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.气候变化在冬季的发展速率快于夏季，导致极端天气事件频次上升，其中包括冬季暖期（winter warm spells, WWS）。这类事件对处于滞育期的昆虫构成显著风险，可能引发生理紊乱与物候错配。我们研究了当前及未来预估的冬季暖期对二角壁蜂（Osmia bicornis）越冬生理与羽化时间的影响——该蜂为独居蜂类，是中欧地区的关键传粉昆虫。在对照实验中，我们将蜂群置于四种越冬环境下：实验室对照组、当前及未来预估的冬季暖期组，以及自然室外环境组。我们于11月至次年3月每月监测蜂群体重，记录其羽化物候，并测定羽化后的脂肪储备量。研究结果显示，冬季暖期显著加快了羽化进程，尤其是在未来预估增温情景与室外环境下——后者相较于基于多年冬季平均值的实验室模拟，温度显著更高。此外，相较于实验室对照组的蜂群，暴露于冬季暖期与室外环境的蜂群，其体重流失与脂肪储备消耗显著更高。但这类生理代价并未导致越冬死亡率上升。上述研究结果表明，冬季变暖可能使二角壁蜂逼近其生理极限，未来气候情景下或将对其适合度与繁殖成功率产生潜在影响。总体而言，本研究凸显了越冬传粉昆虫对冬季变暖的脆弱性，并强调了在预测昆虫对气候变化响应的模型中纳入极端气候事件的重要性。