Incorporation of a novel host plant can result in widespread changes for insect herbivores, impacting both physiological processes and biotic interactions. Investigating the consequences of novel host plant utilization on interactions with natural enemies, including predators and pathogens, in native herbivore populations may provide insight into the evolutionary costs and benefits of dietary expansion. This study evaluated variation in immunocompetence, chemical defense, and exposure to the pathogen Junonia coenia densovirus (JcDNV) in wild populations of the Baltimore checkerspot caterpillar (Euphydryas phaeton), a native North American herbivore that recently incorporated a novel host, Plantago lanceolata, into its dietary repertoire. Caterpillars and associated plant samples were collected from 13 populations throughout the northeastern U.S., where either E. phaeton’s native host plant, Chelone glabra, the novel host plant, P. lanceolata, or both host plants were utilized. Caterpillar immunity, JcDNV prevalence (both in larvae and on host plants), and iridoid glycoside content of plants and caterpillars were evaluated in field-collected samples. E. phaeton caterpillars were also reared in the lab on either C. glabra or P. lanceolata and orally inoculated with JcDNV in order to experimentally assess host plant effects on immunity.
Junonia coenia densovirus was found to be widespread in E. phaeton populations, but more prevalent on the novel host plant, P. lanceolata. Immune assays revealed that caterpillar immunity was also lowest on this novel host plant, both in wild populations and experimental studies. Both phytochemistry and caterpillar sequestration of iridoid glycosides varied substantially between host plants and among E. phaeton populations. These findings reveal the variation in immunocompetence, chemical defense, and natural enemy threat that can exist between herbivore populations utilizing native and novel host plants, highlighting the importance of considering interactions with higher trophic levels in investigations of dietary expansions. Additionally, these results provide groundwork for further studies of the long-term consequences of the incorporation of the novel host P. lanceolata into E. phaeton’s dietary repertoire.