Mass-migrating bumblebees : An overlooked phenomenon with potential far-reaching implications for bumblebee conservation

Bumblebees are one of the most commonly studied pollinators, but they are declining in large parts of their distribution. Whether bumblebees can cope with anthropogenic disturbances such as climate change and habitat loss depends largely on their dispersal capacity. While bumblebee queen dispersal is estimated to be only a few kilometres, bird migration sites have documented mass-migration events with peak migration of 70 bumblebee queens per minute, indicating that bumblebees can migrate over larger distances than previously thought. The open-access database trektellen.org contains 10 daily counts of >1,000 migrating queens past single points in the Netherlands, and one in the United Kingdom (total bumblebee records 65,430; range 1–11,142 individuals), mostly in early spring. Such mass-migration events are poorly documented in the scientific literature, and usually when describing migrations of other insects such as syrphids and social wasps. Most common European bumblebee species were documented to migrate. Bumblebees were observed flying at sea, coming from sea and flying towards the sea, showing that they can cross large water bodies. The wind direction might have helped to concentrate migration at landscape bottlenecks. On 1 day, bumblebee mass-migration was documented on two sites located 200 km apart. Together with the concentrated and directional flight this suggests that they can migrate for several hundreds of kilometres. Because most mass-migration events occur in spring, large-scale shortage in suitable nesting sites may be the trigger for mass-migration (e.g. due to high queen survival or low vole numbers). Future studies should test whether this is the case, or whether bumblebees show annual seasonal migration. Synthesis and applications. Much remains unclear about bumblebee migration, but it may have large consequences for the conservation of bumblebees and the ecosystem services they provide. For example, bumblebees in highly disturbed areas like agricultural landscapes may be continuously supplemented by queens from far-away productive natural areas. This suggests that large-scale conservation initiatives are required to maintain viable populations of common and endangered species. Structured observations and modern-day technology, like isotope analyses or radar monitoring, may shed light on the drivers and consequences of the overlooked phenomenon of bumblebee migration.