Complex effects of climatic variation on bumblebee queen fitness.

Climate change is a global biodiversity threat. To understand how a changing climate affects individual fitness and gain insights into which mechanisms are responsible, we need to establish how climatic variation affects individual life-history traits (e.g. growth, survival and reproduction). Long-term data linking insect life histories and climate parameters are therefore valuable but, unfortunately, rare. Here, we test how climatic variation affects health, survival and reproduction in a key European pollinator, the buff-tailed bumblebee Bombus terrestris. We relate climatic variation experienced by developing queens in the field, caught between 2000 and 2014, to fitness traits assayed in these queens under otherwise constant lab conditions. We show that wet years consistently reduce queen fitness, while warm temperatures have positive and negative impacts. Behind these annual effects lies strong seasonality. In particular, climatic conditions experienced by young queens as they forage, mate and enter hibernation are vital determinants of whether they reproduce the following spring. Results suggest that climatic drivers that reduce queen resource acquisition prior to hibernation, or accelerate resource loss over winter, are especially detrimental to spring queen fitness. This suggests a strategy to mitigate the negative effects of climate change on bumblebees: ensuring high-quality forage late in summer before queens enter diapause.