Apparent competition (defined as a negative effect of one species on the population growth rate or abundance of another species, mediated through the action of shared natural enemies), has been shown to be important in structuring insect communities. While competition has been documented between species at all trophic levels (plant, herbivore, and natural enemies), indirect pathways for competition through natural enemies have rarely been quantified in field studies to date. The sunflower (Liriomyza helianthi) and blotch leafminer (Calycomyza platyptera) along with their community of hymenopteran parasitoids provide a model system for testing the relative strengths of direct and apparent competition in the field. Seven species of parasitoids were reared from the two leafminer species. We developed and analyzed quantitative food webs for ten sites in the Californian central valley.

Results/Conclusions
We found the webs were able to provide detailed, quantitative information about community structure. Our quantitative food webs show the three most common parasitoid genera (Diglyphus, Neochrysocharis, and Chrysocharis) are shared by both herbivore species across a gradient of herbivore and host plant densities. We found that early in the summer herbivore densities were low and Opius dimidiatus was the only common parasitoid reared. As the summer progressed the densities of both herbivore species increased and parasitoids from five genera were reared. In preliminary analysis, at least one of our sites shows that an increase in the more common (L. helianthi) herbivore through time corresponded with an increase in the densities of its associated parasitoids. This increase in shared parasitoids in turn corresponded with a decrease in the density of the less common (C. platyptera) herbivore. Our initial analysis suggests that apparent competition structures this relationship through the shared parasitoids of L .helianthi and C .platyptera. This study demonstrates the power that quantitative webs have for studying apparent competition and developing hypotheses to test in the field.