Nesting biology shapes climate vulnerability of social bees ( <i>Bombus</i> spp.)

Climate warming is a major driver of global pollinator declines, including social bees that live in large colonies and provide critical pollination services. Nesting biology plays a key role in determining a colony's resilience to climate change, but its influence on colony performance has received little attention. We investigate how climate warming affects bumble bee foraging efficiency and thermoregulatory behaviours under mid-century and late-century climate projections. We measured ambient air and nest temperatures in simulated above-ground and below-ground nests, assessed temperature dependence of foraging activity and conducted controlled trials to quantify worker thermoregulatory behaviours (wing fanning and brood incubation). Contrary to predictions, our results show that warming will increase optimal foraging hours for bumble bees and reduce brood incubation needs, potentially improving foraging performance. However, extreme summer temperatures could greatly increase fanning demands, particularly for above-ground nests, which would divert workers from foraging and other essential colony tasks. Our findings highlight that above-ground-nesting bumble bees are particularly vulnerable to warming due to escalating thermoregulatory demands, which could lead to colony failure under extreme conditions. Conservation efforts should consider nesting environment as a critical factor in climate resilience strategies for bees and other animal species.