PS 17-233 - Risk of heat stress in foraging bumble bees increases with pollen load size

Abstract: Climate change has been linked to recent declines in bumble bee populations worldwide, but the specific mechanisms causing thermal stress in these bees have not been well-studied. The goal of this study was to investigate the risk of heat stress in workers foraging for pollen, which is necessary for colony development. Previous studies have demonstrated that pollen foraging increases the thoracic temperatures of honey bee workers in controlled settings, but this relationship has not yet been tested in bumble bees nor in realistic field settings. We addressed this gap in knowledge by examining the effects of increasing pollen load size on the thoracic temperatures of Bombus impatiens workers in the field after accounting for their microclimate and body size. Additionally, to contextualize the effects of pollen load size on bumble bee thermal safety margins, we used our model to predict the thoracic temperatures of small, medium, and large-sized bees, with and without pollen, compared to the critical thermal maximum we measured for B. impatiens. We found a 0.07°C increase in thoracic temperature for every mg of pollen carried (p = 0.007), resulting in a 2°C total effect over the observed range of pollen load sizes. We also found that in the hottest observed microclimate, pollen-carrying bees were predicted to have a 1.7-2.2°C hotter mean thoracic temperature than those without pollen, depending on their size. This increase in thoracic temperature put B. impatiens workers at risk of reaching the critical thermal maximum we measured for this species (41.3°C to 48.4°C). Based on these results, we conclude that bumble bees likely use physiological or behavioral strategies to reduce the risk of thermal stress from pollen foraging and could be limited in foraging options as global temperatures continue to rise.