Impacts of induced plant responses on interactions between an herbivore and its parasite
Upon herbivore damage, plants can systemically induce productions of secondary chemicals as defense strategies, which may mediate effects on subsequent herbivores. In addition, such induced responses can impact higher trophic levels, because interactions between herbivores and their enemies also strongly depend on plant quality. In the current study, we examined effects of induced plant responses on the interaction between the monarch butterfly (Danaus plexippus) and its protozoan parasite Ophryocystis elektroscirrha. Monarch caterpillars use milkweeds (Asclepias) as their larval host plants, which contain cardenolides that can inhibit O. elektroscirrha. In the presence and absence of previous herbivory, we measured life span of both uninfected and infected butterflies (indices of butterfly fitness and parasite virulence), and parasite spore load of infected butterflies reared across three milkweed species (A. incarnata, A. curassavica and A. physocarpa).
Milkweed species differed in their cardenolide concentrations and extent of induction after herbivory. Specifically, A. incarnata had low cardenolide concentrations and previous herbivory did not induce cardenolide production. A. curassavica and A. physocarpa, on the other hand, had high cardenolide concentrations. After damage, cardenolide concentration increased in A. curassavica, but decreased in A. physocarpa.
Effects of plant species, herbivore induction and their interactions on cardenolides were strongly reflected in life span and parasite spore load of infected butterflies. Namely, induced responses in A. curassavica led to increased life span and lower spore load of infected butterflies, whereas in A. physocarpa, induction resulted in decreased life span and higher spore load of infected butterflies. Although previous herbivory did not affect cardenolides in A. incarnata, life span of infected butterflies were lower, suggesting that other plant traits may also play a role in the interaction between monarch butterflies and their parasites. Altogether, our results demonstrated that induced responses in plants can have species-specific and significant effects on virulence and potential transmission of herbivore parasites.