not been submitted which quantify this effect on thidiazuron toxicity.

There is uncertainty regarding the toxicity of thidiazuron to non target plants other than those tested in the laboratory. However, the product labels for thidiazuron include some language which attempts to mitigate potential risk for some of those crops. All labels list precautions when applying the products adjacent to lettuce, citrus, or cantaloupe. Special restrictions to reduce spraydrift potential to lettuce and citrus appear on all the labels.

To explore the apparent risk posed by spraydrift, we used EFED's current AgDRIFT computer model and thidiazuron phytotoxicity data to estimate plant exposure as a function of downwind distances from application areas. Since the current labels lack specific information on droplet size or boom height, we also looked at the effects of varying these. We then compared the various exposure distributions with toxicity level percentiles from the EC10 to EC90 ( in increments of ten).

As can be seen from the sequence of the following bar graphs, there is considerable variation with droplet size or boom height for the EC30 ( the closest toxicity level to the EC25). The no spray zones for aerial applications for the EC30 effect levels range from slightly over 60 feet to greater than 1000 feet for lettuce, the most sensitive crop tested so far. Ground application no spray zones for the EC30 vary from about 3 to 200 feet. Similar trends are demonstrated for the other crops for which slope data could be determined. In addition, according to the labels, cantaloupe and citrus many be more sensitive than lettuce, and no data have been submitted on these crops. All the factors influencing the downwind drift distance, including droplet size, wind speed, boom height, method of application, and targeted toxicity effect should be considered when placing restrictions on labels.

Terrestrial Vertebrate Risks of Concern Page 6 of 129 Thidiazuron may pose chronic risk to mammals, based on a screening level assessment. Conclusions are derived from a 2 generation rat study. From the study, the reproductive NOAEC and LOAEC values are 400 and 1200 mg/ kg diet, respectively, based on delayed sexual maturation and disruption of the estrous cycle. To estimate risk, we used two screening level methodologies, and found that they give different results. Risk quotients based on the daily dietary dose indicated a potential chronic risk to mammals.

II. Introduction Thidiazuron is undergoing review for reregistration under Section 4 of FIFRA as amended. It is currently registered for use as a defoliant to aid in the harvest of cotton. Defoliation, or leaf abscission, is a natural process in all plants, but can be manipulated in cotton plants to cause them to drop their leaves in a relatively short period of time to aid in the efficiency of cotton harvesting.

Pesticide Mode of Action Thidiazuron is a phenylurea herbicide that is said to inhibit regrowth through photosynthetic electron transport inhibition at the photosystem II receptor. This mode of action increases the ethylene concentration in the leaves; this inhibits the transport of the hormone auxin, and results in leaf drop.

While the defoliation process is quite complex, it can be simplified as being governed by two major compounds in the plant, auxin and ethylene. Auxin is a growth promoting hormone that stimulates leaf growth and development. Ethylene can be classified as a ripening agent that causes leaf drop. Leaves fall from the plant once ethylene moves from the leaf blade to the base of the petiole and stimulates the formation of an abscission ( separation) layer. The amount of auxin or ethylene present in the leaves of the cotton plant is related to leaf age. Younger leaves have a higher level of auxin, whereas older leaves have lower levels of auxin and higher levels of ethylene. This is why older leaves are more conditioned for defoliation than younger leaves. Furthermore, because of the hormone balance of younger leaves, low rates of harvest aids often have no effect, and higher rates may actually kill the leaf, leading to desiccation and leaf sticking.

Eventually, all the leaves on a cotton plant age so they will defoliate naturally. However, cotton producers can manipulate these hormone levels with the use of defoliants so all the leaves abscise ( separate and fall) at the same time. When these defoliants are applied, ethylene levels artificially increase so the abscission process begins.

Uses and Use Characterization Thidiazuron is used exclusively as a cotton defoliant to aid in the uniform removal of both mature and juvenile leaves. This facilitates machine harvesting and helps eliminate sources of gin trash and stains. Because of its purpose, thidiazuron is applied during the cotton harvesting period, which is typically in late August and early September. All labels indicate that citrus trees are particularly sensitive to thidiazuron.

All current products containing thidiazuron and their corresponding EPA registration numbers are Page 7 of 129 listed in the following table. Product label statements about use indicate that the areas potentially at direct and/ or indirect risk from the application of thidiazuron in the manner prescribed by the labels are the cotton agronomic environments.

EPA Registration Number Product Name 264 622 Dropp 50WP Cotton Defoliant 264 634 GINSTAR EC Cotton Defoliant1 264 661 Dropp Ultra Cotton Defoliant2 264 700 Dropp SC Cotton Defoliant 264 821 GINSTAR ® 4.5 SC Cotton Defoliant3 400 505 Harvade 4198SC Harvest Growth Regulant for Cotton4 51036 401 Thidiazuron 50W Cotton Defoliant 51036 402 Thidiazuron Technical5 51036 429 Thidiazuron Diuron EC Cotton Defoliant1 51036 430 Thidiazuron 4SC Cotton Defoliant 72167 28 Nation's AG II Thidiazuron Technical5 72167 30 Thidiazuron 50WSB Cotton Defoliant 1 12% thidiazuron and 6% diuron mixture 2 50% thidiazuron and 25% diuron mixture 3 30.3% thidiazuron and 15.1% diuron mixture 4 32.7% dimethipin and 8.4% thidiazuron mixture 5 For cotton defoliant formulation use only As indicated in the table above, thidiazuron is used exclusively as a cotton defoliant or growth regulant. Three of the twelve registered thidiazuron products are mixed with diuron at half the concentration of thidiazuron. In another product ( Harvade 4198SC), dimethipin is present at approximately four times the concentration of thidiazuron.

In the case of the mixed products, care must be taken when calculating the use rates for thidiazuron. This is because the " active ingredient" application rates given on the labels are for the sums both active ingredients. Since each compound is different in physicochemical properties and biological activity, it is more meaningful, useful, and transparent to separate the rates for each. This risk assessment was conducted for that fraction of the application rate contributed by thidiazuron only.

The maximum single application rate for all labels on which thidiazuron is the sole active ingredient is 0.2 lb ai/ A ( EPA Registration Numbers 264 622, 264 700, 51036 401, 51036 430, Page 8 of 129 and 72167 30). A second application is recommended under low temperature conditions when defoliation does not reach the expected level, but the total of the two applications must not exceed 0.3 lb ai/ A. The intervals between applications range from 5 7 days in some of the labels, while the other labels make no recommendations with regard to intervals.

The maximum single application rate when thidiazuron is mixed with diuron is 0.125 lb of thidiazuron per acre and 0.0625 lb of diuron per acre ( EPA Registration Numbers 264 634, 264 661, and 51036 429). The combined rate for both active ingredients is 0.1875 lb ai/ A, and since diuron also has the same mode of action as thidiazuron, the combined total use rate is approximately the same as in the products which contain only thidiazuron as the sole active ingredient. The total number of applications is not specified on any of these labels, however, the maximum combined active ingredient dosage is not to exceed 0.1875 lb ai/ A per season.

When thidiazuron is mixed with dimethipin the maximum single application rate is 0.00625 lb thidiazuron and 0.1 lb dimethipin/ A ( EPA Registration Number 400 505). A second application may be required at a maximum rate per acre of 0.003 lb thidiazuron and 0.05 lb dimethipin 5  7 days after the initial treatment. The maximum yearly application should not exceed 0.01 lb thidiazuron and 0.16 lb dimethipin/ A/ yr.

Risk Assessment Approach For terrestrial risk, we consider the active ingredient thidiazuron only. For aquatic risk, the most sensitive toxicity endpoint measured among parent thidiazuron, aquatic photodegradates AE F132147 and AE C421200 and the thidiazuron metabolite AE F132145 is used to calculate risk quotients. Since environmental concentrations of thidiazuron transformation products would be expected to be less than potential concentrations of thidiazuron, use of the transformation product toxicity data is a particularly conservative component of the screening assessment. The assessment does not consider any additional risk posed by the co formulations of thidiazuron with either of the active ingredients diuron or dimethipin, ( see previous section for specific products and concentrations of each) or by the addition of adjuvants ( surfactants).

Assessment Endpoints The typical assessment endpoints for screening level pesticide ecological risks are reduced survival and reproductive impairment for both aquatic and terrestrial animal species. These effects extend to a consideration of direct acute and direct chronic exposures. The assessment endpoints for aquatic plants are impairments due to a decrease in alga cell density or macrophyte growth. Terrestrial plant assessment endpoints are impairments due to shoot weight, shoot length, or dry weight. There are currently no chronic assessment endpoints for reproductive impairment for plants. While these endpoints are measured at the individual level, they do provide insight about risks at higher levels of biological organization ( e. g., populations).

Measures of Effects The screening level risk assessment process relies on a suite of toxicity studies performed on a limited number of organisms in the following broad groupings: Page 9 of 129 ° Birds ( mallard duck and bobwhite quail) ( see Toxicity to Birds section) ° Acute data are complete for this risk assessment ° Chronic data are not available. ° Mammals ( laboratory rat) ( see Toxicity to Mammals section) ° Acute data are complete for this risk assessment ° Chronic data are available from a 2 generation and a 3 generation rat study ° Freshwater fish ( bluegill sunfish, rainbow trout, and fathead minnow) ( see Toxicity to Fish section) ° Acute data are available for thidiazuron. ° Acute data are available for the AE F132145 metabolite and the AE F132147 and AE C421200 photodegradates, and these data are incorporated into the risk assessment. ° Chronic freshwater fish data are available for thidiazuron ° Freshwater invertebrates ( Daphnia magna) ( see Toxicity to Aquatic Invertebrates section) ° Acute data are available for thidiazuron ° Acute data are available for the AE F132145 metabolite and the AE F132147 and AE C421200 photodegradates, and these data are incorporated into risk assessment ° Chronic data are available for thidiazuron ° Estuarine/ marine fish ( sheepshead minnow) ( see Toxicity to Fish section) ° Acute data are available for thidiazuron. ° No chronic data are available ° Estuarine/ marine invertebrates ( Crassostrea virginica and Mysidopsis bahia) ( see Toxicity to Aquatic Invertebrates section) ° Acute data are available for thidiazuron ° Chronic data are not available ° Algae and aquatic plants ( see Toxicity to Aquatic Plants section) ° Data are available for green algae, blue green algae, marine diatoms, and aquatic vascular macrophytes for thidiazuron. ° Data are not available for freshwater diatoms. ° Data are available for green algae for the AE F132145 metabolite and the AE F132147 and AE C421200 photodegradates, and these data are incorporated into risk assessment. ° Terrestrial plants ( corn, soybean, turnip, oats, wheat, tomato, onion, cabbage, lettuce, cucumber) ( see Toxicity to Terrestrial Plant section) ° Data are available for thidiazuron ° Data are available for thidiazuron SC42 formulated product Within each of these very broad taxonomic groups, an acute and chronic endpoint is selected from the available test data, as the data sets allow. The selection is made from the most sensitive species tested within that organism group.

