Some species have markedly large geographic ranges with distinct populations which experience variation in their local habitat. One such species, Junonia coenia (Nymphalidae, the common buckeye), sequesters the plant secondary metabolites iridoid glycosides for defense against enemies. In addition to these enemies, insect herbivores like the common buckeye must also protect themselves against pathogens and parasitoids, in which their immune system plays a critical role. Recent work suggests there is a trade-off between sequestration and immune function, in which individuals that sequester high amounts of iridoid glycosides have subsequently lower immune function. Understanding how differences among populations may affect this trade-off is important for understanding plant-herbivore and community interactions more broadly. By comparing the sequestration abilities of both immune-challenged and non-immune-challenged caterpillars, I am able to investigate this trade-off. Using the common buckeye as a model to investigate this interaction, I presented caterpillars at various larval stages from four populations across their geographic range (Arizona, California, North Carolina, and Florida) with an artificial immune challenge. I fed these populations the same host plant and monitored their sequestration, growth and development, and immune challenge response.
Results/Conclusions
Preliminary results show that one of our populations, Arizona, has significantly larger pupae than the other three populations, which is an accepted measure of adult fitness. In addition, the Arizona population sequesters iridoid glycosides in different proportions than the other populations. Notably, Arizona caterpillars sequester more of the iridoid glycoside catalpol than other populations, even when fed on the same diet. These findings suggest that Arizona larvae not only exhibit a higher likelihood of adult success, as shown by larger pupae, but they also utilize the iridoid glycosides in their host plants differently than the other three populations. Preliminary data analyses show no differences in immune response among these four populations, but further analyses are in progress. These data suggest that different populations of this widespread butterfly species may differ in their sequestration abilities and thus their susceptibility to enemies. Although population did not have an effect on immune response, there may be trade-offs in immune response and sequestration at the individual level, a speculation I am eager to test in the future. By further understanding the interaction between sequestration and immune function, we can make important inferences about herbivore biology.