PERSPECTIVE ABCs of Insect Resistance to Bt

Genetically engineered crops represent one of the most controversial and rapidly adopted tech-nologies in the history of agriculture. To improve pest control, scientists have engineered cot-ton, corn, and soybeans to make insecticidal proteins from the common bacterium Bacillus thuringiensis (Bt) [1]. These Bt toxins kill some devastating pests, but unlike broad-spectrum insecticides, they do little or no harm to most other organisms, including people [2,3]. The original Bt crops, first commercialized in 1996, each made a single crystalline (Cry) toxin from the Cry1 family effective against certain lepidopteran larvae. However, some of the environ-mental, health, and economic benefits of Bt crops have been lost because of rapid evolution of pest resistance, particularly to single-toxin Bt crops (Fig 1) [4]. To delay resistance and broaden the spectrum of pests controlled, newer “second genera-tion ” Bt crops produce two or more Bt toxins [5]. In particular, Bt toxin Cry2Ab from the Cry2 family is used widely in combination with Cry1 toxins to kill caterpillar pests. For example, the percentage of all cotton planted that was Bt cotton producing both Cry1Ac and Cry2Ab wa