Host-parasitoid interactions in a transgenic landscape: Spatial proximity effects of host density

Widespread management practices such as transgenic insecticidal crops influence the distribution and density of targeted pest species across the agricultural landscape. Natural enemies must cope with this altered distribution, and their response potentially influences the rate of resistance evolution in the pest. The purpose of this study was to examine spatial patterns of parasitism by the specialist parasitoid Macrocentrus grandii Goidanich (Hymenoptera: Braeonidae) in response to the density of its host, the European corn borer (Ostrinia nubilalis Hühner [Lepidoptera: Crambidae]). When we manipulated host distribution and observed resulting patterns of wasp density and parasitism, we found that the smallest host aggregations had the lowest parasitism, but only when not associated with larger host aggregations. A subsequent field experiment confirmed that proximity to large host aggregations increased parasitism in small host aggregations. Theory indicates that such positive density-dependent parasitism should accelerate the evolution of toxin resistance in pest species, but our study suggests that close spatial proximity between insecticidal crops and refuges may help equalize M. grandii parasitism and that simple management techniques such as in-field refuges could potentially increase the complementarity of transgenic and biological control of European corn borer in this system. Further research is needed, however, before extrapolating the results of our small-scale study to field-level patterns and concluding that M. grandii will necessarily hasten resistance evolution in the European corn borer.