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Bacillus thuringiensis (Bt)

Bt is a microbe naturally found in soil. It is a gram-positive, soil-dwelling, spore-forming, rod-shaped bacteria. It makes proteins that are toxic to immature insects (larvae). There are many types of Bt. Each targets different insect groups. Target insects include beetles, mosquitoes, blackflies, caterpillars, and moths.

The different types of Bt create toxins that can only be activated by the target insect larvae. In contrast, when people eat the same toxins, the toxins are not activated and no harm occurs.

Each type of Bt toxin is highly specific to the target insect. For example, the ‘kurstaki’ type targets caterpillars. The ‘israelensis’ type targets immature flies and mosquitoes. Little to no direct toxicity to non-target insects has been observed.

National Pesticide Information Center


Strain and Target Pest:

B.t. kurstaki (B.t.k.): lepidopterous insects, including the gypsy moth and cabbage looper

B.t. israelensis (B.t.i.): mosquitoes, blackflies and some midges

B.t. aizawai (B.t.a.): wax moth larvae in honeycombs

B.t. san diego (BtSD): certain beetle species and the boll weevil

Mode of Action: Bt makes toxins that target insect larvae when eaten. In their gut, the toxins are activated. The activated toxin breaks down their gut, and the insects die of infection and starvation. Death can occur within a few hours or weeks.

Application Tips

Surfaces where caterpillars feed must be well covered because caterpillars must ingest the bacteria to be affected. Careful timing is essential for success. Apply just after caterpillars begin hatching out of eggs. Small caterpillars are more susceptible than older larvae. Treatment is not effective on eggs, pupae, and adults. The material breaks down rapidly in the environment. Try to avoid making applications when the sun is on the plant. Affected caterpillars will stop feeding within hours, but may not die for 2 to 3 days. Wettable powders that you mix up yourself just prior to application will be most effective. Make up a fresh solution each time you treat. Reapplication will be necessary to kill caterpillars hatching one or more days after an application is made. (UC IPM)

Precautions and Safety Equipment

Minimize your exposure to pesticides.  Avoid contact with eyes.  Wear eye protection, long pants, a long-sleeved shirt, and a hat that can be washed after each use.  Always read label of individual product for additional directions. (UC IPM)

Acute Toxicity to People and Other Mammals

  • Toxicity rating: Not Acutely Toxic

Long-Term Toxicity to People and Other Mammals5

  • On US EPA list: Not listed;
  • On CA Proposition 65 list: Not listed

Water Quality Rating

  • Absorbed runoff toxicity risk to fish rating: Low
  • Solution runoff toxicity risk to fish rating: Low
  • Source: UC IPM WaterTox Database (originally NRCS Pesticide Properties Database)

Impact on Natural Enemies

  • Overall toxicity rating: Low
  • Specific impacts: predatory mites (Low), parasitoids (Low), general predators (Low)

Impact on Honey Bees

  • Toxicity category: Apply at any time with reasonable safety to bees

Selected References

Bacillus thuringiensis. A genomics and proteomics perspective. Mohamed A Ibrahim, Natalya Griko, Matthew Junker, Lee A Bulla

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. 

Pesticide Management for Bacillus thuringiensis ssp. kurstaki. UC IPM Statewide Integrated Pest Management Program

Bernstein, L.; Bernstein, J. A.; Miller, M.; Tierzieva, S.; Berstein, D. I.; Lummus, Z.; Selgrade, M. K.; Doerfler, D. L.; Seligy, V. Immune responses in farm workers after exposure to Bacillus thuringiensis pesticides. Environ. Health Persp. 1999, 107, 7, 575-582.

Biopesticide Fact Sheet: Bacillus thuringiensis subspecies kurstaki strain M-200; U.S. Environmental Protection Agency, Office of Pesticide Programs, U.S. Government Printing Office: Washington, DC, 1998.

Biopesticide Registration Action Document: Bacillus thuringiensis subspecies galleriae strain SDS-502; U.S. Environmental Protection Agency, Office of Prevention, Pesticides and Toxic Substances, Biopesticides and Pollution Prevention Division, U.S. Government Printing Office: Washington, DC, 2013.

Viable spores of the microorganism Bacillus thuringiensis Berliner; exemption from the requirement of a tolerance. Code of Federal Regulations, Part 180.1011, Title 40, 2004.

Microorganisms in Biological Pest Control — A Review (Bacterial Toxin Application and Effect of Environmental Factors. Canan Usta Gaziosmanpasa University, Natural Sciences and Art Faculty Department of Biology, Turkey