COS 30-6 - Elevated temperature increases the severity of viral disease in a native insect herbivore

Abstract: Climate change has the potential to impact the prevalence and severity of infectious diseases worldwide. In herbivorous insects, for which dietary composition can be a key mediator of susceptibility to disease, characterizing the interactive effects of elevated temperatures and dietary quality on susceptibility to entomopathogen infection may provide novel insight into the outcomes of climate change for insect fitness and persistence in wild populations. In this study, we characterized the interacting effects of elevated temperature and host plant identity on the development and susceptibility of a native North American herbivore, Euphydryas phaeton (Lepidoptera: Nymphalidae), to pathogen infection. We performed a factorial experiment in which E. phaeton caterpillars were reared in the laboratory using either an ambient temperature regime simulating present-day field conditions (25°C maximum during the summer, decreasing incrementally to a minimum of 0°C in the winter) or an elevated temperature regime (30°C maximum during the summer, decreasing to 4°C during the winter). In addition, caterpillars consumed either a native host plant, Chelone glabra, or a lower quality, introduced host plant, Plantago lanceolata, throughout development. During the third larval instar, a subset of caterpillars from each treatment group was orally inoculated with Junonia coenia densovirus (Parvoviridae: Densovirinae), a pathogen that naturally infects E. phaeton in wild populations. Our study revealed that temperature strongly influenced herbivore survival following viral inoculation, with elevated rearing temperatures resulting in increased mortality across both dietary treatment groups. In addition, elevated temperatures strongly accelerated caterpillar development, while viral inoculation had a weaker accelerative effect. Dietary effects on development time depended upon the temperature at which caterpillars were reared: caterpillars exhibited faster development on the native host plant, C. glabra, at ambient temperatures, but development times were similar on the two host plants at elevated temperatures. These results illustrate the potential for increasing temperatures to intensify the effects of infectious disease in wild insect populations and underscore the importance of incorporating both dietary quality and interactions with natural enemies into evaluations of insect responses to climate change.