Bacillus thuringiensis Fact Sheet
Bacillus thuringiensis (B.t.) is a biological pest control agent. A living bacteria that occurs naturally in soil, B.t. produces poisons that cause disease in insect larvae. B.t. is sprayed in cities and forests, and is commonly used by organic farmers on crops with pest problems. Common targets include mosquitoes, black flies, gypsy moths, spruce budworms, and beetles.
One of the traits that initially favoured B.t. over synthetic pesticides was its selectivity: different subspecies of the B.t. bacteria affect different insects. B.t. has over 19 subspecies, and 5 are used commercially: kurstaki in forest pest control, israelensis for mosquitoes and black flies, and aiyawai, morrisoni, and san diego in crop production. This specificity is also important because it makes B.t. of low toxicity to humans.
Unexpected Inert Dangers
During a 1992 B.t. spraying program for gypsy moths in Oregon, a woman suffered from joint pain and neurological symptoms within 45 minutes after being exposed to the spray. She had a pre-existing allergy to a carbohydrate that was present as an inert ingredient. 
However, low toxicity does not mean no toxicity, and there are important reasons why B.t. should be used with caution. B.t. bacteria should not be confused with genetically engineered B.t. crops such as B.t. corn. These crops are designed to give off the B.t. toxin from every single cell of the plant. They therefore emit a relatively large amount of B.t. into the environment, which has been shown to kill non-target species such as butterflies. B.t. bacteria, on the other hand, do not pose a significant threat to non-targets when sprayed directly onto crops or into catch basins.
How It Works
Like many other bacteria, B.t. forms spores highly resitant, resting forms that protect the bacteria in tough environmental conditions. However, B.t. is unique in that it forms protein crystals along with its spores. This protein is the source of B.t.s toxicity. When the target insect larva eats a B.t. spore and its protein crystal, the protein dissolves in its gut and produces a toxin. The toxin interferes with food digestion, and the insect starves to death. The insect can also die from infection when the bacteria multiply and escape the digestive tract, travelling into the rest of the body. 
Like other larvicides, B.t. is effective because it interferes with regular insect life-cycles. Applying B.t when insects are at their larval stage prevents them from developing into mature adults which are able to reproduce. In contrast, B.t. has no significant effect on adult target insects.
B.t. is of low threat to humans because it does not persist in the digestive tracts of mammals. Classified as slightly toxic, the most common side-effects are skin and eye irritation.
While exposure to the B.t. bacteria may pose little threat to human health, the effects of inert ingredients included in B.t. products are cause for concern. Preparations of B.t. usually contain chemical additives such as emulsifiers, wetting agents and surfactants to improve product quality. Considered trade secrets by manufacturers, they are often not listed on the label.
Inert ingredients are potentially the most toxic component of B.t. products. Chemicals used as B.t. inerts include sodium hydroxide, which can damage the upper respiratory tract, sulphuric and phosphoric acid, recognized as being corrosive, and sodium sulfite, known to cause nausea, diarrhea, lowered blood pressure, hives, shock, and loss of consciousness in sensitive individuals.
In general, B.t. is effective if applied in sufficient quantities to target populations in the larval phase. However, in the case of mosquitoes, the sites that are actually targeted by spraying programs may not be the most likely breeding reservoirs for the mosquitoes that bite people. For example, the most common breeding site for pest mosquitoes is old tires. Prevention methods such as encouraging individuals to roll, cut, or cover old tires may be more effective than pesticide use in general.
Resistance to B.t. can limit the larvicides effectiveness. Initially the complexity of Bts mode of action was thought to prevent insects from developing resistance. However lab and field studies done on at least eight insects have proven this theory wrong. To make matters worse, the development of genetically engineered B.t. crops could dramatically increase the number of B.t.-resistant insects. Since they will greatly increase the amount of B.t. toxin in the environment, the evolution of toxin-resistant pests will be strongly encouraged.
Fate in the Environment
The persistence of B.t. in the environment depends on various factors, including sunlight, humidity, and soil conditions. While most of the applied B.t. persists only a short time, small amounts may remain active on soil particles, on the underside of leaves, and on sediments in the water column. B.t. spores can remain dormant for years, while active bacteria can survive in the environment for months.
B.t. may be considered safe by some because it poses little direct threat to human health. In all actuality, B.t. has significant effects on ecosystems. By killing non-target invertebrates such as caterpillars, B.t. reduces the food supply of fish, and birds, and predatory insects. The food chain can be severely impacted if these predators either starve or shift to alternate prey.
While B.t. may be more specific to target insects than its chemical counterparts, all B.t. products kill at least some non-target species. Beneficial insects are of particular concern; some insects that actually help control pests can be adversely affected. For instance, the number of flies that prey on aphids, a collard-eating pest, was observed to have decreased following B.t. application. B.t. can also kill endangered species of butterflies along with lepidopteron pests.
Although Bt is less toxic to mammals and shows fewer environmental effects than many other synthetic insecticides, it is not safe. B.t. should only be used after all other methods of prevention have been used, only when entirely necessary, and in the smallest quantities possible.
 Carrie Swadener, Insecticide Fact Sheet: Bacillus Thurigiensis (B.t.), Journal of Pesticide Reform 14 (1994):3.
 Jeff Jenkins, Environmental Toxicology and Chemistry Memo, Subject: B.t., Department of Agricultural Chemistry, Oregon State University, January 1992, 8 April 1998, http://ace.ace.orst.edu/info/extoxnet/tics/b_t.txt.
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 Oregon State University, Bacillus thuringiensis, Extension Technology Network Pesticide Information Profile, June 1996, 18 June 2003, http://ace.orst.edu/info/extoxnet/pips/bacillus.htm.
 Irene Novaczek, To B.t. or not to B.t.? A question worth asking! Eco-News Summer 1992: 1-11.
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 D.J. Horn, Selective mortality of parasitoids and predators of Myzus persicae on collards treated with malathion, carbaryl, or Bacillus thuringiensis, Ent. Exp. Appl. 34 (1983): 208-211.
 Bacillus thuringiensis, National Coalition Against the Misuse of Pesticides (NCAMP), June 1988, 18 June 2003, http://www.beyondpesticides.org/main.html.