Beat the heat: Thermal respites and access to food associated with increased bumble bee heat tolerance.

Climate change poses a threat to organisms across the world, with cold-adapted species such as bumble bees (Bombus spp) at particularly high risk. Understanding how organisms respond to extreme heat events associated with climate change as well as the factors that increase resilience or prime organisms for future stress can inform conservation actions. We investigate the effects of heat stress within different contexts (duration, periodicity, with and without access to food, and in the laboratory versus field) on bumble bee (Bombus impatiens) survival and heat tolerance. We found that both prolonged (5 h) heat stress and nutrition limitation were negatively correlated with worker bee survival and thermal tolerance. However, the effects of these acute stressors were not long-lasting (no difference in thermal tolerance among treatment groups after 24 h). Additionally, intermittent heat stress, which more closely simulates the forager behavior of leaving and returning to the nest, was not negatively correlated with worker thermal tolerance. Thus, short respites may allow foragers to recover from thermal stress. Moreover, these results suggest there is no priming effect resulting from short or long duration exposure to heat - bees remained equally sensitive to heat in subsequent exposures. In field caught bumble bees, foragers collected during warmer vs. cooler conditions exhibited similar thermal tolerance after being allowed to recover in the lab for 16 h. These studies offer insight into the impacts of a key bumble bee stressor and highlight the importance of recovery duration, stressor periodicity, and context on bumble bee thermal tolerance outcomes.