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Bt - Bacillus thuringiensis

 

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium used as an alternative to chemical pesticides to control insects. The insecticidal properties of Bt are a means of proliferation for the bacteria and its co-evolution with insects in a variety of habitats worldwide has led to multiple strains of Bt, each with toxicity to only certain groups of insects. By isolating different strains of Bt and identifying their target insects, Bt use can be targeted to specific groups of insects while having little to no effect on other organisms in the ecosystem.

 

The insecticidal properties of Bt were first discovered as far back as 1901 and it has been registered as a general use pesticide in the United States since 1961. Though initial use was limited to Lepidopteran species (moths and butterflies), additional strains began to be discovered in the 1970’s that showed toxicity to different groups of insects. Amongst these was B.t. israelensis (Bti), which was registered by EPA in 1983 for control of black flies and mosquitoes.

 

Mode of action

Bt consists of a spore and a crystal protein that produces toxins to insects.  For the toxin to be activated, Bt must be ingested by an insect larva, where the crystal dissolves in the gut and converts the protoxins to highly toxic endotoxins. These endotoxins bind to gut receptors, perforating the membrane, and result in paralysis, starvation, and ultimately, death of the insect. 

 

Specificity

The properties of the Bt crystal vary between strains of Bt and are responsible for their specificity.  One aspect that affects specificity is pH.  Insects can have widely varying ranges of pH within their guts.  In order for the endotoxin of a specific strain of Bt to be activated, the pH level within the gut must be within a very specific range.  If the gut is too acidic, the toxin will dissolve along with the crystal; too basic and the crystal won’t dissolve enough to release the toxin, which will pass through the organism’s stomach unactivated.  The gut of mosquitoes and black flies, where Bti endotoxins are activated, are much more alkaline than most animals.  Another layer of specificity lies within the receptors of the gut membrane where the endotoxins bind.  These receptors are species-specific, so even when endotoxins are properly activated, they can only cause damage to those species which possess suitable binding sites.

 

Effects to Nontarget Species

Bti is highly toxic to black flies and mosquitoes, and has been found to have activity against some species of midge.  No other insect species, including other dipterans, have been found to be susceptible to Bti toxicity.  No effects to birds, mammals, fish, or other invertebrate species exposed to Bti have been detected.

 

Temporary indirect effects to insectivorous species may occur due to the loss of target insects from the community, though numerous field and mesocosm studies in flowing water systems found no effects to species abundance, richness, or diversity following the application of Bti to the system. 

 

Bti is used in a number of municipalities to control black flies and mosquitoes, perhaps most extensively by the Pennsylvania Department of Environmental Protection, which has used Bti to suppress black flies since 1983.  Currently, this program extends to 1600 miles of 45 rivers and streams, and includes non-target biomonitoring of aquatic macroinvertebrates and fish at selected treatment sites.  A 5-year study investigated the potential effects of this suppression program by measuring the effects of Bti application to fish and invertebrates and found no ecologically significant impacts to these taxa (Jackson et al 2002).

 

2012 Bti Study

 

Solving the Current Challenge: Working for Successful Whooping Crane Nesting in Wisconsin - A Summary of WCEP's ongoing research to ensure a self-sustaining population of whooping cranes.

 

References:

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.

 

World Health Organization, 1999. Environmental health criteria, No. 217: Microbial Pest Control Agent Bacillus thuringiensis Geneva, Switzerland, World Health Organization, 105 p; available from http://whqlibdoc.who.int/ehc/who_ehc_217.pdf

 

Pennsylvania's Black Fly Suppression Web Site, Pennsylvania Department of Environmental Protection
http://www.depweb.state.pa.us/portal/server.pt/community/black_fly/13774

 

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

 

 

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