Additional effects data were available for other taxa and for aquatic community effects measures. These have been incorporated into the risk characterization as other lines of evidence. These data include: Page 10 of 129 ° Acute laboratory toxicity study of technical and formulated product with oligochaete worms ° Toxicity data on beneficial insects ° laboratory tests for honeybee contact toxicity ° laboratory tests for predatory mites and parasoid wasps In addition, a search of EPA's publically accessible Ecotoxicity Database ( http:// www. epa. gov/ ecotox) for relevant toxicological studies revealed no additional data for effects on mortality, growth, behavior, reproduction, population, or ecosystem function for thidiazuron. A complete discussion of all toxicity data available for this risk assessment and the resulting measurement endpoints selected for each taxonomic group are included in Sections VI and VII of this document.

Measures of Exposure Exposures estimated in the screening level risk assessment for non target organisms are not specific to a given species. Rather, general taxonomic exposure assumptions are made that provide separate exposure measures for aquatic and terrestrial plants and animals. The approaches for each are discussed separately below.

Aquatic Organisms In addition to parent thidiazuron, effects data are available for the aqueous photodegradates AE F132147 and AE C421200 and the metabolite AE F132145 in fish, aquatic invertebrates, and aquatic plants. These data suggest that exposure modeling for these degradates is warranted. The following sections describe the general analysis approach used for active ingredient and the three transformation products.

Active Ingredient and Degradate Aquatic Organism Exposure Approach This risk assessment considers thidiazuron exposure in aquatic organisms ( animals and plants) via the fraction of pesticide dissolved in the water column. Thidiazuron is assumed to be introduced to surface waters via runoff and spray drift. Currently, no monitoring data are available that would provide information on thidiazuron levels in surface water bodies receiving runoff or spray drift from agricultural fields treated with the pesticide. Consequently, aquatic organism exposure is estimated through the use of runoff and spray drift models. Aquatic organism exposures are based on a set of standardized receiving surface water body assumptions ( water body size, watershed size, proximity to field, etc.) that result in reasonable upper level estimates of exposure. For this risk assessment the GENEEC model was used for making these exposure estimates. GENEEC is the most conservative exposure model available in OPP. It estimates exposure concentrations in aquatic environments representative of geographic locations where cotton is grown in large quantities. If exposure concentrations resulting from the use of GENEEC yield risk quotients below all aquatic levels of concern, it is concluded that direct and indirect aquatic risks would not occur. In this regard, EFED used the most sensitive toxicological endpoint available from data on thidiazuron and the photoproducts for each of the aquatic organism categories ( i. e., fish, aquatic invertebrates, plants). Page 11 of 129 For acute aquatic risk, no averaging time for exposure was assumed for this assessment. The use of the instantaneous peak assumes that instantaneous exposure is of sufficient duration to elicit acute effects on par with those observed over more protracted exposure periods tested in the laboratory, typically 48 to 96 hours. For assessment of chronic aquatic invertebrate risks with thidiazuron, the peak water concentration was again used. This is a departure from the normal assessment where the 21 day average, 1 in 10 year return frequency values are compared to the measurement endpoint for aquatic invertebrate chronic risks. The decision to use a peak concentration exposure measure was predicated on the belief that the LOCs would not be exceeded even if the higher peak concentrations were utilized.

As discussed in the Environmental Fate section of this document and in Appendix D, thidiazuron undergoes relatively rapid branching photolysis in water into only two photoproducts. Formation of these products is in constant ratio and in complementary stoichiometric yield with parent. The key features of these two photoproducts are as follows:

Photoproduct I ( AE F132347, see chemical structure in Appendix B) is an isomer of parent differing only in the relative positions of the sulfur atom and the two nitrogen atoms in the 5 membered ring. Like parent, photoproduct I is moderately ecotoxic ( slightly more toxic than parent, based on the limited data discussed in the main text of this document). It has a reviewercalculated asymptotic experimental limit of production of 77% of parent at pH 5, 28% at pH 7, and 17% at pH 9. This product was stable to further photolysis under the experimental study conditions and durations ( original MRID 41188201, related follow up MRIDs 41364910 and 43075202, and fate overview document with MRID 44436901). Soil/ sediment sorption data and non photolytic transformation data are available only for parent. However, based on its isomeric chemical structure, photoproduct I, except for photolysis, would be expected to have fate properties similar to parent.

Photoproduct II ( AE C421200, chemical structure in Appendix B) is a substantially degraded, practically non ecotoxic product ( based on the limited data discussed in the main text of this document), and has a complementary asymptotic limit of production to photoproduct I of 23% of parent at pH 5, 72% at pH 7, and 83% at pH 9. This product was also stable to further photolysis under study conditions. Soil/ sediment sorption data and non photolytic transformation data are available only for parent. However, because of its degraded nature and a modicum of direct evidence indicating that photoproduct II is practically non toxic for ecological risk assessment purposes, EFED assumed that photoproduct II does not contribute to ecological risk, and ignored its potential presence.

Aquatic exposure concentrations are derived from a combination of GENEEC model runs for the summation of thidiazuron and its photoisomer as explicitly derived in Appendix D Terrestrial Plants The focus of the terrestrial plant exposure assessment is on terrestrial plants inhabiting dry and semi aquatic areas. Potential routes of exposure to pesticides considered are runoff, spray drift and volatilization. Since thidiazuron is non volatile, volatilization is not a concern. Page 12 of 129 Semi aquatic areas are those low lying wet areas that may be dry at certain times of the year. EFED's runoff exposure estimate in these areas is: ( 1) based on a pesticide's water solubility and the amount of pesticide present in the top one inch of soil, ( 2) characterized as " sheet runoff" ( one treated acre to an adjacent acre) for dry areas, ( 3) characterized as " channelized runoff" ( 10 treated acres to a distant low lying acre) for semi aquatic areas, and ( 4) based on percent runoff values of 0.01, 0.02, and 0.05 for water solubilities of < 10 ppm, 10 100 ppm, and > 100 ppm, respectively.

Spray drift exposure from ground and overhead chemigation applications is assumed to be 1% of the application rate. Spray drift from aerial, airblast, and forced air applications is assumed to be 5% of the application rate with an application efficiency of 60%. The effects of multiple applications are addressed by summing the application rates from individual applications.

Terrestrial Animals The focus of the terrestrial wildlife exposure assessment is on birds and mammals with the exposure route emphasis on uptake through the diet. For exposure to terrestrial organisms, such as birds and small mammals, OPP looks at the residues of pesticides on food items, and assumes that organisms are exposed to a single pesticide residue in a given exposure scenario. For thidiazuron spray applications, estimation of pesticide concentrations in wildlife food items focuses on quantifying possible dietary ingestion of residues on vegetative matter and insects. The residue estimates are based on a nomogram that relates food item residues to a pesticide's application rate. Residues may be compared directly with dietary toxicity data or converted to an oral dose, as is the case for small mammals. The screening level risk assessment for thidiazuron uses an upper bound predicted residue as the measure of exposure. For mammals, the residue concentration is converted to daily oral dose based on fractions of body weight consumed daily, as estimated through mammalian allometric relationships.

Risk Characterization Approach For this assessment of thidiazuron risks, the risk quotient ( RQ) method is used to compare exposure and measured toxicity. Estimated environmental concentrations ( EECs) are divided by acute and chronic toxicity values. The RQs are compared to the Agency's levels of concern ( LOCs). These LOCs are the Agency's interpretative policy, and are used to analyze potential risk to non target organisms and the need to consider regulatory action. These criteria are used to indicate when a pesticide used as directed on the label has the potential to cause adverse effects on non target organisms. Appendix G of this document summarizes the LOCs employed in this risk assessment.

Problem Formulation Analysis Plan In accordance with US EPA's " Guidelines for Ecological Risk Assessment," April 1998, and " Overview of the Ecological Risk Assessment Process in the Office of Pesticide Program," January 2004, an analysis plan and rationale were developed to determine if current label uses of thidiazuron indicate a likelihood that unreasonable adverse effects to nontarget and listed animals and plants could potentially impact the reregistration eligibility decision under the Federal Page 13 of 129 Insecticide, Fungicide, and Rodenticide Act, the Food Quality Protection Act, and the Endangered Species Act. Over the course of its assessment, OPP has attempted to refine its problem formulation by integrating registrant submitted guideline data and various levels of exposure refinements and information on usage to characterize the environmental fate and ecological effects of thidiazuron and its degradates.

Thidiazuron is exclusively used as a cotton defoliant to aid in the machine harvesting of cotton by removing both mature and juvenile leaves. EFED made several assumptions concerning some ambiguities on the labels. Three of the product labels list diuron as an additional active ingredient, and another label lists dimethipin, but none of these labels report the application rates of each active ingredient separately. EFED quantitatively considered the risk resulting from thidiazuron only, rather than the combination of the two active ingredients. Thidiazuron rates were calculated from total product application rates and product chemical composition. The possible additive or synergistic effects resulting from the consideration of the other active ingredients were not evaluated. Additional information provided by BEAD and other sources identified more typical use rates and frequencies in addition to the maximum rates which are routinely used in screening level assessments. These typical rates were considered in the terrestrial plant risk characterization.

The risk assessment considers aquatic and terrestrial risks for parent thidiazuron. In addition, it considers risks posed to aquatic organisms from AE F132347 and AE C421200 ( photodegradates formed in water) and the metabolite AE F132345 by using aquatic exposure concentrations derived from a combination of GENEEC model runs for the summation of thidiazuron and its photoisomer as explicitly derived in Appendix D.

