Thermal carry-over: lasting impacts of early life heat stress on morphology and flight performance in solitary bee Osmia bicornis.

Climate warming is increasing the frequency and intensity of heatwaves, yet the lasting effects of short, extreme thermal events during development on pollinators remain poorly understood. Early life heat stress may generate persistent carry-over effects that influence adult morphology and performance, with consequences for fitness and ecosystem functioning. We experimentally tested how moderate and extreme developmental heatwaves affect adult morphology and flight performance in the red mason bee (Osmia bicornis), a key pollinator across its broad distribution and a model species for solitary bee research. Bees exposed to heat stress emerged with reduced body mass and wing size, with the strongest effects under extreme and prolonged heatwaves. Allometric analyses revealed that heat-stressed bees had systematically smaller wings for a given body mass. These morphological shifts translated into impaired flight performance, including reduced flight initiation, shorter flight distances and lower mean flight speeds under extreme heatwave conditions. In contrast, short moderate heatwaves slightly increased flight speed. Our results demonstrate that developmental heatwaves can induce persistent morphological and functional deficits in pollinators. As extreme thermal events become more frequent, such carry-over effects may reduce pollinator mobility, foraging efficiency and ecosystem service provision, underscoring the need to incorporate thermal extremes into predictions of pollinator resilience.