Heat tolerance in bees: a meta-analysis of ecological, evolutionary, and methodological drivers

Suntaree Karnchananiyom, Natapot Warrit, Patrice Pottier , Kanuengnit Wayo, Carmen da Silva, Vanessa Kellermann, Alyssa Stewart 

Climate change poses a growing threat to bees, which provide essential pollination services in natural and agricultural ecosystems. Although the critical thermal maximum (CTmax) is widely used to assess the vulnerability of pollinators to climate change, we lack a comprehensive understanding of the factors shaping heat tolerance across bee species globally. We conducted a systematic review and meta-analysis of experimental CTmax measurements, extracting 339 effect sizes from 35 studies spanning 198 bee species. Across all moderator variables tested, we found that only three variables were significantly associated with variation in heat tolerance: nesting behaviour, climate zone, and ramping rate. On average, the CTmax of below-ground nesting bees was 1.3oC lower than that of above-ground nesters. This difference was most pronounced in solitary species and weaker in eusocial species, which may benefit from colony-level behavioural thermoregulation. Tropical species had slightly lower CTmax than temperate species, while species distributed across both tropical and temperate regions had the highest thermal tolerance. We also found that CTmax increased strongly with ramping rate, consistent with predictions of heat injury accumulation during dynamic assays. However, sex, body size, and five other methodological factors were not significantly associated with variation in CTmax, which may reflect species-specific responses differing in direction and magnitude, obscuring any overall pattern. We also found that variation in heat tolerance is strongly phylogenetically structured (43% of explained heterogeneity), suggesting that evolutionary adaptation to historical environmental conditions has shaped present-day thermal limits. Our synthesis identified substantial geographic and taxonomic biases, highlighting key priorities for future experimental research. By identifying important ecological drivers of heat tolerance evolution, our study provides a foundation for predicting bee heat tolerance and potential vulnerability to climate change.