Heatwave length and severity drive development disruption in a key pollinator

Heatwaves are becoming more frequent and severe due to climate change, posing a significant threat to global biodiversity and entire ecosystems. Despite alarming projections of population declines for key pollinators like bumblebees, crucial for crop pollination and ecosystem services, surprisingly little is known about the implications of heatwaves on their larval and pupal development. Here, we experimentally exposed Bombus terrestris L. (Hymenoptera: Apidae) pupae in vitro to varying heatwave scenarios in terms of length (3, 5, or 7 days) and intensity (36C, 37C, and 38C) to study the impact on their development at this critical life stage. We assessed their emergence rate, developmental duration, lipid content, sex-specific body size and wing geometrics morphometrics. Our major findings revealed a critical heat stress threshold at 38C, where pupal emergence was drastically reduced by almost twofold and fivefold for 3 and 5 days, respectively. Adults emerged from these heatwave treatments exhibited a fourfold and sevenfold increase in wing deformities. Even excluding these extreme cases, we found clear evidence for heatwave-induced effects on female wing shape and directional asymmetry (DA) among treatments. Given the relative heat tolerance of our model species B. terrestris, we expect even greater vulnerability in more cold-adapted bumblebee species. Our findings underscore the profound ecological consequences of heatwaves on bumblebee pupal development, with potentially devastating cascading effects on colony fitness, population viability, crop pollination, and ultimately, ecosystem services essential for human well-being.