In many cases, an herbivore’s food plant provides primary host selection cues to which parasitoids and other natural enemies respond. Therefore, the host plant can influence an herbivore’s risk of parasitism. Some parasitoids are more likely to attack unrelated herbivores in a particular habitat on one plant species. Significant differential parasitism of macrolepidopteran herbivores feeding on different host plants has been demonstrated. Although differential parasitism has been shown in several systems, the mechanisms are still unclear. Our data suggests that host plant-associated differential immune response may be an alternative interpretation of differential parasitism. Parasitoids typically inject a polydnavirus when they oviposit which alters the herbivore immune response. This parasitoid-associated strategy alters the herbivore’s immune response by affecting the hemocyte (“defense cells”) number or morphology. An objective of the study was to examine the effect of host diet on immune response of caterpillars to parasitism. To measure the immune response of caterpillars, we injected latex beads into larvae and measured the degree to which each bead was obscured by blackened hemocytes. To examine the effect that polydnaviruses have on herbivore immune response, we compared caterpillars injected with beads and polydnavirus to caterpillars injected with beads alone.
Results/Conclusions
There are significant differences in encapsulation capacity between larvae feeding on some, but not all of the host plants. Further, there appears to be a significant interaction between genotype and host plant indicating that cohorts demonstrate differential immune response on host plants. Experiments demonstrate that polydnavirus affects the encapsulation capacity of some of the herbivore species by altering their immune response. Polydnaviruses appear to suppress the herbivore’s immune response in some of the species examined. The variation in the pattern from species to species may be due to the absence of polydnavirus in some of the parasitoids species or that other parasitoid species employ alternative counter-defenses, such as venom or maternal proteins. This research provides evidence that host plant strongly influences an herbivores vulnerability to parasitoids. Few studies have investigated the influence of host plant consumed on immune response. The results of these experiments will also help to evaluate a widely used approach for assessing immune responses in insect herbivores.