Using infrared thermography to understand the thermal ecology of wild bumblebees

As global warming intensifies, pollinators such as bumblebees may experience increasing exposure to temperatures near their thermal limits. Heat stress impairs foraging and survival, making it essential to understand bumblebee body temperature in natural conditions. This study tested the feasibility of using infrared (IR) thermography as a non-invasive technique to measure the thoracic temperature of wild, foraging bumblebees and to evaluate how body temperature relates to environmental variables, including ambient air and floral surface temperatures. Thermographic measurements were validated against internal thoracic temperatures recorded by thermocouples in static bees, revealing a strong correlation (r = 0.98) with an average absolute difference of <1 °C. We analysed thermal images of live Bombus individuals (n = 98) collected over five observation days in late summer. Bee body temperatures routinely exceeded both ambient and floral temperatures and approached the critical thermal maximum (CT_max) during midday foraging. A linear mixed-effects model revealed that bee temperature increased significantly with both ambient air and floral temperature, and a significant interaction term indicated that warmer floral surfaces amplified the effect of high ambient temperatures. These findings demonstrate that IR thermography can reliably measure bumblebee body temperature in-situ, bridging the gap between laboratory-derived thermal limits and field conditions. By capturing the combined effects of microclimate and physiology, this method offers new insight into pollinator heat stress at the organismal level and highlights the importance of fine-scale thermal data for assessing species' responses to climate change.