Risk to terrestrial plants from the formulated product, Thidiazuron SC42 ( DROPP) is considered in the risk assessment. Exposure from foliar routes of exposure are primarily explored for terrestrial and aquatic plants and animals. However, this risk assessment does not take into account atmospheric transport, other than spray drift, in estimation of environmental concentrations. Further, it does not account for ingestion of residues by animals in drinking water or contaminated grit, ingestion through preening activities, or uptake through inhalation or dermal absorption by terrestrial animals. Exposure to terrestrial animals is based primarily on dietary consumption of foliar residues, whereas aquatic exposures assume that all potential routes of direct exposure are accounted for and are based on standard surface water modeling used in EFED.

As previously discussed, exposure to terrestrial plants inhabiting dry and semi aquatic areas involves runoff assumptions based on the water solubility and the amount of pesticide present in the top one inch of soil. This runoff exposure is characterized as " sheet runoff" ( one treated acre to an adjacent acre) for dry areas, and " channelized runoff" ( 10 treated acres to a distant lowlying acre) for semi aquatic areas. Terrestrial plant spray drift exposure assumptions are based on 1% of the application rate for ground applications and 5% for aerial applications with an application efficiency of 60%.

Maximum application rates on vulnerable soils for representative crops are selected for modeling environmental concentrations for this screening level deterministic ( risk quotient based) risk Page 14 of 129 assessment. This assessment is not intended to represent a site or time specific analysis; i. e., assessments are intended to represent a national level exposure based on vulnerable soils as opposed to being regionally specific. However, if exposures exceed LOCs, the assessment will be refined to include higher tiered models with spatially specific use scenarios. Likewise, the most sensitive toxicity endpoints are used from surrogate test species to estimate treatment related direct effects on acute mortality and chronic reproductive, growth, and survival assessment endpoints. Toxicity tests are intended to determine pesticidal effects on birds, mammals, fish, terrestrial and aquatic invertebrates and plants. These tests include short term acute, subacute and reproduction studies and are typically arranged in a hierarchical or tiered system that progresses from basic laboratory tests to applied field studies. These studies are used to evaluate the potential of a pesticide to cause adverse effects, determine whether further testing is required, and to determine the need for precautionary label statements to minimize the potential adverse effects to nontarget animal and plants ( CFR 40 § 158.202, 2002).

The conceptual model used to depict the ecological risk associated with thidiazuron was initially generic and assumed that thidiazuron was capable of affecting terrestrial and aquatic animals provided environmental concentrations were sufficiently elevated as a result of labeled uses. This initial conceptual model is presented below. Page 15 of 129 Conceptual Model for ThidiazuronRisk Assessment Thidiazuronapplied to agricultural field Runoff Spray drift Water body Aquatic plants Aquatic invertebrates Aquatic vertebrates Riparian Zone Terrestrial plants Terrestrial insects Terrestrial Mammals Birds Individual fish Reduced survival Reduced growth Reduced reproduction Food chain Decrease in abundance Shift in prey base Habitat integrity Decreased water quality Reduced cover Stream destabilization Stressor Receptors Attribute Change Groundwater Atmospheric deposition Source Page 16 of 129 Refined Conceptual Model for a Screening level Assessment of Thdiazuronum on Cotton Source: Aerial and ground Application to Cotton Thidiazuron/ Degradates/ Products Terrestrial Plant Exposure from Spray Drift, Runoff, and Sheet Runoff Receptor: Plants Adjacent to treated sties Receptor: Plants in Semi Aquatic Areas Acute Effects to Non endangered and Endangered Plants Acute Effects to Non endangered and Endangered Plants Receptor: Mammals in and Adjacent to treated sites Direct and Indirect Chronic Effects to Non endangered and Endangered Plants However, through an iterative process of examining fate and effects data, the conceptual model has been refined to reflect the exposure pathways and the organisms for which risk is most likely. Based on OPPs screening level assessment, the refined conceptual model for depicting the risks of thidiazuron primarily focuses on risk to terrestrial plants. Additionally, there are concerns about possible chronic effects to small mammals, depending on the methodology and toxic endpoints used to calculate the risk quotients. Further, since there are no chronic data currently available for birds, chronic risk to birds is undetermined. The refined conceptual model for depicting risks to nontarget terrestrial plants is presented below. Page 17 of 129 III. Integrated Environmental Risk Characterization Risks to Aquatic Organisms The results of the risk assessment suggest that acute and chronic risk quotients for all freshwater and marine fish and invertebrates are below all levels of concern established by the Agency's screening level assessment.

The acute risk quotients ranged from < 0.0003 ( marine fish) to 0.002 ( freshwater fish and estuarine invertebrates). Chronic risk quotients ranged from 0.002 ( freshwater fish) to > 0.1 ( freshwater invertebrates). In addition, the acute risk quotients for the non endangered and endangered species are well below all established levels of concern for aquatic plants. The definitive acute risk quotients ranged from 0.0005 to 0.013 and the endangered species risk quotients ranged from 0.006 to 0.2.

Although there may be some uncertainty with respect to chronic risk to freshwater and marine invertebrates because a definitive NOAEC value could not be obtained, it is possible to estimate chronic risk based on a 21 day EC 10 value which was obtained in the chronic freshwater invertebrate study. The 21 day EC 10 value, was 720 g/ L based on the length of the test organisms. Using this generated EC 10 value, the definitive 21 day RQ value of 0.015 is well below the chronic level of concern. In order for the chronic aquatic invertebrate LOC to be exceeded, the test would have to be repeated to test levels lower than 11 g/ L ( nearly 10 fold the current test level of 100 g/ L). It is doubtful that definitive NOAEC/ LOAECs would exceed the chronic levels of concern. Even if the persistence and intermediate mobility were taken into account where some year to year build up in soil would occur ( asymptotic limit twice that of annually applied amounts), the resulting RQs would still not exceed any aquatic LOCs.

Risk to Birds Acute Risks  Birds Acute risks to birds for this use are below all levels of concern.

To determine potential acute risk to birds, we use exposure data from Hoeger and Kenaga ( 1972), as modified by Fletcher et al. ( 1994). These data provide maximum and mean residue concentrations of a pesticide on various terrestrial food items on the basis of a one pound per acre application in the field. Predicted residue concentrations are available for short grass, tall grass, broad leaved plants/ small insects, and seeds/ large insects.

Toxicant concentrations on food items following multiple applications are predicted using a firstorder residue decline method, EFED's " ELLFATE" model, which allows determination of residue dissipation over time incorporating degradation half life. Predicted maximum and mean EECs resulting from multiple applications estimates the highest one day residue, based on the maximum or mean initial EEC from the first application, the total number of applications, interval between applications, and a first order degradation rate, consistent with EFED policy. The input parameters for the cotton scenario were based on a maximum single application of 0.2 lbs a. i./ A. Page 18 of 129 For multiple applications, a follow up application of 0.1 lb ai/ A at an interval between 5 and 7 days is allowed on all labels, and for this scenario EFED used the ELLFATE program to estimate the maximum residue concentrations occurring over a 99 day period based on a 35 day foliar dissipation half life and a 7 day interval between applications. The first application was run at 0.2 lb ai/ A. To factor in the second application of 0.1 lb ai/ A ( on day 6), the maximum residue concentrations from it were added to those remaining from the first application ( on day 0).

Acute data submitted for thidiazuron indicate that the dietary LC50s were greater than 5000 mg/ kg diet for bobwhite quail and mallard duck. The LD50s were greater than 3160 mg/ kg bw for both species. Since the results from both the dietary LC50 and the oral gavage LD50 classify thidiazuron as practically non toxic to birds and the formulations are non granular, the dietary LC50 for bobwhite quail was selected for risk quotient calculations.

The resulting acute risk quotients ( Appendix G) at all sites were below all levels of concern using the most sensitive LC50 of > 5000 mg/ kg diet. The highest RQ observed for two applications of 0.2 lb ai/ A for birds foraging in short grass was < 0.013.

Chronic Risk  Birds Chronic risks to birds cannot be quantitatively evaluated at this time because chronic data were not submitted. However, available data showing that chronic levels of concern for mammals are exceeded and that thidiazuron is persistent in the field suggest that there is also potential for chronic risk to birds should they be of equal of greater sensitivity than mammals.

Risks to Mammals Acute Risks  Mammals Acute risks to mammals were evaluated using the rat LD50 of > 2000 mg/ kg bw. There were no acute LOCs exceeded ( Appendix G). The highest risk quotient observed for the 15 g mammal foraging on short grass is < 0.03, well below the endangered species LOC of 0.1.

Chronic Risk  Mammals The results from the 2 generation rat study was used to assess chronic risk to mammals. The NOAEL for both parental and offspring toxicity in this study was 400 ppm diet ( 35.4/ 39.8 mg/ kg/ day [ M/ F]) and the LOAEL for these endpoints was 1200 ppm diet ( 108.5/ 121.1 mg/ kg/ day [ M/ F]). Although the reproduction performance in the laboratory was not adversely affected there were observed delays in female sexual maturation and irregular estrous cycles or acyclicity at the 1200 ppm dose, F1 females). Effects on the timing to reach sexual maturity and disruptions in estrous cycling could pose adverse reproduction effects in wild mammals because of the seasonality of reproduction periods in the wild and seasonal influences on the survival of offspring. Therefore the NOAEL for wild mammal reproduction risk assessment purposes would be 400 ppm diet. Page 19 of 129 A 3 generation rat reproduction study was also performed. This study was classified as coresupplemental in 1995 due to several deficiencies in the study design. Most notable of these was that there was no measure of food consumption and the amount of food consumed was estimated by using default food consumption values developed by the FDA many years ago. Since this study is the only chronic study currently available, EFED will use this data for this assessment.

The reproductive NOAEC and LOAEC values based on reduction of litter size are 200 and 600 mg/ kg diet, respectively. By standard conversion factors the NOAEL and LOAEL are 10 and 30 mg/ kg bw/ da, respectively. The mean litter size data from the study is summarized in the following table and indicate that significant reductions are occurring in the F3b generation at the highest dose, and possibly the lowest as well.

The quality of the 2 generation study, devoid of the inter litter mating problems associated with the 3 generation study, suggests that the effects measurement endpoints derived from this study be used in preference to those from the 3 generation study.

Using the NOAEL of 35.4 mg/ kg bw/ day is reasonable because the percent of reproductive impairment that is significant for a given species would be based on their reproductive strategy. For long lived ( K selected species), episodic reductions in reproduction might not be of the same significance as short lived ( r selected) species. Reproductive impairment for a short lived species could be enough to impact any number of species with low generation survival rates and a dependance on high reproductive output for population continuance ( r selected species).

