Feeling the heat: Bumblebee workers show no acclimation capacity of upper thermal tolerance to simulated heatwaves

Climate change is our most significant challenge in the 21st century and among the main drivers of biodiversity loss. Recent distributional shifts and declines in crucial pollinators, such as bumblebees, seem to be associated to this phenomenon. However, despite future climate projections on climate warming, few studies have assessed the ability of temperate bumblebees to acclimate to extreme weather events, such as heatwaves. This study estimates the upper critical thermal limits (Critical Thermal Maximum (CTmax) and Heat Coma Temperature (HCT)), of the bumblebee subspecies Bombus terrestris audax, and assesses whether CTmax increases following exposure to a simulated heatwave. The critical thermal maximum occurred between 48.9 and 52.7 degrees C, while the heat coma temperature varied between 50.7 and 53.4 degrees C. After measurement of HCT, around 23% of bees survived 24 h or longer, but coordination was never recovered. There was no significant association between upper critical thermal limits and body mass, which highlights the need to investigate other factors to comprehend the mechanisms behind thermal tolerance limits. Furthermore, the heatwave treatments had no significant effect on the CTmax of bumblebee workers, indicating no acclimation capacity of upper thermal tolerance to simulated heatwaves. Our study provides insights into the upper thermal tolerance limits of Bombus terrestris audax and reveals that exposure to heatwave-like events does not change the upper thermal tolerance of bees, highlighting the need to develop effective strategies that might enable them to cope with extreme weather events.