First documented stingless bee mortality and related colony demographics during an unprecedented heatwave

Vulnerability assessment of climate change has mainly focused on organism responses to gradual, long-term effects. Although heatwaves can be catastrophic, studies linking extreme warming to population declines and analyses of related factors are scarce. Here, we document the immediate impact of a long-lasting heatwave (May-June 2024) on stingless bees, the most abundant pollinators in tropical and subtropical regions, but potentially susceptible to extreme heat due to their ectothermic behavior. We gathered data from stingless beekeepers across the Yucatan Peninsula, Mexico and simultaneously analyzed demographic and environmental factors linked to mortality in the iconic Mesoamerican Melipona beecheii. During the heatwave, ambient and intracolonial temperatures were recorded together with estimations of adult and brood numbers to analyze demographic factors associated with colony detriment. Brood mortality was detected in all colonies surveyed albeit at different levels. Average colony mortality reached 9.6 % and was positively correlated with the maximum environmental temperatures in the surveyed localities. Temperatures inside colonies increased above estimates for tolerance of the brood, which explains extensive immature abortion. Nonetheless, average temperatures inside colonies with low populations varied closer to environmental ones, compared with colonies with strong populations. In addition, brood abortion was negatively correlated with colony worker population indicating that there may be mechanisms of passive temperature diffusion associated with adults acting as a shield against high environmental temperatures. We discuss the implications of heat-related stingless bee declines and measures to ameliorate their effects on these key stone pollinators.