Bt is widely used as a traditional, sprayed insecticide even by organic farmers. It gets its name from the bacterium Bacillus thuringiensis, which naturally produces the toxin. As with any pesticide, if Bt is used long enough, some insects are likely to evolve to be unharmed by it. But because Bt crops produce the insecticide constantly, resistant insects may appear even faster. If they mate with other resistant insects, they may spawn uncontrollable pests.
Research continues on the best way to delay insect resistance.
The few resistant insects are likely to mate with susceptible bugs, producing susceptible progeny. The Environmental Protection Agency requires that growers plant at least 20 percent non-Bt crops, a fraction that has been agreed upon by scientific panels from the government, industry, and the Union of Concerned Scientists, as well as researchers working with farmers, according to Fred Gould, a professor of entomology at North Carolina State University. But insecticide-producing crops might also harm wildlife other than the pests they target. Last spring, much publicity arose around a Cornell University study of monarch-butterfly larvae that fed on milkweed coated with pollen from Bt corn. Although almost half of the caterpillars died, many researchers say the study, which was conducted in a laboratory, has little relevance for butterflies living in the wild, near corn fields.