Host plant effects on a specialist caterpillar-parasitoid interaction

Alternate hostplants can affect caterpillar behavior, performance, and defensive traits, which could in turn impact interactions with natural enemies. We used the Baltimore checkerspot, Euphydryas phaeton (Nymphalidae), and its specialist braconid parasitoid, Cotesia euphydryidis (Braconidae), to explore how an introduced hostplant impacts parasitoid-host interactions in this sequestering, specialist caterpillar. We used a combination of field-based collections and experiments in the lab to determine if parasitoid pressures vary between populations using different hostplants and if hostplants impact parasitoid preference. We tested parasitoid preference for caterpillars reared on either a native hostplant, Chelone glabra (Plantaginaceae), or an introduced hostplant, Plantago lanceolata (Plantaginaceae). We then compared mortality and survivorship of parasitized and unparasitized caterpillars on both hostplants. We found that C. euphydryidis, while present at most aggregations of pre-diapause caterpillars in the field, was found in low frequencies in post-diapause larvae. Parasitized late instar larvae also showed interesting behavioral changes, enclosing themselves in silk nests in leaves of non-hostplants.  In the lab, parasitoids preferred E. phaeton caterpillars reared on the introduced P. lanceolata compared to those reared on the native, C. glabra.  However, caterpillars suffered twice the mortality when reared on P. lanceolata and were three times more likely to die when parasitized. This conflict between parasitoid preference and host survivorship could be mediated by hostplant chemistry, a mechanism currently under exploration. Future work studying the demographic consequences of these interactions could provide important insights into the role of multi-trophic interactions in driving novel host shifts and the evolution of herbivore diet-breadth.