To calculate mammalian risk quotients, we preferentially use the Nagy small mammal assumptions of percent body weight consumed for the 15, 35, and 1000 g mammals and divide the dietary EEC adjusted for food consumption by the NOAEL ( expressed as mg/ kg bw/ day). The resulting risk quotients are presented in Tables 7 and 8 of Appendix G and indicate that LOCs are exceeded for mammals ( herbivores/ insectivores) foraging in short grass and broadleaf forage and small insects. The risk quotients range from 1.8 ( short grass) to 0.01 ( large insects). The chronic LOCs are not exceeded for mammalian granivores.

Another method of calculating risk quotients is to divide the dietary EEC for each of the forage groups by the NOAEC ( expressed as mg/ kg diet). Using the NOAEC of 400 mg/ kg diet, the chronic LOCs are not exceeded for any of the food categories. These risk quotients are presented in Table 9 of Appendix G.

There are uncertainties associated with the risk quotient exceedances. As mentioned above, there was no measure of food consumption in the study or the amount of pesticide consumed. Further, there are no measured foliar dissipation half life data to evaluate residue concentrations; therefore, the EFED standard default foliar half life of 35 days was used.

Chronic risk can also be evaluated for single applications for maximum Fletcher values for predicted maximum and mean residues. Residues calculated for mean residues did not exceed the LOC. Page 20 of 129 Risks to Non Target Insects EFED currently does not estimate risk quotients for terrestrial non target insects. Whenever an LD 50 is < 11 g/ bee, an appropriate label statement is required to protect foraging honeybees. The acute contact toxicity study to honeybees revealed a contact LD 50 > 100 g/ bee for the technical thidiazuron and a contact LD 50 > 98.1 g/ bee for the SC42 formulated product. This classifies thidiazuron technical and the formulated product as practically non toxic to honeybees.

The two other non target insect OECD studies, which investigated the lethal and sub lethal effect of the SC42 water miscible suspension concentrate 500 g/ L on the parasitoid wasp ( Aphidius rhopalosiphi) and the predatory mite ( Typhlodromus pyri), were found not to show statistical differences from the controls for mortalities or reproduction effects. The wasp study showed the percent mortality was 12.5% in the highest exposed concentration of 0.2 lb ai/ A compared to 10% in the controls. The percent reduction when compared to the control was 27.3 and 38.1% in the 100 and 200 g ai/ ha treatment groups, respectively. In the predatory mite study the mortalities were 8% in the treated group compared to 13% in the untreated group, and the mean number of eggs per female was 9.32 in the control group compared to 8.99 in the 200 g ai/ ha treatment. Since these studies showed no statistically significant effects at the highest maximum single rate ( 0.2 lb ai/ A), there is little concern for non target insects.

Risks to Terrestrial Plants RQs for terrestrial plants in dry and semi aquatic areas are calculated for multiple and single spray applications for endangered and non endangered species. As mentioned in the exposure section, the runoff scenarios are based on solubility, and since the solubility of thidiazuron is 31 mg/ L ( ppm), the percent runoff value used in the risk quotient calculations was 0.02. A 60% application efficiency factor is also included in the runoff calculations for aerial applications. The detailed procedures for terrestrial exposure and plant risk quotient calculations are discussed in the terrestrial exposure section of this assessment. The detailed exposure and risk quotient calculations are tabulated in detail in Appendix G, and a summary of the resulting RQs for single applications is presented below. Page 21 of 129 Terrestrial Plant Risk Quotients for Single Applications Plant Group ( non Endangered / Endangered) Type of Plant Study Type of Plant ( monocot / dicot) Most Sensitive EC25 ( Non Endangered) or NOAEC / EC05 ( Endangered) Risk Quotient Range ( Plants Adjacent to Treated Sites) Risk Quotient Range ( Plants in Semi Aquatic Areas)

Nonendangered Seedling Emergence Monocot < 0.1783 > 0.03  > 0.07 > 0.17  > 0.22 Dicot 0.0152 0.39  0.79 1.97  2.63*

Vegetative Vigor Monocot > 0.1783 < 0.01  < 0.06 < 0.01  < 0.06 Dicot 0.0011 1.82  9.09* 1.82  9.09*

Endangered Seedling Emergence Monocot 0.1783 0.03  0.07 0.17  0.22 Dicot 0.0111 0.54  1.08* 2.7  3.6*

Vegetative Vigor Monocot 0.1783 0.01  0.06 0.01  0.06 Dicot 0.00019 10.53  52.63* 10.53  52.63*

* Exceeds terrestrial plant Levels of Concern ( LOC)

Multiple spray applications  All thidiazuron labels allow two applications. The sum of these is a maximum of 0.3 lb ai/ A/ yr ( 0.2 lb ai/ A for the first application and up to 0.1 lb ai/ A for the second application). Only one label specifies that intervals between applications should be between 5 7 days.

The uncertainty in the magnitude of exposure to multiple applications of thidiazuron is great. The TERR PLANT exposure model calculates the potential exposure for terrestrial plants for only a single application of thidiazuron. Since thidiazuron labels allow multiple applications to each crop, the magnitude of the risk to plants is uncertain, and potentially underestimated. The exposure calculations for terrestrial plants assume exposure both through spray drift and runoff from a treated field after a heavy rain event. The likelihood of co occurrence of such events is uncertain, especially for multiple applications, and as a result the estimated exposure values may be conservative. In addition, while the likelihood of exposure to non target plants through drift alone is significantly greater, the same plants may not be exposed to spray drift from each application, because wind speed and direction could be different at the time of each application.

Additional information on the effect of thidiazuron on terrestrial plants would be needed to evaluate the uncertainty in multiple application terrestrial plant RQs. The effects of multiple applications could only be additive if the affected plants could not recover from the effects of successive applications. If the plants could recover over time, the effect of multiple applications could be greater when crops are treated at the shortest application intervals.

Potential risks to terrestrial plants are possible because RQs exceed the LOCs. This is especially true for dicots for single applications to plants inhabiting dry and semi aquatic areas. The RQs Page 22 of 129 resulting from single applications to dicots range from 0.39 to 9.09 for non endangered plants and up to 53 for endangered plants.

Risk to monocot species appear to be substantially lower. However, definitive EC25s and NOAECs have not been determined because tier II tests were not conducted for some of the more sensitive species such as onion and oats. These EC25s range from > 0.05 to > 0.35 and are presented in more detail in the Appendix C.

Definitive tier II EC25s and NOAECs are lacking for the seedling emergence of two monocot species tested because test concentrations were not low enough to establish these endpoints. However, risk quotients from the tests for which definitive endpoints were established were below levels of concern. Furthermore, compared to the labeled application rates for field use and corresponding estimated field exposure concentrations, the lowest concentrations for the tests for which endpoints were not determined were adequate to establish that risk quotients would not exceed any of the terrestrial plant LOCs at current application rates for thidiazuron.

Thidiazuron inhibits regrowth by uptake through the roots and foliage. When thidiazuron is applied, ethylene levels artificially increase so the defoliation ( leaf drop) process begins. If nontarget plants which are more sensitive than cotton come into contact with thidiazuron, the leaves of these plants may be killed or severely damaged. As a result, the plant may die, or the damage may be sufficient to prevent the plant from reproducing or successfully competing with other plants for resources. The use of thidiazuron could apply selective pressure to dicots along field edges resulting in changes in species composition.

Even though thidiazuron is only applied at cotton harvest, application timing is a consequential factor to consider. Reproduction abnormalities are some of the plant injuries that can occur due to thidiazuron exposure during particular developmental stages. Late season annuals complete their life cycle in one year beginning with seed germination, plant growth, and reproduction. Late season spraying of thidiazuron could cause defoliation during the plant growth phase which can directly affect populations and may reduce their capability to reproduce. Biennials are plants that complete their life cycles in two years, producing foliage the first year and flowers the following year. Defoliation resulting from applications of thiadiazuron in the late season may inhibit these plants from reproducing the following year. Perennials are plants that grow throughout the year and produce flowers and/ or seeds at some point in their growing season. Late blooming perennials may be impacted by spraying of thidiazuron during their blooming phase and thidiazuron may inhibit the plants' ability to adequately reproduce. Although the plant may survive, sterile florets or non viable seed production can occur. This could have effects on nontarget plant populations' ability to sustain a viable population in future years.

Plant material serves as a primary food source for many species of animals. If the available plant material ( including seeds) is reduced due to the effects of thidiazuron, this may have negative effects throughout the food chain.

The exceedances discussed above were calculated using maximum applications rates derived from the thidiazuron labels. EFED completed a characterization of potential risks by assessing the effect of calculating RQ using EECs predicted from modeling with average application rates ( 0.05 Page 23 of 129  0.07 lb ai/ A) as reported in the BEAD QUA report. In the table below the average application rates are compared to the maximum allowable application rates. Although the RQs at the average rates are reduced by at least one half, only the risk to non endangered species caused by the single ground application would be eliminated in all categories expect dicot plants ( vegetative vigor).

The RQ values for both maximum and average application rates were calculated using toxicity data for the most sensitive monocot and dicot species. Therefore, although RQs for dicots in the table below exceed the acute and endangered species levels of concern, not all plants will be as sensitive as lettuce, the most sensitive species tested. In fact, only 2 of the 6 dicots tested exhibited effects in the seedling emergence test, and 4 of the 6 dicots tested exhibited effects in the vegetative emergence test.

However, although lettuce was the most sensitive among tested plants, it is uncertain where it would fall on a sensitivity distribution among plants which would be exposed in the field. Some would presumably be more sensitive than lettuce, and some less. In addition, Bayer has indicated that " thidiazuron is most efficacious when the target plant is vegetatively dormant and through the reproductive cycle." Seedling emergence and vegetative vigor tests are acute toxicity tests which do not test plants when they are vegetatively dormant nor through the reproductive cycle. Therefore, the test plants in the laboratory may less sensitive than non target plants in the field, and the results of the tests may underestimate thidiazuron toxicity. Furthermore, Bayer indicated that " defoliants work best with high temperatures and high humidity, night temperature > 16C, and low nitrogen supplies." Non target plants would be exposed to thidiazuron under the same climatic conditions which make it an effective cotton defoliant when it is applied. Since laboratory tests are not conducted under these conditions, test plants may be exposed under conditions which make them less sensitive to thidiazuron than they are in the field.

