Bumble bee species display contrasting phenological responses to climate variation

Predicting how phenology shifts with climate change requires knowing how climate affects key phenological events (e.g., activity onset and end). We investigated this in bumble bees (Bombus spp.), a widespread group of social insects. Queens form annual colonies in spring, produce workers in summer, and produce males and new queens by fall (reproduction). While queen emergence tracks rising spring temperatures, we know less about the timing of late-season events like reproduction. Life history models, however, have explored the relationships between environmental conditions and bumble bee reproduction. These imply that if colony growth is density-independent, reproduction should be later in longer growing seasons, but if it is density-dependent, reproduction should be earlier (occurring when demographic limits are reached). To evaluate these predictions and understand how climate influences bumble bee phenology in general, we quantified how temperature and precipitation affected flight periods of 14 North American bumble bee species using quantile regression. In warmer conditions, flight period onset was earlier in all species and flight period end was earlier in most (consistent with density dependence), though two species ended later (consistent with density independence). Precipitation effects were mixed and weaker than temperature effects. Most species had longer flight periods in longer seasons, and two common species also had longer reproductive periods. Overall, our study reveals interspecific phenological variation in bumble bees (suggesting life history variation) that could influence climate change responses. We show how life history models can inform studies of phenological shifts and, more broadly, better our knowledge of phenological drivers across taxa.