Host-associated immune response and genetic differentiation in generalist species of Lepidoptera

Background/Question/Methods Host diet is an important factor in the fitness of insect herbivores. Feeding on a particular host plant species can affect the life history traits of an herbivore, e.g. growth, survival and reproduction. These differences between potential host plants pose a particular challenge to polyphagous herbivores that can choose from a wide variety of plant species, each of which may result in differential fitness. If survival and other fitness components are affected by the choice of a host plant as well as the genetic lineage of the herbivore, generalist species may exhibit host-associated genetic differentiation. There were two objectives for this study: 1) to examine the effect of host diet on immune response to parasitism, an important fitness component thought to differ by host, and 2) to examine representative generalist species for evidence of host-associated genetic differentiation. Lepidopteran larvae were collected from four host tree species at four different sites in Maryland. To measure the immune response of caterpillars, we injected latex beads and measured immune response using the degree each bead was obscured by blackened hemocytes. We also extracted DNA from the adult moth and used Amplified Fragment Length Polymorphisms (AFLPs) to examine each species for host-associated genetic differentiation.

Results/Conclusions Encapsulation studies demonstrate significant difference in encapsulation capacity between larvae feeding on box elder and black willow. Further, there appears to be a significant interaction between genotype and host plant indicating that cohorts demonstrate differential immune response on host plants, especially on a novel host plant. This research provides evidence that host plant strongly influences an herbivore's vulnerability to parasitoids with respect to immune response. Initial genetic results suggest no significant evidence of host-associated genetic differentiation in all of the species examined. The data suggest some genetic structure within and between sites that does not appear to be host-associated. This study provides further evidence that the plant host has an indirect effect on differential survival in insect herbivores through immune response. Although we were unable to demonstrate host-associated genetic differentiation, differential immune response may be a mechanism for host-associated genetic differentiation of herbivores.