No abstract

The refuge strategy is used worldwide to delay the evolution of pest resistance to insecticides that are either sprayed or produced by transgenic Bacillus thuringiensis (Bt) crops. This strategy is based on the idea that refuges of host plants where pests are not exposed to an insecticide promote survival of susceptible pests. Despite widespread adoption of this approach, large-scale tests of the refuge strategy have been problematic. Here we tested the refuge strategy with 8 y of data on refuges and resistance to the insecticide pyriproxyfen in 84 populations of the sweetpotato whitefly (Bemisia tabaci) from cotton fields in central Arizona. We found that spatial variation in resistance to pyriproxyfen within each year was not affected by refuges of melons or alfalfa near cotton fields. However, resistance was negatively associated with the area of cotton refuges and positively associated with the area of cotton treated with pyriproxyfen. A statistical model based on the first 4 y of data, incorporating the spatial distribution of cotton treated and not treated with pyriproxyfen, adequately predicted the spatial variation in resistance observed in the last 4 y of the study, confirming that cotton refuges delayed resistance and treated cotton fields accelerated resistance. By providing a systematic assessment of the effectiveness of refuges and the scale of their effects, the spatially explicit approach applied here could be useful for testing and improving the refuge strategy in other crop-pest systems.pesticide resistance | predictive evolutionary models | pest management | resistance management P opulation growth will continue to favor agricultural intensification for decades. Because agricultural intensification is associated with increased pest pressure, pesticides generally help to increase yield (1-3). Although significant progress has been made to reduce reliance on pesticides (4, 5), an increasing number of insects and mites exhibit field-evolved resistance to synthetic pesticides, Bacillus thuringiensis (Bt) sprays, and transgenic Bt crops (6, 7). Negative consequences of resistance include increased pesticide use, disruption of food webs and ecosystem services, increased risk to human health, and loss of profits for farmers and industry (1, 3).One of the main strategies for delaying resistance promotes survival of susceptible pests by providing refuges, which are areas of host plants where pests are not exposed to an insecticide. Theory predicts that refuges will slow the evolution of resistance by reducing the fitness advantage of resistant individuals (7-9). Refuges can also reduce the heritability of resistance when susceptible individuals mate with resistant individuals surviving exposure to an insecticide (7). Empirical support for the refuge strategy was provided by short-term laboratory and greenhouse experiments (10, 11). Although these experiments test the hypothesis that mating between susceptible and resistant individuals delays the evolution of resistance, they do not consider several fact...

Fitness costs associated with insect resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) reduce the fitness on non-Bt refuge plants of resistant individuals relative to susceptible individuals. Because costs may vary among host plants, choosing refuge cultivars that increase the dominance or magnitude of costs could help to delay resistance. Specifically, cultivars with high concentrations of toxic phytochemicals could magnify costs. To test this hypothesis, we compared life history traits of three independent sets of pink bollworm, Pectinophora gossypiella (Saunders), populations on two cotton cultivars that differed in antibiosis against this cotton pest. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. Confirming previous findings with pink bollworm feeding on cotton, costs primarily affected survival and were recessive on both cultivars. The magnitude of the survival cost did not differ between cultivars. Although the experimental results did not reveal differences between cultivars in the magnitude or dominance of costs, modeling results suggest that differences between cultivars in pink bollworm survival could affect resistance evolution. Thus, knowledge of the interaction between host plants and fitness costs associated with resistance to Bt crops could be helpful in guiding the choice of refuge cultivars.

Transgenic crops producing Bacillus thuringiensis (Bt) proteins kill key insect pests, providing economic and environmental benefits. However, the evolution of pest resistance threatens the continued success of such Bt crops. To delay or counter resistance, transgenic plant “pyramids” producing two or more Bt proteins that kill the same pest have been adopted extensively. Field populations of the pink bollworm (Pectinophora gossypiella) in the United States have remained susceptible to Bt toxins Cry1Ac and Cry2Ab, but field-evolved practical resistance to Bt cotton producing Cry1Ac has occurred widely in India. Here we used two rounds of laboratory selection to achieve 18,000- to 150,000-fold resistance to Cry2Ab in pink bollworm. Inheritance of resistance to Cry2Ab was recessive, autosomal, conferred primarily by one locus, and independent of Cry1Ac resistance. We created a strain with high resistance to both toxins by crossing the Cry2Ab-resistant strain with a Cry1Ac-resistant strain, followed by one selection with Cry2Ab. This multi-toxin resistant strain survived on field-collected Bt cotton bolls producing both toxins. The results here demonstrate the risk of evolution of resistance to pyramided Bt plants, particularly when toxins are deployed sequentially and refuges are scarce, as seen with Bt cotton and pink bollworm in India.

For delaying evolution of pest resistance to transgenic corn producing Bacillus thuringiensis (Bt) toxins, limited data are available to compare the effectiveness of refuges of non-Bt corn planted in seed mixtures versus blocks. Here we addressed this issue in the ear-feeding pest Helicoverpa zea Boddie by measuring its survival and development in the laboratory on ears from field plots with 90% Cry1A.105 + Cry2Ab corn and 10% non-Bt corn planted in a seed mixture or blocks. We compared a strain of H. zea selected for resistance to Cry1Ac in the laboratory, its parent strain not selected in the laboratory, and their F1 progeny. The relative survival of the F1 progeny and dominance of resistance were higher on ears from Bt plants in the seed mixture than the block. Half of the kernels in ears from non-Bt plants in the seed mixture produced both Cry1A.105 and Cry2Ab. However, survival on ears from non-Bt plants did not differ between the block and seed mixture. In simulations based on the observed survival, resistance to Cry1A.105 + Cry2Ab corn evolved faster with the seed mixture than the blocks, because of the higher dominance of resistance in the seed mixture. Increasing the refuge percentage improved durability of Cry1A.105 + Cry2Ab corn more for the blocks than the seed mixture. These findings imply that, for a given percentage of non-Bt corn, resistance of H. zea and other ear-feeding pests to multi-toxin Bt corn is likely to evolve faster for seed mixtures than blocks.