Glucosinolate induction by parasitized caterpillars

Herbivore feeding damage induces increased production of chemical defenses in many plants and the amount of induction is often correlated with the amount of feeding damage. Many parasitoids are thought to increase plant fitness by decreasing herbivore feeding in the near term or reducing herbivore populations in the long term, making defenses which negatively impact parasitoids typically disadvantageous for the plant. Solitary parasitoids (where only one individual completes development per host) generally cause their hosts to feed less than unparasitized herbivores. However, gregarious parasitoids (where more than one individual develops per host) can decrease plant fitness if parasitized herbivores feed more than unparasitized herbivores, which should then also induce greater defense chemistry. Yet, it is unclear whether differential induction effects are primarily due to differences in feeding amount or parasitoid identity.

We explored the relationship between plant defense induction and caterpillars parasitized by a solitary versus a gregarious parasitoid. We wanted to determine whether: 1) caterpillars parasitized by a gregarious parasitoid induced higher concentrations of chemical defense and 2) differential responses to feeding by herbivores parasitized by different parasitoids were based on differences in the amount of feeding damage or the identity of parasitoids developing within the parasitized caterpillars. We measured the induction of glucosinolate defenses in Brassica rapa using Pieris rapae caterpillars of equivalent size and development stage parasitized by either a solitary (Cotesia rubecula) or a gregarious (C. glomerata) parasitoid. We demonstrate that C. glomerata-parasitized P. rapae elicit an increase in glucosinolate production independent of the amount of feeding damage, suggesting qualitative rather than quantitative differences in feeding damage are responsible for the observed patterns. Our results indicate that plants can differentially induce chemical defenses in response to the identity of a parasitoid inside an herbivore and demonstrate the importance of parasitoid identity in determining plant defense expression.