Understanding herbivore population regulation has been a goal of ecologists for decades. Here we try to tease apart the relative importance of indirect (apparent) and direct competition in regulating a native dipteran leafminer community in California. 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 - may 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 indirect interactions.

Results/Conclusions

In the summer of 2008 and 2009 populations of L. helianthi and C. platyptera, along with their community of hymenopteran parasitoids, were sampled at seven sites in the Central Valley of California. Quantitative food webs for each site were used to visualize host-parasitoid interactions. We found that the parasitism of C. platypera, the less common leafminer species, is significantly positively correlated with L. helianthi densities when a time lag is incorporated. This time lag corresponds with the development time of the most common shared parasitoid species. We hypothesize that in this system populations of C. platyptera are suppressed through apparent competition with L. helianthi. These complicated indirect interactions are not unique to our system, and we believe that understanding the factors driving the importance of apparent competition in this case will provide support for theories that may generalize to other communities.