The role of interspecific competition in structuring herbivorous insect communities has been the subject of debate. Current opinion, which is based on recent reviews of experimental work, appears to favor competition as a widespread and frequent process. However, many studies have examined only a few species pairs within larger multi-species assemblages. In addition, the frequency of competitive interactions at the population level has rarely been evaluated in the field at natural herbivore densities. Thus, the importance of competition as a process structuring these communities remains unclear. We examined interspecific competition among 9 species in the seed-feeding insect guild of wild sunflower. In density manipulation experiments using typical high densities of herbivores, we found that only 2 of 18 species pairs interacted competitively within an inflorescence. Moreover, a population-level experiment found no evidence of negative effects between species in either of 2 years. When considered together with several years of herbivore abundance data, our findings suggest that herbivore populations are only infrequently (~ every 5-10 yr) regulated by competitive interactions. Thus, although interspecific competition may have strong negative effects when it does occur, it is uncommon when all potential species pairs are evaluated and is infrequent given typical densities of herbivorous insects.
Species 1: Lepidoptera Noctuidae Plagiomimicus spumosum
Species 2: Coleoptera Curculionidae Smicronyx fulvus
Species 3: Diptera Cecidomyiidae Neolasioptera helianthi
Keywords: interspecific competition, multi-species
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