Growers of Bacillus thuringiensis (Bt) crops often use refuges of non-Bt plants to delay pest resistance, but plant-to-plant gene flow between Bt and non-Bt crops could affect this strategy. Here we used simulation modeling to explore the consequences of pollen- and seed-mediated gene flow in cotton fields on the evolution of resistance in a generic pest. We modeled a landscape of 0.5-ha fields where growers used farm-saved seed, as could often occur in the developing world. Specifically, we examined the effects of moderate and high gene flow rates, larval feeding behavior, dominance of resistance, refuge type and abundance, and the interactions among these factors. With either completely dominant or completely recessive inheritance of resistance, gene flow among plants and larval feeding behavior had limited practical impact on resistance evolution. With intermediate dominance, however, moderate or high gene flow among plants substantially accelerated resistance evolution in some simulations where non-Bt cotton refuges were 5 or 20% of the cotton acreage. The acceleration was usually greater when larvae moved and fed indiscriminately among Bt and non-Bt cotton plants than when larvae were sedentary or discriminated among plant types. Adding alternative host plant refuges to the landscape delayed resistance, while increasing the non-Bt cotton refuge from 20 to 50% of the cotton acreage had positive, negative, or neutral effects, depending on dominance, the amount of alternative host plant refuges, and larval feeding behavior. The results suggest that, under certain conditions, reducing gene flow between refuges and Bt crops could help delay pest resistance.
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