Professor Colorado State University, Colorado, United States
Background/Question/Methods
Insect herbivores are constantly challenged by plant defense compounds and natural enemies like parasitoids. While herbivores deal with these toxic phytochemicals using specialized enzymes, their immune responses are likely to be the key physiological defense against parasitoids. One clear gap in our knowledge is the cascading effect of phytochemicals on parasitoid via herbivore immunity. Here we addressed this gap by asking two major questions: 1) what is the role of variable plant defense chemistry on herbivore physiology-immunity (?), and 2) can host plant driven herbivore immune status alter the success of parasitoids with different life history traits (?). We used two variable glucosinolate (GSL) producing plants (Brassica oleracea var. viridis and Brassica rapa) to grow cabbage white butterfly (Pieris rapae) and examined the effect of host plant chemistry on larval physiology and performance.
Results/Conclusions
We used two variable glucosinolate (GSL) producing plants to grow cabbage white butterfly (Pieris rapae) and examined the effect of host plant chemistry on larval physiology and performance. Our data showed that higher GSL concentration of B. rapa was negatively correlated with larval body size but had positive impact on their immune status. Furthermore, the higher concentration of GSL prolonged the overall larval development duration of P. rapae. The cellular immune status, in terms of total hemocytes number was higher on B. rapa fed larvae throughout their developmental stages in comparison to B. oleracea var. viridis. When we measured the hemocytes diversity in circulation, plasmatocytes, granulocytes and oenocytoids appeared to be increased on second and third instar larvae that were grown on B. rapa. The melanization capacity was enhanced during second and fifth instar larval stages on B. rapa. This heightened immune status showed an elevated encapsulation rate against the gregarious endo-parasitoid Cotesia glomerata, leading to a 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 similar impact on all parasitic wasps.
