Effects of parasitism and competition on Asphondylia borrichiae

Background/Question/Methods

The gall midge Asphondylia borrichiae competes for resources with a guild of stemboring insects. The presence of stemborers results in a reduction in gall volume, potentially increasing susceptibility to parasitoid natural enemies. Previous studies of A. borrichiae on its three host species showed that differences in mean gall diameter resulted in size differences of eclosed adults among hosts, and the size differences corresponded to differences in fecundity among hosts. Thus, both direct effects of competition on fecundity and indirect effects of competition mediated through natural enemies must be measured to understand the full cost of competition incurred and whether it exceeds the effect of parasitism acting alone. Examining the variation within a single host species, Iva frutescens, parasitism rates were determined on galls co-occurring with stemborers and in the absence of stemborers, and mortality components were apportioned among members of the gall parasitoid guild as a metric of the direct effects of parasitism and indirect effects of competition mediated by natural enemies. Wing lengths of A. borrichiae, a proxy for body size, both correlates of fecundity, were measured to determine the direct effect of competition on potential fitness.

Results/Conclusions

Previous research has demonstrated that gall size is positively correlated with fecundity and therefore potential fitness. Preliminary evidence found no difference in mortality between galls on stems possessing or lacking stemborers, although the composition of the gall parasitoid guild was altered, with the proportion of mortality due to Torymus umbilicatus reduced in the presence of stemborers and a concomitant increase in Tenuipetiolus teredon. Estimates of parasitism rates exceed 70% and far outweigh the decrease in fecundity resulting from reduction in plant resources caused by competition with stemborers. Although competition from stemborers has minimal effect on mortality and fecundity of the gall former, it may have the unexpected effect of altering guild structure at a trophic level higher than that of the competitor.