Bee microbiomes in a changing climate: Investigating the effects of temperature on solitary bee life history and health.

Climate change is rapidly warming thermal environments, an important abiotic stimulus governing interactions between microbial symbionts and their hosts. Increasing evidence suggests that solitary bees rely on pollen provision microbes for successful development. However, the effects of heat stress on provision microbiota and the resulting consequences for larval health and development remain to be examined. We performed an in vitro study to investigate the effects of the thermal environment on provision microbiome composition and measured fitness outcomes for Osmia lignaria larvae. While pollen sterilisation removed bacteria from microbe-rich provisions, larval survivorship did not significantly differ between bees reared on microbe-rich (unmanipulated) diets and provisions treated with ethylene oxide (EO) gas. In contrast to previous research in solitary bees, larvae reared on EO-treated provisions weighed more and had higher total fat content, with temperature moderating the degree of difference. As anticipated, we observed a negative relationship between the duration of larval development and temperature. Our results indicated that an intact provision microbiota may not always improve bee fitness and that bee-microbe interactions during larval development may contribute to the size-shrinking effect observed for cavity-nesting bees under warming conditions.