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.