Climate Change Impacts on Diapause Outcomes in Bombus terrestris Across an Environmental Gradient

Most temperate insects survive winter by entering a period of dormancy called diapause. Some of the greatest climate change impacts are likely to occur during this period, as entire populations of some species are represented by a relatively small number of hibernating individuals. Bumblebee populations are particularly vulnerable because only queens overwinter and any decline in the winter survival of queens, or reduced post-diapause fitness, could have significant consequences on population dynamics the following spring. This study investigated the impact of different overwintering conditions on Bombus terrestris queens across an altitudinal gradient in the German Alps. Snow pack coverage was manipulated to simulate advanced snow melt and delayed snow melt, and maintained for current/control conditions. We found that changes in snow pack and altitude affect the level and variability of soil temperature across the winter season but that these have no effect on hibernation survival (very high in all cases) or subsequent colony establishment (very low in all cases). However, queen weight loss during diapause did differ across altitudes, with bumblebees hibernating at lower levels and being exposed to higher and more widely fluctuating temperatures, losing significantly more weight. Our results therefore confirm the potential for negative physiological impacts of exposure to higher and more variable winter temperatures during diapause, as is increasingly common under climate change. But we also find that bumblebees have some resilience to these effects, at least in terms of hibernation survival, within the range experienced in our experiments. We conclude that further field studies targeted at the range of conditions likely to be experienced by European insects over winter are necessary to anticipate potential population impacts, but that existing levels of climate change are likely sufficient to affect many of these insects, even if at sub-lethal levels.