Herbivores induce plant chemical defenses based on the identity of their attacking parasitoid

Many parasitoids increase plant fitness by decreasing herbivore feeding damage. Solitary parasitoids (where only one individual completes development per host) generally cause their hosts to feed less than unparasitized herbivores, reducing pressure on plants to induce chemical defense production. However, gregarious parasitoids (where more than one individual develops per host) can cause their hosts to 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. Furthermore, gregarious parasitoid broods were reduced when their hosts fed on plants with higher glucosinolate concentrations, indicating the potential of glucosinolates to reduce brood size and potentially feeding damage by the host. 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.