Overwintering challenges for solitary bee Osmia bicornis in the face of global warming-induced warm spells

Climate change is progressing faster in winter than in summer, contributing to an increased frequency of extreme weather events, such as winter warm spells (WWS). These events pose a significant risk to diapausing insects, potentially leading to physiological disruptions and phenological mismatches. We investigated the effects of current and projected WWS on the overwintering physiology and emergence timing of Osmia bicornis, a solitary bee and key pollinator in Central Europe. In a controlled experiment, we exposed bees to four overwintering conditions: laboratory control, current and projected WWS, and natural outdoor conditions. We monitored body mass monthly from November to March, recorded emergence phenology, and measured fat reserves post-emergence. Our results show that WWS, particularly under projected warming scenarios and outdoor conditions, which were notably warmer than laboratory simulations based on multiannual winter averages, significantly accelerated emergence. Also, body mass loss and fat reserve depletion were significantly higher in bees exposed to WWS and outdoor conditions compared to those kept under laboratory control. However, these physiological costs did not translate into increased overwintering mortality. These findings suggest that warming winters may push O. bicornis closer to its physiological limits, with potential consequences for fitness and reproductive success under future climate scenarios. Overall, our finding highlights the vulnerability of overwintering pollinators to warming winters and emphasizes the importance of incorporating extreme climatic events into models predicting insect responses to climate change.