Bti

Introduction

Bacillus thuringiensis subsp. israelensis (Bti ) is a naturally occurring soil bacteria used as a microbial insecticide to control the spread of vector-borne diseases, protect public health, and manage insect pest species. Bti was first discovered in a stagnant pond in the Negev Desert of northcentral Israel in 1976 (Margalit and Dean 1985). Initial testing of Bti revealed acute toxicity to mosquitoes (Goldberg and Margalit 1977) and black flies (Undeen and Nagel 1978) that can carry and transmit diseases. Further research demonstrated that Bti is nontoxic to humans, mammals, birds, beneficial insects, fish, plants, and most aquatic organisms (EPA 1998 Bti EG2215 Factsheet). Bti was first registered by the U.S. EPA as an insecticide in 1983. Species-specific properties, safety to nontarget organisms, rapid breakdown, and reduced insect resistance make Bti an ideal pesticide (de Barjac and Sutherland 1990), with greatly reduced environmental impacts in comparison to man-made chemical insecticides.

Trade Names: Vectobac, Teknar, Aquabac, Bactimos, LarvX, etc. (26 Bti products in U.S.)

Application Procedures and Mode of Action on Target Organisms

Bti is applied by aerial spraying or backpack spraying at a safe rate specified by the United States Environmental Protection Agency. Black flies and mosquitoes must actively ingest Bti in order for the material to be effective. Bti is a gram positive, aerobic, spore-forming bacterium that produces protoxins in the form of parasporal protein crystals. In the alkaline digestive tract of black flies and mosquitoes, the protoxins become activated into highly toxic delta-endotoxins. The endotoxins cause a rapid breakdown in the lining of the midgut and necrosis of skeletal muscles, resulting in paralysis and mortality of target insect pests. Bti is nontoxic to most nontarget organisms due to their acidic digestive systems and lack of suitable tissue receptor sites.

Impacts on Nontarget Organisms

Most nontarget scientific studies conducted since the discovery of Bti in 1976 have concluded that the material is harmless to most nontarget organisms when applied at recommended label rates. Research has demonstrated that Bti is nontoxic to humans, mammals, birds, fish (trout and bluegill), and most invertebrates when properly applied (EPA 1998 Reregistration Eligibility Decision). Moderate toxicity in Daphnia and honeybees has been attributed to inert formulation ingredients rather than active ingredients. In fish studies, juvenile trout species and fathead minnows were unaffected by normal dosages, and were impacted only at very high Bti concentrations. Studies have demonstrated limited impacts to several aquatic Diptera families (Chironomidae and Blepharoceridae midges) when exposed to Bti treatments in mosquito and black fly habitats (Laird et al. 1990). Sensitive mayflies, stoneflies, caddisflies, crustaceans, snails, and bivalves, important members of the aquatic food chain in freshwater fisheries, are not significantly affected by Bti applications (Jackson et al. 2002). Data from a large number of studies indicate that Bti can be used in a carefully managed treatment program to selectively control insect pest and vector species with minimal adverse environmental impacts.

Further Reading

• Undeen, A.H. and W.L. Nagel. 1978. The effect of Bacillus thuringiensis ONR-60A strain (Goldberg) on Simulium larvae in the laboratory. Mosquito News 38: 524-527.
• de Barjac H. and D.J. Sutherland (eds.). 1990. Bacterial control of mosquitoes and black flies: Biochemistry, genetics and applications of Bacillus thuringiensis israelensis and Bacillus sphaericus. Rutgers University Press, New Brunswick, NJ. 349 pp.
• Environmental Protection Agency (EPA). 1998. EPA Bacillus thuringiensis subspecies israelensis stain EG2215 Factsheet; available from http://www.epa.gov/pesticides/ biopesticides/ingredients/factsheets/factsheet_006476.htm.; Internet.
• Environmental Protection Agency (EPA). 1998. EPA Reregistration Eligibility Decision (RED) Bacillus thuringiensis EPA738-R-98-004; available from http://www.epa.gov/oppsrrd1/REDs/ 0247.pdf
• Goldberg, L.J. and J. Margalit. 1977. A bacterial spore demonstrating rapid larvicidal activity activity against Anopheles sergentii, Uranotaenia unguiculata, Culex univitattus, Aedes aegypti and Culex pipiens complex. Mosquito News 37: 355-358.
• Jackson, J.K., R.J. Horwitz and B.W. Sweeney. 2002. Effects of Bacillus thuringiensis israelensis on black flies and nontarget macroinvertebrates and fish in a large river. Transactions of the American Fisheries Society 131: 910-930.
• Laird, M., L.A. Lacey and E.W. Davidson (eds.). 1990. Safety of Microbial Insecticides. CRC Press, Inc. Boca Raton, Florida. 259 pp.
• Margalit, J. and D. Dean. 1985. The story of Bacillus thuringiensis var. israelensis (B.t.i.) Journal of American Mosquito Control Association 1: 1-7.