The risk assessment for terrestrial plants was based on RQs calculated from toxicity studies using a TEP ( typical end use product). The product used in this case was " Thidiazuron SC42", presumably DROPP SC ( EPA Registration Number 264 700). However, it appears that no surfactants or adjuvants were added to the product during the study, and all labels clearly state the addition of adjuvants has been shown to improve the performance. Therefore, the risk to terrestrial non target plants may be greater than RQs indicate. The production of actual tank mix data would reduce some of the uncertainties associated with risk. Page 24 of 129 Comparison of Maximum and Average Application Rate Risk Quotients ( RQ) Which Exceed the Terrestrial Plant Levels of Concern ( LOC)

Test Plant Group Maximum Application Rate RQ Average Application Rate RQ Adjacent Plants Semi Aquatic Areas Adjacent Plants Semi Aquatic Areas Single Ground Application  Non Endangered Species ( max. 0.2 lb ai/ A; average 0.07 lb ai/ A)

Seedling emergence Monocot Dicot 2.63 Vegetative vigor Monocot Dicot 1.82 1.82 Single Aerial Application  Non Endangered Species ( max. 0.2 lb ai/ A; average 0.07 lb ai/ A)

Seedling Emergence Monocot Dicot 1.97 Vegetative Vigor Monocot Dicot 9.09 9.09 3.18 3.18 Single Ground Application  Endangered Species ( max. 0.2 lb ai/ A; average 0.07 lb ai/ A)

Seedling Emergence Monocot Dicot 3.6 1.32 Vegetative Vigor Monocot Dicot 10.53 10.53 3.68 3.68 Single Aerial Application  Endangered Species ( max. 0.2 lb ai/ A/ yr; average 0.07 lb ai/ A/ yr)

Seedling Emergence Monocot Dicot 1.08 2.7 1.07 Vegetative Vigor Monocot Dicot 52.63 52.63 18.42 18.42 1 The following equation is used to calculate the various EC x levels: [ EC 25/ 10 0.67/ slope] x 10 a/ slope = EC x where a = 1.28, 0.84. 0.54, 0.25, 0,  0.25,  0.54,  0.84,  1.28 for EC 10, EC 20, EC 30, EC 40, EC 50, EC 60, EC 70, EC 80, EC 90, respectively. A lognormal toxicity distribution is assumed. This assumption of normality in log space is a source of some uncertainty in this assessment. For example, the lettuce vegetative vigor response (% inhibition relative to control) for shoot weight was 12, 27, 12, 22, and 40% for lowest to highest doses in the study.

Page 25 of 129 Additionally, as previously stated, four thidiazuron products are formulated as mixtures with other herbicides ( see table in section on Use and Use Characterization). In three of the mixed products, thidiazuron comprises 67% of total active ingredients and diuron the remaining 33%. In another co formulated product, thidiazuron comprises only 20% of the total active ingredients, while dimethipin comprises the remaining 80%. Plant data on these products with typical adjuvants would reduce the undetermined risk associated with these mixtures. An alternative to the submission of such data, assuming no synergism or antagonism among compounds, would be to simply add the known effects for these other two chemicals to those for thidiazuron.

Finally, all labels list precautions when applying the products adjacent to lettuce, citrus, or cantaloupe. The labels further state that mixing with organophosphates may increase non target crop phytotoxicity. Special restrictions to reduce drift potential to lettuce and citrus appear on all the labels. Thidiazuron products cannot be applied by air within ½ mile of lettuce or 5 miles downwind of the point of application when citrus is in flush in the Rio Grande valley of Texas. Ground applications are restricted to 100 ft downwind from the point of application for lettuce or ½ mile downwind from the point of application when citrus is in flush. No restrictions are prescribed for cantaloupe.

Phytotoxicity and Downwind Distance  Spray drift from aerial and ground applications may also impact non target plants or other crops grown adjacent to cotton, although the density of the spray droplets is expected to be considerably lower in these areas. Thidiazuron labels do not have many restrictions on how and under what conditions the product may be applied. For example, there are no restrictions on droplet size or boom height. Using EFED's current AgDRIFT computer model and registrant phytotoxicity data, it is possible to estimate distances downwind from application areas at which a particular toxic effect level would be experienced by a particular tested plant species. These estimates can be presented for combinations of aerial and ground applications for varying spray droplet size in the form of three dimensional bar charts. To calculate the EC 10  90 values in increments of 10, the EC 25 values from the seedling emergence and vegetative vigor studies used for RQ calculation are used in conjunction with the toxicity slopes from each of these studies1. Unfortunately slopes for only two of the ten crops for seedling emergence tests and four of the ten crops tested for vegetative vigor are currently available. These EC 10  90 values and the maximum application rate for cotton were entered into an Excel spreadsheet with Tier 1 Ag DRIFT ( version 2.01). Calculated downwind deposition levels were then compared to the EC 10  90 values to identify the downw distance at which the EC 10  90 values would be reached.

The few crops for which downwind distances could be calculated include lettuce and turnip for the seedling emergence tests, and lettuce, cucumber, tomato, and soybean for the vegetative vigor tests. Although these crops were the most sensitive in the available two tests, the current labels Page 26 of 129 contain warnings and specific instructions concerning the use of thidiazuron on lettuce, cantaloupe, and citrus. The language from one of the labels is the following:

" Particular care should be taken when applying Dropp ® UltraTM adjacent to lettuce, citrus, or cantaloupe."

" In addition, for citrus crops, do not apply Dropp ® UltraTM by air when citrus in flush is within five ( 5) miles downwind of the point of application. Do not apply Dropp ® UltraTM by ground when citrus in flush is within one half ( 1/ 2) mile."

The bar graphs for the data for which we have slopes are presented on the following pages for aerial and ground application under varying droplet sizes. Following the graphs is a discussion of their interpretation and EFED policy for terrestrial plants. Page 27 of 129 10 20 30 40 50 60 70 80 90 t urnip let t uce 0 50 100 150 200 Percent Effect No Spray Zone ( f t ) Aerial: Coarse Spray ( Seedling emergence) Page 28 of 129 10 20 30 40 50 60 70 80 90 t urnip let tuce 0 50 100 150 200 250 300 Percent Effect No Spray Zone ( ft) Aerial: Medium Spray ( Seedling emergence) Page 29 of 129 10 20 30 40 50 60 70 80 90 0 200 400 600 800 1000 Pe rce nt Effe ct Aerial: Fine Spray ( Seedling emergence)

turnip lettuce Page 30 of 129 10 20 30 40 50 60 70 80 90 soybean tomato cucumber let tuce 0 200 400 600 800 1000 Percent Effect No Spray Zone ( ft) Aerial: Coarse Spray ( Vegetative vigor) Page 31 of 129 10 20 30 40 50 60 70 80 90 soybean t omato cucumber let t uce 0 200 400 600 800 1000 Percent Effect No Spr ay Zone ( ft) Aerial: Medium Spray ( Vegetative vigor) Page 32 of 129 10 20 30 40 50 60 70 80 90 soybean cucumber 0 100 200 300 400 500 600 700 800 900 1000 Percent Effect No Spray Zone ( ft) Aerial: Fine Spray ( Vegetative vigor) soybean tomato cucumber lettuce Page 33 of 129 10 20 30 40 50 60 70 80 90 turnip lettuce 0 1 2 3 4 5 6 7 8 9 10 Percent Effect No Spray Zone ( ft) Ground: Low Boom, Medium Coarse Spray ( Seedling emergence) Page 34 of 129 10 20 30 40 50 60 70 80 90 turnip lettuce 0 2 4 6 8 10 12 14 Percent Effect No Spray Zone ( ft) Ground: HIgh Boom, Medium Coarse Spray ( Seedling Emergence) Page 35 of 129 10 20 30 40 50 60 70 80 90 turnip lettuce 0 2 4 6 8 10 12 14 Percent Effect No Spray Zone ( ft) Ground: Low Boom, Medium Spray ( Seedling emergence) Page 36 of 129 10 20 30 40 50 60 70 80 90 t urnip let t uce 0 1 0 20 30 40 50 60 70 No Spr ay Zone ( ft) Ground: HIgh Boom, Medium Spray ( Seedling emergence) Page 37 of 129 10 20 30 40 50 60 70 80 90 soybean tomato cucumber let tuce 0 50 100 150 200 250 300 350 400 Percent Effect No Spray Zone ( ft) Ground: Low Boom, Medium Coarse Spray ( Vegetative vigor) Page 38 of 129 10 20 30 40 50 60 70 80 90 soybean tomato cucumber lettuce 0 100 200 300 400 500 600 Percent Effect No Spray Zone ( ft) Ground: HIgh Boom, Medium Coarse Spray ( Vegetative vigor) Page 39 of 129 10 20 30 40 50 60 70 80 90 soybean cucumber 0 100 200 300 400 500 600 700 Pe r cent Effect No Spray Zone ( ft) Ground: Low Boom, Medium Spray ( Vegetative vigor) Page 40 of 129 10 20 30 40 50 60 70 80 90 soybean t omato cucumber let t uce 0 1 00 200 300 400 500 600 700 800 900 1 000 Pe r ce n t Eff e ct No Spray Zone ( ft) Ground: HIgh Boom, Medium Spray ( Vegetative vigor) Page 41 of 129 EFED policy with regard to non endangered terrestrial plants is that the LOC is exceeded when the environmental exposure concentration is greater than the EC 25. In the case of thidiazuron, aerial application is the most common mode of application, and, as demonstrated in the charts above, no spray zones for the EC 30 effect levels range from > 60 ft for course sprays to over 1000 ft. for fine sprays for the most sensitive crop tested so far ( lettuce). Ground applications indicate that no spray zones for the EC 30 effect level range from < 3 ft for low boom, medium course spray to < 200 ft for high boom medium sprays. The charts also show similar trends for the other crops.

The endangered species terrestrial plant LOC is exceeded when the environmental concentration is greater than the EC 05. Aerial applications of thidiazuron indicate that the no spray zone the EC 10 effect level ranges from > 95 ft. for course sprays to > 1000 ft for fine sprays for lettuce. Ground applications ranges from > 95 ft for low boom, medium course sprays to < 1000 ft for high boom, medium sprays.

