Introducing genetically-modified insect-protected crops into the agricultural landscape has profound effects on the abundance and distribution of targeted herbivores and their specialized parasitoids. Vast acreage of crops no longer contain appropriate hosts, but hosts should still be abundant in refuge plantings designed to prevent the spread of Bt resistance in herbivore populations. The parasitoid's response to this heterogeneous host distribution can in turn influence resistance evolution in the herbivore. If parasitoids show a density-dependent response to herbivores, rare resistant herbivores in the transgenic crop experience reduced parasitism and increased fitness relative to their susceptible counterparts in the refuge, potentially accelerating the spread of resistance. We experimentally tested the density-dependent response of Macrocentrus grandii, a specialist parasitoid of Ostrinia nubilalis. We created a matrix of different sized host patches within a field of Bt sweet corn by infesting outplanted non-Bt plants with O. nubilalis larvae, and released M. grandii throughout the field. We found that parasitism of O. nubilalis was approximately 50% lower in the smallest patch size relative to the larger patches, indicating that isolated resistant O. nubilalis in transgenic Bt corn may experience reduced parasitism, thus promoting the relative fitness and spread of the resistant genotype. However, we also found that O. nubilalis in small patches located near larger patches received higher parasitism than in more remote small patches. We therefore suggest that tight spatial coupling of transgenic crops and refuges could be used to reduce the isolation of resistant hosts and minimize differential parasitism.
Species 1: Hymenoptera Braconidae Macrocentrus grandii
Species 2: Lepidoptera Crambidae Ostrinia nubilalis (European corn borer)
Keywords: resistance management, foraging behavior
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