Heat exposure in the nesting cavities of the solitary bee Megachile rotundata

Ambient air temperature are not a reliable predictor of the microclimates where insects live. Nesting cavities can have significantly higher temperatures than ambient due to the physical properties of the nesting material and radiant heating from sun exposure. In solitary bees, multiple life stages are confined to nesting cavities. If temperatures rise to stressful levels, these life stages are unable to move to more favorable microclimates. We examined field nesting cavity temperatures in the solitary bee Megachile rotundata to determine whether temperature affect diapause rates, larval survival, adult body size and overwintering nutritional reserves. We found that heat exposure causes early life mortality and can cause low rates of diapause aversion. In the range of cavity temperatures measured in the field, higher temperatures did not affect adult body weight or nutritional reserves. We conclude that nest cavities can reach temperatures that are regularly higher than ambient and have a negative effect on M. rotundata survival. Increasing summer temperatures may impact the yield of agricultural populations of M. rotundata.