Indirect interactions among phytophagous insects can play an important role in structuring communities. In particular, negative interactions mediated by shared natural enemies (�apparent competition�) are well documented. In contrast, we describe a positive interaction between herbivores mediated by a specialist parasitoid. Corn rootworm, Diabrotica spp. (Coleoptera: Chrysomelidae), reduces parasitism of the European corn borer, Ostrinia nubilalis H�bner (Lepidoptera: Crambidae), by the specialist wasp Macrocentrus grandii Goidanich (Hymenoptera: Braconidae). We observed 95% fewer wasps in a Diabrotica infested cornfield than an adjacent uninfested area, and found that experimental O. nubilalis larvae experienced 99.5% less parasitism. While O. nubilalis larval densities were 57% lower per plant in the infested area, this was insufficient to explain the near absence of wasps. Wasp non-preference for the area with high Diabrotica density may have arisen from altered microclimate: Diabrotica damage resulted in a 33% decrease in corn height, and a 20% reduction in plant density, creating a more open habitat. Experimental data supports this hypothesis, but alternative factors such as plant volatile production may also have played a role. The net effect of Diabrotica on O. nubilalis in this study was negative because losses due to exploitative competition outweighed the benefits of reduced parasitism. It�s possible, however, that under other circumstances this balance could shift, and we suggest that this sort of indirect facilitation could act to stabilize strong herbivore/natural enemy interactions by creating a non-intuitive refuge for the herbivore.
Species 1: Lepidoptera Crambidae Ostrinia nubilalis (European corn borer)
Species 2: Hymenoptera Braconidae Macrocentrus grandii
Species 3: Coleoptera Chrysomelidae Diabrotica (corn rootworm)
Keywords: Indirect interactions, parasitism
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