Implications of Cross‐resistance amongBacillus thuringiensisToxins in Resistance Management

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Abstract
Insect resistance is now recognized as a serious threat to the long‐term usefulness of Bacillus thuringiensis (Bt) toxins in pest management. Because of the great diversity among Bt toxins found in nature, one of the most tempting resistance management strategies is to use two or more of these toxins in mixtures, rotations or sequences. Cross‐resistance among toxins and the ability of insects to develop resistance to multiple toxins will limit the success of this approach. Studies have shown that there are large differences in the cross‐resistance spectrum of the insect species that have been selected for resistance using single toxins or simple mixtures. Other studies have demonstrated that some species can readily develop resistance to mixtures of toxins, and that the use of strains of Bt that produce a wide array of toxins can cause broad‐spectrum resistance to most Bt toxins. These factors tend to be specific to individual insect species and must be considered when attempting to manage insect resistance using multiple toxin strategies. Polygenic inheritance and the existence of multiple mechanisms of resistance may be involved in broad‐spectrum resistance, and may limit the use of multiple toxin strategies for managing resistance.