Direct and indirect competition between two species of leaf-mining fly

Understanding herbivore population regulation has been a goal of ecologists for decades. The importance of competition in structuring herbivore communities, especially phytophagous insects, has been debated for decades. Here we 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. We assess direct competition with a series of laboratory experiments and assess indirect competition with both natural history and manipulative field experiments. We find evidence for both asymmetric direct and indirect competition between the two species of leaf-miner. These complicated indirect and direct interactions are not unique to our system, and we believe that understanding the factors driving the importance of both types of competition in this case will provide support for theories that may generalize to other communities.