Thermal responses of within-host infection dynamics in a butterfly-parasite system

Isabella G. Ragonese, Richard J. Hall and Sonia Altizer, Odum School of Ecology, University of Georgia, Athens, GA, Isabella G. Ragonese, Richard J. Hall and Sonia Altizer, Center for the Ecology of Infectious Diseases, University of Georgia, Athens, GA, Maya Sarkar, College of Biological Sciences, University of Minnesota, St. Paul, St. Paul, MN, Richard J. Hall, Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, GA

Background/Question/Methods
Insect physiology, survival, activity patterns and large-scale distributions are all shaped by environmental temperature. Shifts in temperature can also impact the interactions between insects and other organisms, including their parasites. Given that insects are important pollinators, pests, and vectors of disease, it is vital to explore how temperature influences insect-parasite interactions. Monarch butterflies (Danaus plexippus), known for their transcontinental migration, are parasitized by a protozoan, Ophryocystis elektroscirrha (OE). OE infection can decrease the insects’ survival, reproduction, and flight ability. Here, we examine the temperature-dependence of within-host parasite replication, host immunity, and fitness outcomes. Monarchs from replicate genetic lineages were inoculated with one of two parasite strains (high and low virulence) and reared from hatching to adult eclosion under one of five constant temperatures (18, 22, 26, 30, and 34°C). We collected data on monarch development, survival, size, immune function and OE infection status and intensity.
Results/Conclusions
Monarch size and survival declined sharply at the hottest temperature, and no inoculated individuals reared at 34°C became infected, suggesting that hot temperatures decrease both host and parasite fitness. The low infection probability at 34°C is not due to greater innate immunity, but could instead reflect the thermal limits of OE invasion and replication. In the context of global climate change, longer growing seasons that facilitate year-round breeding of monarchs are known to result in high infection prevalence, but our experiment suggests rising temperatures at the upper thermal range could reduce the abundance of both monarchs and their parasites.