Professor Colorado State University Fort Collins, Colorado
Recent studies have shown a correlation between phytochemical concentrations and herbivore immune functions but comparatively less attention has been paid to the cascading effect of plant defense metabolites on parasitoids via herbivore immunity. Here we examined the role of glucosinolate (anti-herbivore defense compounds produced by members of the Brassicales) concentrations on the ontogenetic profile of herbivore immune status modulating the success of two species of parasitoids with different life-history traits. We used two plants (cultivated collards Brassica oleracea var. viridis and wild field mustard B. rapa) that vary in glucosinolate concentrations to examine the effects of host plant chemistry on the immune responses of the small cabbage white butterfly (Pieris rapae) against parasitoids. We show that P. rapae caterpillars that fed on plants containing higher levels of glucosinolates attained lower body weights and prolonged larval development but had enhanced cellular immune response. Caterpillars that fed on B. rapa, had higher total hemocyte counts throughout their development from first through fifth instars. Counts of plasmatocytes, granulocytes, and oenocytoids were each elevated in second and third instar larvae that fed on B. rapa. Correspondingly, caterpillars that fed on B. rapa exhibited elevated melanization capacity. This heightened immune status enhanced encapsulation rate against gregarious endo-parasitoid Cotesia glomerata, leading to reduced brood size. When tested against solitary endo-parasitoid Cotesia rubecula, we found no correlation between larval immunity and parasitoid success. Therefore, our results illustrate that plant defense metabolites can strongly influence herbivore immunity, but might not have a similar impact on all parasitic wasps.
