Chronic elevated temperatures intensify viral infection and reduce survival following winter diapause in a North American caterpillar

Climate change has the potential to impact the prevalence and severity of infectious diseases worldwide. Characterizing the effects of elevated temperatures on interactions between insects and entomopathogens may provide novel insight into the ecological factors mediating insect fitness within changing climates. In this study, we evaluated the effects of persistent elevated temperatures on the susceptibility of a native North American herbivore, Euphydryas phaeton (Lepidoptera: Nymphalidae), to pathogen infection. We performed a yearlong 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, decreasing to 4°C). 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. We found that temperature strongly influenced caterpillar survival following viral inoculation, with elevated temperatures resulting in increased mortality within 30 days of inoculation. In caterpillars that were not inoculated with the virus, elevated temperatures resulted in accelerated development, reduced overwintering body mass, and increased mortality; however, mortality within this group primarily occurred later in larval development, following emergence from winter diapause. 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 interactions with natural enemies into evaluations of insect responses to climate change.