Degree days and crop management

Mark Twain once said, "Everybody talks about, but no one can do a thing about the weather!" And there is no group of people that this statement applies to more than those dealing with midwestern agriculture. Each week during the growing season, integrated pest management (IPM) staff at Iowa State University (ISU) publish seasonal weather data in the Integrated Crop Management newsletter to alert producers to potential pest problems and to track weather developments across the state.

Degree days. The growth rates of insects and plants are dependent upon the amount of heat the organism receives. Each species, whether a crop, weed, insect, or disease organism, is adapted to grow best over certain minimum temperatures and essentially ceases growth at its own maximum temperature. For example, giant foxtail germinates at lower temperatures and thus becomes a weed problem earlier than shattercane; and seedcorn maggots actively feed at lower temperatures than corn or soybean grows. Scientists at ISU and neighboring land-grant universities have studied the minimum temperatures required for growth of many economically important pests. This knowledge can be used to predict when pests are likely to show up in a given area. The heat accumulation for a given base temperature corresponding to a specific species is reported as growing degree days (GDDs) that are cumulative within the growing season. However, when using GDDs, producers must remember that each base temperature will have its own degree day accumulation that is independent of other base temperatures.

Climatic data. The Midwest Climate Information Center collects automated climatic information from reporting stations in the midwestern states. The Center uses proven geostatistical methods to calculate temperature and precipitation averages for climate divisions within each state. These divisions correspond with and are reported as crop reporting districts. The nine Iowa climate or crop reporting districts are shown in the map.

What to expect during the season. Most of Iowa's major crop pests develop according to GDDs that fit into the following schedule:

Black cutworm overwinter in Louisiana, Texas, and Mexico, and fly north in the spring to lay eggs in Iowa fields. For over 15 years, ISU Extension staff has used pheromone traps to determine when and where moths arrive and females begin laying eggs, and they have calculated base-50 GDDs to determine when producers should scout for the presence of larvae and their damage. When a significant moth flight arrives in an area, we start measuring the base-50 GDDs that accumulate from that day. When we reach 300 base 50-GDDs, newly hatched larvae will have grown large enough to start cutting corn plants and potentially cause economic damage. ISU Extension specialists prepare one or more predictive maps to let the Iowa farm community know these dates so that the information can be used in management plans, rather than applying planting time preventative treatments. Corn and soybean also develop according to base-50 GDD accumulations.

Alfalfa weevils overwinter as eggs and adults. Time of hatch is dependent upon base-48 GDD accumulations since January 1 (little GDD accumulation occurs in the winter, so January 1 is used as a base starting point). Research has shown that alfalfa weevils can approach economically damaging populations when 250 base-48 GDDs have accumulated. Areas in the southern third of Iowa have occasional late-fall and winter days when the temperature is above 48o F, but such temperatures are less common in the northern two-thirds of the state. Therefore, we add a measure of safety for scouting projections for counties in the southern one-third of Iowa (climate reporting districts 7, 8, and 9), and begin scouting after 200 base-48 GDDs. It is difficult to know when in the fall the eggs are actually laid, and so the arbitrary January 1 start date is effective.

Stalk borer moths lay eggs in grassy field edges and similar areas in the fall. The eggs overwinter and hatch the next spring and the larvae initially feed on available grass stems and other weeds. As with alfalfa weevils, we start tracking GDD accumulation on January 1, but for stalk borer, base-41 GDDs are used. Egg hatch occurs around 575-750 accumulated GDDs, but we rarely target egg hatch because the larvae move directly into noncrop plants. When GDD accumulations reach 1,400 to 1,700 base-41 GDDs, the larvae outgrow the initial host plants and move into corn and occasionally soybean fields. For areas where stalk borer populations have caused significant problems in the past, we watch for movement into corn border rows between the 1,300 and 1,400 base-41 GDD, and treat if necessary. Treatments at other times are not effective and result in increasing costs and unnecessay insecticide use.

Other pests, including green cloverworm (base-52) and seedcorn maggot (base-39) also can be monitored with GDDs, but we generally have other methods available to determine the likelihood of damaging pest populations, and the occurrence of these pests is historically less frequent or more site specific.

Remember that the predictions based on GDD accumulations are regionally based, and a given location may gain more or less than the district average. The ability to monitor and predict economically damaging pest populations has significantly improved the effectiveness of rescue treatments both economically and environmentally--the hallmark principle of IPM.

See our Degree Day Information System for current degree days during the growing season.

This article originally appeared on pages 36-37 of the IC-480 (4) -- April 6, 1998 issue.

Updated 04/05/1998 - 1:00pm