The results of this analysis indicate that downwind distance for a specific EC x can be determined for four of the crops which were tested. All the factors influencing the downwind drift distance including droplet size, wind speed, boom height, method of application, and targeted toxicity effect should be considered when placing restrictions on labels.

Endocrine Disruption Assessment The 2 generation reproduction study with rats indicates the potential for endocrine disruption, based on delayed sexual maturation and disruption of the estrous cycle. The reproductive NOAEC and LOAEC values based on reduction of litter size are 400 and 1200 mg/ kg diet, respectively. The measured NOAEL and LOAEL were 35.4 and 108.5 mg/ kg bw/ da, respectively. This reproductive effect could be an indicator of potential endocrine disruption in mammals or other organisms.

EPA is required under the Federal Food, Drug, and Cosmetic Act ( FFDCA), as amended by the Food Quality Protection Act ( FQPA), to develop a screening program to determine whether certain substances ( including all pesticide active and other ingredients) " may have an effect in humans that is similar to an effect produced by a naturally occurring estrogen, or other such endocrine effects as the Administrator may designate." Following the recommendations of its Endocrine Disruptor Screening and Testing Advisory Committee ( EDSTAC), EPA determined that there was scientific bases for including, as part of the program, the androgen and thyroid hormone systems, in addition to the estrogen hormone system. EPA also adopted EDSTAC's recommendation that the Program include evaluations of potential effects in wildlife. For pesticide chemicals, EPA will use The Federal Insecticide, Fungicide, and Rodenticide Act ( FIFRA) and, to the extent that effects in wildlife may help determine whether a substance may have an effect in humans, FFDCA authority to require the wildlife evaluations. As the science develops and resources allow, screening of additional hormone systems may be added to the Endocrine Disruptor Screening Program ( EDSP). When the appropriate screening and/ or testing protocols being considered under the Agency's EDSP have been developed, thidiazuron and its degradates may be subjected to additional screening and/ or testing to better characterize effects related to endocrine disruption. Page 42 of 129 Endangered Species Assessment The Agency's level of concern for endangered and threatened non target terrestrial plants and mammals is exceeded for the use of thidiazuron on cotton.

Action Area For listed species assessment purposes, the action area is considered to be the area affected directly or indirectly by the Federal action and not merely the immediate area involved in the action. At the initial screening level, the risk assessment considers broadly described taxonomic groups and so conservatively assumes that listed species within those broad groups are collocated with the pesticide treatment area. This means that terrestrial plants and wildlife are assumed to be located on or adjacent to the treated site and aquatic organisms are assumed to be located in a surface water body adjacent to the treated site. The assessment also assumes that the listed species are located within an assumed area which has the relatively highest potential exposure to the pesticide, and that exposures are likely to decrease with distance from the treatment area. Section II of this risk assessment presents the pesticide use sites that are used to establish initial collocation of species with treatment areas.

If the assumptions associated with the screening level action area result in RQs that are below the listed species LOCs, a " no effect" determination conclusion is made with respect to listed species in that taxa, and no further refinement of the action area is necessary. Furthermore, RQs below the listed species LOCs for a given taxonomic group indicate no concern for indirect effects upon listed species that depend upon the taxonomic group covered by the RQ as a resource. However, in situations where the screening assumptions lead to RQs in excess of the listed species LOCs for a given taxonomic group, a potential for a " may affect" conclusion exists and may be associated with direct effects on listed species belonging to that taxonomic group or may extend to indirect effects upon listed species that depend upon that taxonomic group as a resource. In such cases, additional information on the biology of listed species, the locations of these species, and the locations of use sites could be considered to determine the extent to which screening assumptions regarding an action area apply to a particular listed organism. These subsequent refinement steps could consider how this information would impact the action area for a particular listed organism and may potentially include areas of exposure that are downwind and downstream of the pesticide use site.

Species Affected The preliminary risk assessment for listed species indicates that thidiazuron exceeds the endangered species LOCs for the following combinations of analyzed uses and species:

° Use of thidiazuron on cotton indicate that chronic LOCs are exceeded for mammals foraging on short grass and broadleaf forage and small insects.

° Use of thidiazuron on cotton results in potential risks to listed species of plants. Page 43 of 129 County level location data for listed species ( terrestrial plants and mammals) were compared with county level information on crop production to identify coarsely overlaps of listed species with the proposed labeled uses of thidaizuron. This analysis was limited to those counties with 10 or more acres of land in production for the labeled crops. The results are presented below:

Numbers of Listed Plant and Mammal Species in States with at Least 10 Acres of Cotton State Number of Listed Species Plants Mammals Arkansas 2

California 49 9 Florida 5 4 Georgia 14 3 Kansas 1

Louisiana  2 Mississippi  1 Missouri 5 1 New Mexico 5 5 North Carolina 17 3 Oklahoma  1 South Carolina 10

Tennessee 4 2 Texas 19 3 Virginia 2 1 IV. Environmental Fate Assessment Basis This assessment is based on an integration of: 1) environmental fate study data from previously reviewed studies, in some cases with subsequent amendments, addenda, or other written explanatory responses or updated information; 2) environmental fate study data from the current review of new fate studies submitted to support the re registration of thidiazuron; 3) intrinsic physical/ chemical properties; and 4) information from product labels. As with all EFED fate assessments, it does not take into consideration any alteration of the environmental fate properties of thidiazuron that may be associated with the addition of co formulated pesticides ( currently Page 44 of 129 diuron or dimethipin); so called " inert" ingredients ( formulants); tank mixtures with other pesticides, for example, organophosphates, for which label statements indicate increased phytotoxicity; or tank mixes with adjuvants, such as surfactants or oils, for which product label statements indicate " improved performance."

Status of Environmental Fate Data Although some of the submitted fate studies had deficiencies, taken as a whole and within the constraints stipulated in the fate assessment, the data enable the Agency to adequately assess the environmental fate of thidiazuron. Although no foliar dissipation studies are available to the Agency at this time ( these are not generally required), having such data would make any fate assessment more precise, especially in the case of a defoliant such as thidiazuron. However, as discussed in the fate assessment, if dislodgement from or transformation in/ on treated plants were to be an important fate pathway, then EFED's inferences from existing fate studies and assumptions for exposure modeling for parent and potential degradates should reasonably and conservatively cover the possibility. If the registrant wishes to demonstrate that definitive data on foliar dissipation would substantially affect any of EFED's inferences, assumptions, or environmental risk conclusions, the registrant should submit appropriate data. Otherwise, except for spray drift requirements, the Agency needs no additional environmental fate studies at this time.

Required field spray drift and spray droplet size spectral data have not been submitted. However, since the applicant is a member of the Spray Drift Task Force ( SDTF), the registrant may be able to satisfy the spray related requirements with data the SDTF has submitted to the Agency.

Fate Assessment Abstract. In soil, thidiazuron is persistent, as evidenced by laboratory and field half lives and rotational crop intervals on the order of one year. It has intermediate soil sorption coefficients. Such persistence and intermediate mobility would allow some year to year accumulation over time and, therefore, more opportunity for favorable conditions for runoff from application sites to occur. With a one year half life, build up in soil could asymptotically approach twice that of annually applied amounts.

When thidiazuron reaches surface water photolysis is expected to be the major route of transformation; other degradative processes are essentially negligible by comparison. Aqueous photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of the photodegradates is a structural isomer of parent, whereas the other has a substantially altered chemical structure. The photolysis process requires special modeling considerations.

Based on its solubility, vapor pressure, and other laboratory evidence, thidiazuron is non volatile. Based on its relatively low octanol/ water partitioning coefficient, thidiazuron should not bioconcentrate.

No data on foliar dissipation are available to EFED. Since thidiazuron is used to defoliate cotton prior to harvest, dislodgement from or transformation in/ on leaves may be an important fate pathway. The effect of this uncertainty and alternative assumptions on the risk assessment are considered. Page 45 of 129 N H N H N N S O Physical/ Chemical Properties Thidiazuron's chemical structural formula, various names and table of physical/ chemical properties are as follows:

Common Name: Thidiazuron Other Designations: AE B049537, SN 49537, NC 19211, Hoe 080279 IUPAC name: 1 Phenyl 3( 1,2,3 thiadiazol 5 yl) urea CAS name: N Phenyl N' 1,2,3, thiadiazol 5 yl urea CAS No.: 51707 55 2 ( 330 54 1 also found) PC Code No.: 120301 Empirical formula: C 9 H 8 N 4 O S Molecular Weight: 220.25 SMILES string: O= C( Nc1ccccc1) Nc1cnns1 Page 46 of 129 Physical and Chemical Properties of Thidiazuron ( from product chemistry review)

GLN Requirement MRID Status Result or Deficiency 830.6302 C o l o r 459340 01 A TGAI: yellow Brown; PAI: Brown 830.6303 Physical state " " " A TGAI: amorphous solid; PAI: crystalline solid 830.6304 Odor " " " A TGAI & PAI: characteristics Sulfur like odor 830.6313 Stability to normal and elevated temperatures, metals, and metal ions " " " A TGAI: None PAI: None 830.6314 Oxidation/ reduction: chemical incompatibility " " " A TGAI: No reaction with H 2 O, KMnO 4, Zn or CO 2 PAI: No reaction with H 2 O, KMnO 4, Zn or CO 2 830.6315 Flammability NA 830.6316 Explodability 459340 01 A TGAI: exotherm onset observed at 116.30C PAI: NA 830.6317 Storage stability 459340 01 I 1 year under progress 830.6319 Miscibility NA 830.6320 Corrosion characteristics 459340 01 I 1 year under progress 830.7000 pH 459340 01 A TGAI: 5.21 @ 20 0C PAI: 6.50 830.7050 UV/ Visible absorption 459340 01 A   max = 238 nm;   = 985 M 1 cm 1 830.7100 Viscosity NA 830.7200 Melting point 459340 01 A PAI: 2230C ( decomposition)

830.7220 Boiling point NA 830.7300 Relative Density 459340 01 A TGAI: 0.324 gm/ cc at 20 0C 830.7370 Dissociation constants in water 413649 01 ( cited) A PAI: pKa = 8.86 25 0C 830.7550 Partition coefficient ( n octanol/ water), shake flask method 413659 06 ( cited) A PAI: K o/ w = 50.0 ( 25 0C, pH = 7.3)

