Impact of thermal stress during pupal development in a key pollinator

The increasing frequency and intensity of extreme thermal events like heatwaves, driven by climate change, threaten global biodiversity and entire ecosystems. Despite alarming projections of population declines for key pollinators like bumblebees, little is known about the direct impact of varying elevated temperatures during their pupal development. To gain a better understanding of the dose-response to thermal stress, we experimentally exposed Bombus terrestris L. (Hymenoptera: Apidae) pupae in vitro to varying thermal stress lengths (3-7 days) and intensities (36-38°C). This mechanistic approach allowed us to uniquely isolate the direct physiological effects of temperature on pupal development from complex colony dynamics and assess individual emergence success, developmental duration, body size, relative lipid content and wing geometrics morphometrics. At the critical threshold of 38°C, pupal emergence was drastically reduced (2-5-fold) and wing deformities increased (4-7-fold). Even excluding wing deformities, we found thermal stress-induced effects on emerged worker wing shape and directional asymmetry. Hence, our findings suggest that increased extreme thermal stress negatively impacts bumblebee pupal development. Given the relative heat tolerance of B. terrestris, we expect even greater vulnerability in more cold-adapted bumblebee species.