In her seminal work, Silent Spring, Rachel Carson writes: "If Darwin were alive today the insect world would delight and astound him with its impressive verification of his theories of survival of the fittest. Under the stress of intensive chemical spraying the weaker members of the insect populations are being weeded out." (1)rnEvolution of insecticide resistance in >400 species of insects not only confirms Darwin's theories, it threatens agriculture and human health worldwide. To reduce reliance on insecticide sprays, corn and cotton have been genetically engineered to produce insecticidal crystal (Cry) proteins derived from the bacterium Bacillus thuringiensis (Bt). Transgenic Bt corn and Bt cotton grew on 42 million ha during 2007, with a cumulative total of >200 million ha planted worldwide since their commercialization in 1996 (3). However, the history of insecticide resistance informs us that adaptation by insects could diminish the long-term efficacy of Bt crops and the associated economic, health, and environmental benefits (4-6). To date, field-evolved resistance to Bt crops has been documented in only 3 insect species (Fig. 1) (7-10). Along with other evidence, the report by Meihls et al. (11) in this issue of PNAS suggests that refuges of plants that do not produce Bt toxins may be useful for delaying insect resistance to Bt crops.
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