Some herbivorous insects can sequester plant secondary metabolites from their host plants to defend against predators. However, sequestration can also be costly, affecting immunological responses important for other types of natural enemies, such as parasites and pathogens. Given that both immune function and sequestration abilities can change across development, these costs could also vary across herbivorous insect development, though few studies have assessed variation in the cost of sequestration across development. The common buckeye, Junonia coenia (Nymphalidae), is a specialist butterfly that sequesters iridoid glycosides (IGs) from its host plants, including the introduced weed, Plantago lanceolata (Plantaginaceae). To determine how immune function and sequestration vary across development, and if they are impacted by hostplant, we reared caterpillars on a native host, Mimulus guttatus (Phrymaceae), which does not contain IGs, and on the introduced P. lanceolata, which does. We assayed immune function across 3rd, 4th, and 5th instar caterpillars, measured hemocyte density, and then estimated the ability of hemocytes to encapsulate foreign bodies by challenging the immune system with nylon filaments. For caterpillars reared on P. lanceolata, we then explored the relationships between IG sequestration and immune function across instars.
Results/Conclusions
We found that hemocyte density and melanization were highly variable across development and experimental periods, were sometimes host-dependent, and that the impacts of IG sequestration varied depending on host-plant chemistry during experimental periods. In general, hemocyte density tended to increase with instar regardless of host plant. However, caterpillars reared on P. lanceolata had lower encapsulation ability, and encapsulation decreased with increasing IG sequestration, though patterns varied between instars and across experimental periods. Interestingly, immune challenged caterpillars consistently sequestered more IGs than unchallenged caterpillars, suggesting that caterpillars responded to immune challenge by sequestering or retaining more IGs. We validated these results with an additional experiment varying levels of manipulation and immune challenge. These results suggest that sequestration can have important consequences for immune function across caterpillar development, and that caterpillars can respond to immune challenge through altering sequestration. While the mechanisms driving increased sequestration in response to immune challenge remain unknown, the potential tradeoffs suggest that incorporation of novel hosts may have important consequences for defense against natural enemies.