Causes of colony mortality in bumblebees

Despite considerable interest in bumblebees and their conservation, few data are available on basic life-history parameters such as rates of nest predation and the proportion of wild nests that survive to reproduction. Here, we use a combination of data collected by volunteers and our own direct observations which together describe the fate of 908 bumblebee nests in the UK between 2008 and 2013. Overall, 75% of nests produced gynes, with marked differences between species; the recently arrived species, Bombus hypnorum, had the highest proportion of colonies surviving to gyne production (96%), with the long-tongued Bombus hortorum having the lowest success in reaching gyne production (41%). There were also large differences between bumblebee species in the timing of nesting, gyne production and nest mortality, with B. hypnorum and Bombus pratorum nests starting early, producing most gynes before mid-summer, and then dying off in June, whereas at the other end of the spectrum Bombus pascuorum nests started late and produced gynes mainly in August. There was evidence for the partial or complete destruction of 100 nests. The main reported causes were excavation by a large mammal, probably primarily Meles meles (50%). Human disturbance was the second greatest cause of nest mortality (26%), followed by flooding (7%). Wax moth infestations were common (55% of nests), with B. hypnorum nests most frequently infested. However, infestation did not results in reduced likelihood of gyne production, perhaps because infestations often do not become severe until after some gynes have been produced. Our study provides novel insights into the little-studied biology of wild bumblebee nests and factors affecting their survival; collecting similar datasets in the future would enable fascinating comparisons as to how parameters such as nest survival and reproduction are changing over time, and are affected by management interventions for bees.