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 Hübner (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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