830.7840 Water solubility: column elution method; shake flask method 413649 06 417862 02 ( cited) A PAI: water = 31.0 mg/ L ( pH = 7.0; 25 0C) organic solvents ( g/ l): acetone, 6.67; CH 2 Cl 2,0.003; ethyl acetate, 1.10; hexane, 0.002; MeOH, 4.20; Toluene, 0.40 ( all measured at 20 0C)

830.7950 Vapor pressure 942460 01 ( cited) A PAI: 3.5 x 10 9 mPa @ 20 0C A = Acceptable; N = Unacceptable ( see Deficiency); N/ A = Not Applicable.; G = Data gap; I = In progress or need upgrade As the table shows, thidiazuron is a weak organic acid with a pKa of approximately 8.9. Therefore, at pHs found in most aquatic and terrestrial environments, it exists mainly in neutral form (> 99% neutral at pH 7 or below, 88% at pH 8, and 42% at pH 9). Thidiazuron is nonvolatile as indicated by its reviewer calculated Henry's Law constant ( H = 2.5 x 10 16 atm m3/ mol and dimensionless K H = 1.0 x 10 14 at 20 C) and by collateral data showing the absence of thidiazuron in collection traps for volatile products in various laboratory degradation and 2Half lives are based on first order linear regression of time ( x axis) versus the logarithm of the relative concentration ( percent of radioactivity) of thidiazuron ( y axis). These experimental half lives were then adjusted by the reviewer to the EFED standard temperature of 25

C and to the standard soil moisture content of 75% of 1/ 3 bar water potential ( suction) by the methods described in the next footnote.

3MRID 46119601, Allan, J. G., 2003; EFED review 24 May 2004. Experimental half lives were adjusted by the reviewer to the EFED standard temperature of 25

C by use of the Arrhenius equation and to the standard soil moisture content of 75% of 1/ 3 bar water potential ( suction) by use of the Walker equation, as described in the Data Evaluation Record ( DER). Actual study conditions were 20

C and 40% maximum water holding capacity ( MWHC); under these conditions half lives were 163, 355, and 322 days, respectively, as extrapolated from study periods lasting 160 168 days.

4MRID 41950101, Feyerabend, M., 1991; EFED review 28 Apr 1993. Previous reviewer incorrectly reported the half life as 111 days at 21

C and 78% of 1/ 3 bar. The 111 day value was the registrant reported DT50, not the half life. Based on this reviewer's calculation using standard semi logarithmic transformed data, the experimental half life is 182 days ( 95% confidence range of 155 221 days). After adjustment to 25

C and 75% of 1/ 3 bar soil water potential by the method described by the reviewer in the DER for MRID 46119601 ( see previous footnote), the standard half life becomes 140 ( 119 170) days. ( The registrant's calculated half life using untransformed, simple first order data was 147 days ( no confidence range reported.)

Page 47 of 129 metabolism studies. Its relatively low n octanol to water partitioning ratio of approximately 50 indicates little likelihood of bioconcentration.

Persistence Degradation/ Metabolism Half lives In standard laboratory studies, thidiazuron did not hydrolyze ( MRID 42069203), and was stable against anaerobic metabolism in water ( MRID 4266601) and soil ( MRID 41945201).

Thidiazuron was relatively long lived in aerobic soil. As a measure of its persistence, laboratory aerobic soil metabolism half lives2 in four soils were approximately 206 ( 173 254), 253 ( 187 393), 436 ( 310 737), and 140 ( 119 170) days ( 95% confidence intervals in parentheses as a measure of precision). The first three listed half lives are derived from MRID 461196013, the fourth from MRID 419501014. The average of these four values is 259 days, and the one tailed upper 90% confidence bound for the mean, which is used for EFED standard environmental modeling purposes, is approximately 363 days ( 259 day average plus a confidence level of 104 days). Soil metabolites are identified in a Transformation Products ( degradates/ metabolites) subsection below.

Crop rotational intervals also provide a measure of thidiazuron's persistence in soil. Product labels list rotational crop intervals as long as one year.

Another index of thidiazuron's persistence in soil is dissipation in terrestrial field studies. It should be recognized, however, that field dissipation is punctuated by variable climatic events, and generally involves a combination of physical dispersal/ transport ( leaching, runoff, volatilization) and chemical and/ or biochemical transformation. Furthermore, transformation does not necessarily imply detoxification, since byproducts may also be toxic. Results from terrestrial field studies are as follows: Page 48 of 129 In three supplemental, bare ground field studies ( MRID 44454001), each conducted for 18 months, linear first order dissipation half lives were 237 days ( California), 173 days ( Florida), and 161 days ( North Carolina). Reviewer calculated respective times required for 90% loss, based on simple first order kinetics were approximately 800, 600, and 500 days. Corresponding registrantcalculated DT90 values, based on a two compartment model, were 1190, 655, and 475 days. Although these measures of field persistence are conventionally reported, because of the inherent variability and randomness in dissipation processes, they should be interpreted only in a crude, qualitative sense. Three transformation products were sporadically identified, but were most frequently below the limit of quantitation; therefore, a kinetic pattern could not be established. In another supplemental study in Florida ( MRID 41761105), there was no significant dissipation of thidiazuron during a nine month study period in which only small amounts of two transformation products were detected. Identified soil transformation products and observations concerning mobility are discussed separate subsections below.

Thus, laboratory aerobic soil metabolism studies, product label statements, and field dissipation studies show a general consistency and a similar degree of persistence for thidiazuron. The demonstrated persistence would allow some year to year accumulation. With the hypothetical assumption that there is no significant physical movement of thidiazuron from a site of application for several years, then, with the upper 90% confidence bound half life of approximately one year and with uniform annual applications, the asymptotic limit of build up in soil would result in soil concentrations that would approach twice that of the yearly application amount.

Photolysis of thidiazuron requires special consideration. In water ( original MRID 41188201, related follow up MRIDs 41364910 , 43075202, and fate overview document MRID 44436901), it undergoes relatively rapid ( see below), pH dependent, branching photolysis into two photoproducts. Formation of these products is in constant ratio, and in complementary stoichiometric yield with parent. A product originally identified as " photoproduct I" and subsequently identified as AE F132347 ( see chemical structure in Appendix B) is an isomer of parent differing only in the relative positions of the sulfur atom and the two nitrogen atoms in the 5 membered ring. Like parent, photoproduct I is moderately ecotoxic ( slightly more ecotoxic than parent, based on limited data). It has a reviewer calculated asymptotic experimental limit of production of approximately 77% of parent at pH 5, 28% at pH 7, and 17% at pH 9. This product was stable to further photolysis under the experimental study conditions and durations.

Photoproduct II ( AE C421200) is a significantly degraded, practically non ecotoxic product, and has a complementary reviewer calculated asymptotic limit of production to photoproduct I of 23% of parent at pH 5, 72% at pH 7, and 83% at pH 9. This product was also stable to further photolysis under study conditions.

Under standard, ideal laboratory exposure conditions, the pH dependent photolysis half lives were approximately one to two hours ( adjusted to twelve hours of sun per day). [ Note: When these half lives are put directly into current EFED aquatic modeling scenarios, because of water depth, clarity, and other factors, the effective or practical photolysis half lives become equivalent to approximately five to ten days.] For the purpose of aquatic ecological exposure assessment, we protectively assumed the pH 5 results for which 77% of parent reaching water is converted to the stable isomer photoproduct I ( AE F132347). Under these conditions, the thidiazuron photolytic Page 49 of 129 half life was 1.44 hours ( 0.0602 days). Half lives at higher pHs were somewhat shorter ( approximately 6% shorter at pH 7 and approximately 40% shorter at pH 9.

To further simplify exposure estimates for aquatic ecological assessment, rather than separately apportioning parent and photoproduct I exposures and toxicities, we chose to add exposure concentrations of parent and photoproduct I ( ignoring the practically non toxic ecotoxic photoproduct II), and use the toxicity of the slightly more ecotoxic photoproduct I for parent also. Whether we use this simplified procedure or add the separate contributions from parent and photoproduct I makes no meaningful difference in the aquatic ecological risk assessment in this particular case. Appendix D describes in detail the special quantitative procedure used for estimating aquatic exposure concentrations. For surface drinking water sources for humans, we provide two other alternatives, depending on potential human health concerns for parent and photoproducts; these alternatives are discussed in the Drinking Water Assessment Appendix A. Because of thidiazuron's demonstrated persistence in soil, modeled concentrations in water could be underestimated by as much as a factor of two.

A unacceptable laboratory soil photolysis study conducted on thin plated soil samples had numerous flaws and inconsistencies ( MRID 00156241 and MRID 41364902, the latter being essentially a re submission of a published version of the unpublished former; and fate overview document MRID 44436901, which summarized study issues and responses). The previously cited unacceptability of the study is confirmed by the current reviewer ( who has noticed even additional study irregularities). Ostensibly, the study authors maintained that soil photolysis was occurring at a relatively rapid rate, with a half life on the order of one hour or less, and that the only significant product was the isomer photoproduct I ( AE F132347). The reviewer notes that photoproduct I is the same photoisomer found in the aqueous photolysis study discussed above. In view of the study unacceptability and the inherent marked physical differences between the effectiveness of photolysis on soil that is plated thin ( 0.5 mm) on glass an that applied on soil in the field, the registrant and the Agency are in agreement that terrestrial field studies would serve to determine the relative importance of soil photodegradation. In all field studies ( the four studies discussed above), thidiazuron was sprayed on bare soil without incorporation. In these studies thidiazuron was long lived, and there was only minimal evidence of photolysis. Hence, to whatever extent photolysis may occur in/ on field soil, it is definitely a slow process.

Transformation Products ( degradates/ metabolites)

Chemical structures, chemical names, and other forms of past and present identifiers are in Appendix B. Naming conventions, different company codes for the same compounds, and incorrectly named compounds have been and can easily be tedious sources of confusion and potential error. Major photodegradates in water and soil have already been discussed.

