Gotta Fan 'em All: Using computer vision based tracking to understand the impacts of heatwaves on social thermoregulation in bumble bees

Bumble bees are a diverse group of important wild and managed pollinators in temperate regions across the globe. Climate and land use change–namely heatwaves and landscape simplification–are negatively impacting bumble bee populations, but how these stressors interact to cause population declines is not well understood. Elucidating these mechanisms can help us focus conservation efforts and understand where bumble bees will be most vulnerable to interacting global change stressors. Here, we investigate the interacting impacts of heat and nutrition stress on the behavior and survival of bumble bees (Bombus impatiens). We fed B. impatiens microcolonies high or low nectar quality and challenged them with a 3-day heatwave. We quantified impacts of nutrition stress on behavioral thermoregulation (fanning), brood care behaviors, and worker and brood survival. To record temporally dynamic changes in behavior, we integrated tag-based tracking of individual workers with deep learning-based behavioral classification. This data revealed a non-linear trend in fanning investment. Fanning peaked approximately one hour after the heatwave started, but declined throughout the heatwave. Bees in the low quality nutrition treatments fanned less than those in high quality nutrition treatments as the heatwave progressed–despite consuming more nectar throughout the experiment. In combination, simulated heatwaves and low quality nutrition reduced worker and brood survival. Our study leverages novel techniques to understand how combined stressors can alter behavior and survival, providing compelling evidence for the importance of high quality nutrition for bumblebee survival of heat stress.