Plant Influences on Parasitism of Two Leafminers: A Test of Enemy‐Free Space

Abstract

We evaluated the importance of host plant traits as causes for a much greater parasitoid species richness on Tildenia inconspicuella (Lepidoptera: Gelechiidae), a leafminer on horsenettle, Solanum carolinense, than on tildenia georgei, a leafminer on groundcherry, Physalis heterophyllaa var. ambigua. In three field experiments, we found no evidence for short—term effects of host plants on parasitoids. None of the parasitoid species normally associated with the horsenettle miner successfully parasitized groundcherry miners transferred to horsenettle. Rates of parasitism and composition of parasitoid species did not differ for horsenettle miners on horsenettle vs. on eggplant, nor for groundcherry miners in tied vs. untied leaves. Instead, long—term effects of host plant morphology on leafminer behavior explained differences in parasitism between the two leafminer species. Whereas the mine structure of the groundcherry miner allows the larva to leave its mine and move freely over the leaf surface, the horsenettle miner stays within its mine, even when disturbed. We found that this difference has made the horsenettle miner vulnerable to more parasitoid species; it can be exploited, not only by parasitoids that can attack vagile hosts, but also by parasitoids restricted to concealed, sedentary hosts. Evidence further indicated that horsenettle trichome morphology has favored the evolution of this obligate endophytism in the horsenettle miner by selecting against mine architectures that would allow miners easy access to the leaf surface. When experimentally placed on leaf surfaces, miner larvae could not feed on horsenettle because they could not penetrate its meshwork of stellate trichomes. On groundcherry, where trichomes are thin and flexible, larvae easily initiated mines. We argue that obligate endophytism, which is uncommon among related gelechiids, is a specific adaption to horsenettle morphology that has made the horsenettle miner available to more parasitoid species than its vagile relatives.