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 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,
altitudinal level did affect weight loss during diapause, with bumblebees
hibernating at lower levels, and so 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.