Does chemistry make a difference? Milkweed butterfly sequestered cardenolides as a defense against parasitoid wasps

Plant allelochemicals have important roles in plant defense as well as ecological and co-evolutionary dynamics within tri-trophic systems of plants, herbivores, and natural enemies. Milkweed butterflies represent a model system for chemical ecology because they sequester cardenolides semi-proportionally to the concentration in their host plants, yet little is known about the role of sequestered cardenolides in interactions with invertebrate natural enemies. We experimentally tested the preference and performance of two species of parasitic wasps (Pteromalus cassotis Walker and Pteromalus puparum Linnaeus) on milkweed butterfly pupae (monarchs, Danaus plexippus Linnaeus, and Euploea core Cramer) reared on plants to contain variable concentrations of sequestered cardenolides. We measured host survival and parasitoid reproductive success to determine whether greater concentrations of herbivore-sequestered plant toxins provide a defensive benefit or influence parasitoid success. We found that P. puparum was unable to develop from monarchs, regardless of toxicity. Monarchs containing more cardenolides (those fed Asclepias curassavica) were more likely to survive encounters with P. cassotis than those containing fewer cardenolides (fed Asclepias incarnata), but only because this parasitoid was less likely to attack more toxic monarchs. Once attacked, host toxicity had no effect on the likelihood of monarch survival nor the emergence of parasitoids. Host toxicity affected parasitoid performance in more subtle ways, however, decreasing P. cassotis brood size and survival to adulthood. When attacking cardenolide-free E. core pupae, P. cassotis reproduced successfully, but P. puparum did not, suggesting that milkweed butterflies may employ other defenses against parasitoids, perhaps in addition to cardenolides.