Preserving the Effectiveness of Bt Crops

Since their introduction in 1996, the area grown to transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has grown rapidly. In 2004, Bt plants were grown on over 13 million ha in the U.S. and 22.4 million ha worldwide1,2. Bt corn and Bt cotton comprised about one-third and one-half, respectively, of the U.S. corn and cotton markets in 2004. Worldwide, approximately 15 million ha of Bt corn and 7.4 million ha of Bt cotton were grown in 2004. The economic and environmental benefits of Bt crops, compared to other technologies, have been well-documented3. Reports from China have also shown that fewer insecticide poisonings occur when Bt cotton is used4. Despite the extensive use of Bt crops over an 8-year period, there have been no reports of product failure or increased resistance in insect pests5. To put this in perspective, Bt crops have already exceeded the length of time that typically passes in the field before resistance is first documented with most conventional neurotoxic pesticides, despite undergoing what has been hailed as one of the world’s largest selection for resistance6. However, because of the demonstrated ability of two insect species (the diamondback moth and the cabbage looper) to evolve resistance to sprays of Bt proteins in commercial settings, there is concern that some insects may also evolve resistance in the field to Bt plants. To prevent or delay resistance to Bt crops, various insecticide resistance management (IRM) strategies have been proposed. These strategies include manipulation of the transgenic elements of the plants, such as the genes and promoters, and the manner in which plants are deployed in the landscape. The most widely employed tactic to delay resistance is the use of plants expressing a high dos