Indirect competitive effects of stemborers on a gall community

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Interspecific competition between phytophagous insects using the same host plant occurs frequently and can strongly affect population densities of competing species. Competition between gallmakers and stemborers could be especially intense because both types of herbivore are unable to avoid competition by relocation during their immature stages. For apical meristem gallmakers the main result of competition is likely to be the interruption of resources to the gall by the stemborers' devouring of stem contents. The proximate effect of such competition could be to reduce gall size, thereby increasing the number of chambers per gall unit volume, and reducing the size and potential reproductive output of the gallformer. In addition, smaller galls may be more susceptible to attack from size-limited parasitoids, resulting in a second indirect effect of competition. Using a community of galling and stemboring insects on the saltmarsh shrub Iva frutescens L. (Asteraceae), we measured for indirect effects of competition. We examined the primary indirect effect of competition on gall midge crowding and the secondary effects on parasitism rates and parasitoid guild composition. Results indicated that galls co-occurring with stemborers were smaller, crowding of gall inhabitants was 22% greater, and the composition of the parasitoid guild was altered relative to galls on unbored stems. The overall parasitism rate was not different between galls on bored vs. unbored stems. These results show that competition resulting from the presence of stemborers has the potential to affect the gall midge Asphondylia borrichiae Rossi & Strong (Diptera: Cecidomyiidae) and secondarily to affect its guild of hymenopteran parasitoids.

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