Coevolutionary theory predicts that particularly strong plant-herbivore interactions may lead to the evolution of specific defense strategies. Here we tested some of the prerequisites of this theory and also a long-standing prediction of defense allocation. We used a quantitative genetics approach to investigate induced responses and specificity of elicitation in common milkweed (Asclepias syriaca) to two lepidopteran herbivores (Danaus plexippus and Euchetias egle). In a field experiment, we grew 20 full-sib families of milkweed and treated replicates from each family as controls, damaged by D. plexippus or damaged by E. egle.
Results/Conclusions Latex, a potent defense of the milkweed, showed nearly five-fold variation among genetic families and, on average more than doubled in response to herbivory by D. plexippus. Induction of latex by E. egle was 40% weaker than that of D. plexippus. Although constitutive levels of latex exudation were equivalent at top and bottom leaf positions on plants, only the top leaves showed an induced response to herbivory. In addition, only for D. plexippus herbivory did we find a significant family by herbivory interaction (i.e., heritable variation for the induced response). Finally, we found no evidence for a tradeoff between constitutive and induced latex production. In summary, common milkweed shows heritable variation in its latex defense. This defense is most strongly induced by D. plexippus feeding and plants show heritable variation in their responses to this specific herbivore. Contrary to common wisdom, we found no tradeoff between constitutive and induced latex. Thus, as natural selection by specific herbivores varies in space and time, we expect that milkweeds will respond in both their constitutive and induced defenses.
