Deferred mortality: cyclic thermal stress during pupation triggers irreversible carry-over costs in a key pollinator

Extreme weather events, such as heatwaves, pose a concerning threat to global biodiversity, particularly to terrestrial ectotherms like insects. While lethal effects of thermal stress are widely studied, very little is known about sublethal carry-over effects of heatwaves experienced during development on adult fitness. Addressing this fundamental knowledge gap is crucial for key pollinators such as bumblebees, which face alarming population declines. Therefore, we used a highly controlled in vitro approach to expose Bombus terrestris pupae to ecologically relevant cyclic thermal stress, and studied their emergence success, subsequent adult longevity, and morphological traits. We found that cyclic thermal stress caused significant acute pupal mortality (up to 30% reduction in emergence). Most notably, this developmental stress resulted in deferred mortality, significantly reducing adult longevity (HR = 1.81) in both workers and males. Furthermore, we identified antennae and wing deformations as a powerful hazard indicator (HR = 2.50) that strongly predicts premature adult mortality. Our findings reveal that cyclic thermal stress during pupation imposes irreversible developmental damage that undermines adult physiological resilience. We argue that life-stage specific carry-over effects should be considered more in the future to avoid underestimating the long-term impact of climate change on crucial insect pollinator populations.