Extensive cultivation of crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has suppressed some major pests, reduced insecticide sprays, enhanced pest control by natural enemies, and increased grower profits. However, these benefits are being eroded by evolution of resistance in pests. We report a strategy for combating resistance by crossing transgenic Bt plants with conventional non-Bt plants and then crossing the resulting first-generation (F 1 ) hybrid progeny and sowing the second-generation (F 2 ) seeds. This strategy yields a random mixture within fields of three-quarters of plants that produce Bt toxin and one-quarter that does not. We hypothesized that the non-Bt plants in this mixture promote survival of susceptible insects, thereby delaying evolution of resistance. To test this hypothesis, we compared predictions from computer modeling with data monitoring pink bollworm (Pectinophora gossypiella) resistance to Bt toxin Cry1Ac produced by transgenic cotton in an 11-y study at 17 field sites in six provinces of China. The frequency of resistant individuals in the field increased before this strategy was widely deployed and then declined after its widespread adoption boosted the percentage of non-Bt cotton plants in the region. The correspondence between the predicted and observed outcomes implies that this strategy countered evolution of resistance. Despite the increased percentage of non-Bt cotton, suppression of pink bollworm was sustained. Unlike other resistance management tactics that require regulatory intervention, growers adopted this strategy voluntarily, apparently because of advantages that may include better performance as well as lower costs for seeds and insecticides.sustainability | evolution | resistance management | genetically modified | refuge G enetically engineered crops that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have been planted globally on a cumulative total of over 732 million ha since 1996 (1). The benefits of these Bt crops include pest suppression, reduced insecticide use, enhanced biological control, and increased farmer profits (2-7). However, increasingly rapid evolution of resistance to Bt crops by pests has eroded these benefits (8-11). The main strategy for delaying pest resistance to Bt crops aims to increase the survival of susceptible insects with "refuges" of host plants that do not produce Bt toxins (9). Although refuges can delay insect adaptation to Bt crops (2, 3, 9, 12), the optimal spatial scale for planting refuges remains unresolved (13-15). Also, because refuges are often perceived to cause short-term economic sacrifices for growers, they are usually imposed by regulations.Before 2010, regulations in the United States mandated refuges of non-Bt plants in blocks consisting of separate fields, rows, or strips (14). In 2010, the regulations were modified to include mixtures of Bt and non-Bt seeds generating a random array of Bt and non-Bt plants side by side within...

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