Effects of heatwaves and larval host plant on overwinter performance of a generalist herbivore

Abstract Heatwaves are increasing in frequency and severity due to anthropogenic climate change and may play a role in recent insect declines. Heatwaves can occur during any season, and how insects are affected by heatwaves may depend on an insect's phenology and life stage when the heatwave occurs. Most research investigates summer heatwaves; however, winter heatwaves may have greater negative impacts on insects than summer heatwaves, but further investigation is necessary. We designed an experiment to test the impact of winter heatwaves on a dietary generalist (fall webworm, Hyphantria cunea ) to test the individual and interactive effects of heatwave occurrence, timing and host plant on insect performance. Previous research has shown fall webworm are negatively affected by summer heatwaves, and responses varied by host plant, but whether winter heatwaves affect fall webworm has not been tested. We reared fall webworm larvae on six different host plants and exposed them as pupae to one of four winter heatwave treatments: no heatwave (control), early‐, middle‐, or late‐season heatwave. We found that larval host plant played a much stronger role in overwintering performance than heatwave treatment, yet the presence of a heatwave significantly affected the phenology of fall webworm eclosion and marginally affected pupal survivorship during the winter. Host plant also significantly affected adult longevity and mating success, but heatwave treatment had no effect on these performance measures. For three separate metrics (adult longevity, mating success and egg cluster production), there were effects of host plant, sex and their interaction. Overall, we found that winter heatwaves had some moderate effects on performance for this dietary generalist herbivore, but that the host plant upon which larvae were reared played a much bigger role in overwinter success. More research is needed on how winter heatwaves affect different species to determine the relative effects of a winter heatwave on insects and how these effects may vary with other factors, such as host plant.