Competition and community interactions of two generalist web-building caterpillars: Western tent caterpillar (Malacosoma californicum) and fall webworm (Hyphantria cunea)
Competition is one of the fundamental structuring forces in many communities, yet its role has historically been controversial in herbivorous insects. We study host-plant mediated competition between two common, destructive herbivorous insects: Western tent caterpillars (Malacosoma californicum) and fall webworms (Hyphantria cunea). Tent caterpillars feed on chokecherry early in the spring while fall webworms feed on the same plant species in the late summer and both species construct highly visible webs/tents on their host-plants. We studied the effects of competition at three different levels within the same study system: interspecific competition between tent caterpillars and fall webworm, intraspecific competition among tent caterpillars, and the season-long effects of tent caterpillars on arthropod community on chokecherry. To test interspecific competition, we reared larvae, both in lab and field conditions, to assess the effect of early-season tent caterpillar damage on late-season fall webworm fitness. We assessed the effect of intraspecific competition on oviposition behavior of tent caterpillars by surveying trees with and without tent caterpillar damage for the presence of tent caterpillar egg masses. Finally, to assess the community effects of this tent building species, we conducted an arthropod survey on trees with and without tent caterpillar damage.
We found that mechanical damage to chokecherry trees has a significant negative effect on fall webworm fitness in lab rearing trials, but tent caterpillar damage did not reduce fitness significantly. This result suggests that fall webworms and tent caterpillars do not compete indirectly through their host-plant, at least at the densities that we tested. In our field trial, we found significantly more fall webworm mortality on trees with tent caterpillar damage than on trees without tent caterpillar damage. This result suggests tent caterpillar damage or tent caterpillar tents attract shared predators with fall webworms. We found significantly more tent caterpillar egg masses on trees without tent caterpillar damage, which indicates that tent caterpillar females avoid using trees with spring tent caterpillar damage as oviposition sites. This finding suggests that tent caterpillars incur a fitness cost either when ovipositing on or when their larvae develop on trees with tent caterpillar damage. Finally, we observed a higher density of spiders and other predators in mid to late summer on trees with tent caterpillar tents. We suggest that the presence of tent caterpillar tents on chokecherry has a strong impact on arthropod communities and that tent caterpillars may be ecosystem engineers.