Bt Fact Sheet

Bacillus thuringiensis

How does Bacillus thuringiensis work?

Bacillus thuringiensis (Bt) is an insect disease causing bacterium, which can only be found in the soil or in infected caterpillars. It is commercially produced in laboratories for use as an insecticide. It can be purchased by homeowners and is sold under various trade names. During the commercial production of Bt, spores (resting reproductive structures) and crystals (delta-endotoxins) are produced. Bt must be eaten by the caterpillars to be effective. Once ingested, the crystal is dissolved by the enzymatic activity induced by the high gut pH of the caterpillar. This activity releases a toxin from the crystal which interferes with the gut cells and feeding stops within hours. Death of the insect will occur in three to seven days from either the toxin or from the bacterial spore invading the insect's blood system.

What are Bt's uses?

The EPA has registered the use of Bt for control of gypsy moth. Bt is also used for the control of the following insects: fall webworm, spring and fall cankerworm, bagworm, spruce budworm, eastern tent caterpillar, cotton leafworm, loopers, fruit tree leafroller, cut worm, horn worm, cabbage butterfly, armyworm, apple leaf skeletonizer and a variety of other actively feeding Lepidoptera larvae.

What are the environmental effects of Bt?

Commercially produced Bt, when used at labeled rates, has a very short life in the environment. It can remain active from one to twenty days depending on weather conditions and exposure to ultra violet light. Under normal weather and exposure conditions, formulations of Bt usually remain active from three to seven days after application. Due to the specific conditions required within the insect, Bt only affects leaf feeding caterpillars (Lepidoptera larvae) that are actively feeding at the time spray material is on the foliage.

Bt is very specific in its mode of action and does not bioaccumulate in the food chain. If other animals, such as birds or small mammals, feed on insects killed by Bt, the spore and the crystal pass through their digestive system with no adverse effect. This occurs because the enzymes and high gut pH needed for release of the toxin are not present.

What is the toxicity of Bt?

  • Bt has very low mammalian toxicity through oral, dermal and inhalation routes. Tests conducted on guinea pigs, mice, rabbits and humans showed no toxic reactions. Test results from human volunteers eating Bt showed no adverse effects.
  • Bt has no adverse effect on wildlife. Tests done in laboratories, field trials and during actual control projects showed no effects on birds, mammals or fish.
  • Bt has no effect on earthworms, parasitic wasps, honey bees, praying mantis or nearly all other insects except moth and butterfly caterpillars.
  • Based on extensive testing conducted on Bt and its very specific mode of action, the EPA has exempted Bt from tolerance requirements for residues on food crops. Tolerance requirements are the amount of pesticide residue permitted by federal regulations to remain on or in a crop at the time of harvest.
  • No long term effects were determined to exist in studies done on Bt.

Effectiveness of Bt

Bt is generally about sixty to eighty percent effective in killing caterpillars. This is usually sufficient to prevent defoliation; however, some caterpillars will usually remain. It is most effective in areas of low gypsy moth populations.  It is safe to use in areas containing open bodies of water and in other environmentally sensitive areas.

Precautionary Statement

Bt is an insecticide and should be treated as such. If you feel you have been sprayed by any insecticide, remove and wash clothing, take a shower with soap and water and wash hair. When rinsing, use lots of clean water. If materials get in your eyes, rinse eyes with clean water to flush out the insecticide. If irritation persists, call your doctor or nearest poison control center.

This publication was compiled by Forest Pest Branch staff based on information contained in documents prepared by the Environmental Protection Agency, Virginia Department of Health and information obtained from the manufacturers. For further information on the products, please contact the Fairfax County Forest Pest Branch at 703-324-5304, TTY 711, or via e-mail.



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