Effects of climate change on fungal-pathogen-induced mortality of the spongy moth

The dynamics of a host-pathogen relationship can be strongly influenced by weather conditions. The fungal pathogen Entomophaga maimaiga is the dominant source of larval mortality for the spongy moth, an invasive species that is one of the most destructive forest pests in North America. Previous research indicates that larval infection rates by E. maimaiga increase with environmental moisture and tend to decrease as temperatures rise, but their interactive effects remain unclear. To test the independent and interacting effects of temperature and precipitation on rates of spongy moth larval infection by E. maimaiga, we manipulated temperature and precipitation according to a factorial design. We caged laboratory-reared 4^th instar spongy moth larvae on experimentally warmed forest plots, exposing the larvae to soil inoculated with E. maimaiga resting spores. In total, there were 36 plots, with 2 cages per plot. Our experiment utilized three temperature treatments: ambient, +1.7°C, and +3.4°C. For precipitation, at each plot, we randomly added supplemental precipitation to 1 of the 2 cages. We assessed fungal mortality with phase contrast microscopy techniques. Our results indicate a statistically significant positive effect for the highest temperature treatment (+3.4°C) on rates of larval infection. This finding is in alignment with previous modeling results that indicate suboptimal temperatures for E. maimaiga in parts of North America, such as northern Minnesota where this experiment occurred. This study highlights the potential for climate change to increase spongy moth larval infection rates in areas that are currently too cold for E. maimaiga persistence.