In the two submitted laboratory aerobic metabolism studies identified above, no major ( defined as > 10% of applied radioactivity) organic transformation products were isolated or identified in any of a total of four soils tested. In the most recent study ( MRID 46119601), the three soils tested were treated with thidiazuron radiolabeled with 14C in the thiadiazol ring position; in the other study ( MRID 41950101), the one soil was treated with thidiazuron uniformly radiolabeled with 14C in the phenyl ring. Such labeling insures that major molecular changes can be monitored. Page 50 of 129 In the three soils treated with the thidiazol ring label ( MRID 46119601), the only organic transformation product identified was AE F132345 ( thidiazolylurea or 1,2,3 thiaiazol 5 ylurea). In the soil in which metabolism was fastest, AE F132345 appeared to reach an approximately flat maximum concentration of roughly 6 9% of applied radioactivity during the period from approximately 88 days to study end at 168 days. The broad plateau is evidence that AE F132345 would not form in greater yield, and that it may be a persistent metabolite. Maximum concentrations of AE F132345 in the other two soils were roughly 1% to 3%. Extracted substances that were not identified in any soil at any sampling interval comprised no more than roughly 1 6% of applied radioactivity. Non extracted 14C material ( so called " bound" residues) in these soils increased variably from approximately 16 32% of applied at study end. In one soil, 14CO 2 reached a broad total plateau of 5 12% of applied radioactivity during later study periods. In the two other soils, 14CO 2 totaled around 1 2% and 6 8%, respectively, during later study periods. For all three soils, volatile organics totaled 0.1% of applied radioactivity. The data evaluation record ( DER) for the three soil study has additional details for individual soils, and a brief comment on a postulated transformation pathway.

In the one soil treated with the uniformly labeled phenyl ring ( MRID 41950101), three very minor metabolites were identified, and at least one these may have been an initial impurity. All concentrations of each were < 0.8% of applied radioactivity. The three minor metabolites were: AE F132347 ( 0.8%), the compound that may have been an initial impurity and which is the same compound identified above as photoproduct I; phenylurea ( 0.3%); and AE F147706 ( 0.8%). ( Appendix B shows chemical structures and has various past and present forms of identification.) Of extracted substances, approximately 3% of applied radioactivity was not identified. By the end of the 361 day study, unextracted (" bound") residues increased to approximately 45% of applied, and 14CO 2 accounted for approximately 21%.

In the supplemental terrestrial field studies cited above ( MRID 44454001 and MRID 41761105), three transformation products were detected, but rarely above the limit of quantitation of 0.01 parts per million ( ppm). The three compounds were AE F132345, AE F132347 ( photoproduct I), and phenylurea. Since they were almost never detected above the limit of quantitation of 0.01 ppm for any sampling interval, no kinetic analysis is possible for them. The maximum concentrations for the sporadic detections of these products ranged, without correlation, from roughly < 4% to perhaps as high as 19% of the maximum concentration ever detected for parent.

Thus, based on laboratory and field studies, thidiazuron undergoes very slow transformation in soil by aerobic metabolism, with an indefinite contribution from photolysis. Laboratory studies further suggest that the identified transformation products will slowly be converted in soil into the carbon pool of bound residues, carbon dioxide, and other products of mineralization.

Mobility in Soil Based on batch equilibrium sorption coefficients in four soils ( MRID 41364909, three German standard soils and one United Kingdom soil), thidiazuron has intermediate mobility, as indicated by Freundlich adsorption coefficients of approximately 4.4, 7.3, 16, and 19 Freundlich units. Slopes ( 1/ n) of the isotherms ranged from approximately 0.7 0.8. Sorption correlated reasonably well Page 51 of 129 with organic carbon, and respective Freundlich coefficients normalized for organic carbon ( Kfoc) were approximately 900, 800, 800, and 500. Soil textural identification and detailed sorption values are in Appendix C.

In the four supplemental terrestrial field studies in bare soil discussed above, no significant leaching was apparent for parent or transformation products. However, because of imposed experimental limitations that were common to each study ( for example, limits of quantitation and the absence of a water balance or use of a tracer) potential for leaching was not clearly established. However, based on the intermediate range of its sorption coefficients and the supplemental field evidence, leaching should not be major route of dissipation. Model estimated potential exposure to thidiazuron from surface and ground drinking water source are presented in the separate Drinking Water section of this document.

The one major metabolite found in laboratory aerobic soil studies, 1,2,3 thiadiazol 5ylurea ( also identified as thidiazolylurea, AE 132345, and ZK 85290 ), had somewhat lower sorption coefficients than parent. In four United States soils, its Freundlich adsorption coefficients were approximately 1.5, 8.3, 2.0, and 1.7 Freundlich units. Values for 1/ n ranged from approximately 0.8 1. Sorption also correlated reasonably well with organic carbon, and respective Freundlich coefficients normalized for organic carbon ( Kfoc) were approximately 200, 280, 170, and 340. Soil textural identification and detailed sorption values are in the Data Evaluation Record ( DER) for MRID 44436902. As was the case for parent in the same supplemental field studies, no significant leaching pattern was evident for this metabolite, which was detected sporadically and without correlation at < 0.05 ppm. However, as previously stated, potential for leaching was not clearly established in the field.

Assumptions/ Limitations/ Uncertainties No data on foliar dissipation are available to EFED. Since thidiazuron is used to defoliate cotton prior to harvest, dislodgement from or transformation in/ on leaves may be an important fate pathway. Although photolysis was the rapid, major route of transformation in water, in field soil parent was long lived with only minimal evidence of photolysis or leaching. However, in laboratory plates of thin soil, the photoisomer AE F132347 ( photoproduct I) was a quickly formed major degradate. Thus, whether photolysis in or on cotton plants is a major fate process is unclear, and can be decided only by experiment. However, based on available data, if photolysis were to be substantial on foliage, it is a reasonable inference that the same major structural isomer formed in water and soil also be produced on foliage. In this case, EFED's modeling assumptions for parent and the photoisomer should cover the possibility, and, within standard model limitations, our estimated water exposure concentrations remain essentially the same.

It bears repeating that thidiazuron is persistent in soil, as evidenced by laboratory and field halflives of the order of one year, and that it has intermediate soil sorption coefficients. Such persistence and intermediate mobility allows some year to year build up in soil over time and, therefore, greater opportunity for favorable conditions for runoff from application sites to occur. With the hypothetical assumption that there is no significant physical movement of thidiazuron from a site of application for several years, then, with a one year half life and uniform annual applications, the asymptotic limit of build up would result in soil concentrations that would 5EFED's Pesticides in Ground Water Database has no thidiazuron data. The United States Geological Survey ( USGS), as part of a large water monitoring program, does not analyze for thidiazuron. The EPA STORET water quality databases have no useful information.

Page 52 of 129 approach twice that of the yearly application amount. Runoff concentrations would be affected similarly.

V. Drinking Water Assessment Summary There are no useful water monitoring data for thidiazuron in EFED's standard monitoring sources5. The exposure estimates in this assessment come from the two EFED Tier 1 screeninglevel models: 1) FIRST ( Ver. 1.0; Aug. 1, 2001) for surface water concentrations resulting from runoff and spray drift into an Index Reservoir and 2) SCI GROW ( Ver. 2.3; Nov. 4, 2003) for ground water concentrations resulting from leaching. These models and their descriptions are available at the EPA internet site:

http:// www. epa. gov/ oppefed1/ models/ water.

As described in the Fate Assessment, when thidiazuron reaches surface water photolysis is expected to be the major route of transformation; other degradative processes are essentially negligible by comparison. Aqueous photolysis is rapid, and occurs by branching in quantitative yield into the two photoproducts identified as AE F132347 and AE C421200. The first is a structural isomer of parent, while the latter has a substantially altered chemical structure. The photolysis process, chemical identification, and special modeling and computational considerations are in Appendix A. Ground water is not expected to be impacted by these degradates because all field studies show that parent is long lived in soil ( half lives of the order of one year) with only minimal evidence of photolysis.

Modeling Results. The following table shows the FIRST derived potential surface water concentrations for the three respective combinations of 1) parent plus both aqueous photodegradation products ( most conservative), 2) parent plus photodegradate 1, and 3) parent only ( least conservative). Degradate modeling is based on mass balance using simultaneous decline and formation over time. All reported concentration combinations are in terms of parent mass chemical equivalents. However, concentrations for each photodegradate can easily be separated.

The tabulated exposure concentrations are for a single aerial application at a maximum rate of 0.3 lb a. i./ acre. However, as specifically illustrated in Appendix A, whether the total annual maximum is applied all at once or is split in two ( as specified on product labels), differences are negligible for the particular case of thidiazuron because of its long life in soil. Again, complete model input/ output tables with detailed explanations of the computational procedures used to handle the photodegradates are in Appendix A. Page 53 of 129 Surface Water Concentrations ( derived from FIRST model, Ver. 1.0)

THREE ALTERNATIVE DEGREES OF PROTECTION PEAK DAY ( ACUTE) CONCENTRATION ( ppb or µ g/ L) ANNUAL AVERAGE ( CHRONIC) CONCENTRATION ( ppb or µ g/ L)

Parent + Both Photoproducts 3.5 1.0 Parent + AE F132347 ( photoproduct 1) 3.5 0.82 Parent Only 3.4 0.068 The next table shows the SCI GROW estimated potential groundwater exposure concentration. Groundwater is not expected to be impacted by photodegradates because all field studies show that parent is long lived in soil ( half lives on the order of one year) with only minimal evidence of photolysis or leaching. However, as previously discussed, dislodgeability, photolysis ( or other chemical changes) in or on plant surfaces are uncertain. Therefore, it is possible that photodegradates identified in water and soil or other compounds may form in or on cotton plants, reach the soil surface at unknown rates, and be subject to unknown degrees of leaching or runoff. EFED's assessment cannot specifically account for this absent information. However, based on all available fate evidence, it appears that photodegradation would be the most likely candidate for any rapid degradation of thidiazuron that may occur on plants. If this is the case, then, to a first approximation, the physical and chemical fate properties of the photodegradates should be similar to parent, and any resulting loss in parent would be offset by production of photodegradates. Therefore, combined concentrations of all these compounds should approximate those given in the table for parent only.

Groundwater Concentration ( derived from SCI GROW regression model, Ver. 2.3 )

Acute and Chronic Concentration*: 0.066 ppb ( µ g/ L)

* As also noted in Appendix A, the SCI GROW estimated groundwater concentration given here is an extrapolation beyond the regression model limits for Koc. SCI GROW was developed using Koc values ranging from approximately 32 to 180 mL/ g of organic carbon, whereas the median Koc value for thidiazuron is 783 mL/ g. Extrapolation increases uncertainty, but without ability to estimate confidence.