Bumbles disperse to spread the seed far away

Precious bumblebees have been on the wing since January, perhaps courtesy of climate change. Only females survive the winter.

Awakening from hibernation, each must find a place to nest and to lay eggs that will hatch into infertile workers. These will help her build a colony. But how does she house-hunt? Do bumblebees set up home locally or head for pastures new?

At Queen Mary University London, James Makinson and his colleagues fitted tiny radar antennas to the backs of captive bumblebees. When the bees emerged from their winter sleep, they were released to the wild at a research site in Hertfordshire.

Birds and mammals have a ‘gold-rush’ mentality; a ‘first come, first served’ approach. It was thought that bumblebees did likewise, opportunistically grabbing the best seats, the weaker ones losing out. But, the Queen Mary research shows otherwise.

The experimental bumbles, newly emerged from hibernation, spent most of their time hiding in vegetation on the ground. Two or three times an hour, they made furtive flights of ten to twenty seconds duration, before going to ground again.

Although they have internal compasses that allow them to fly in straight lines, the bees did not travel in a particular direction, nor did they return to their natal sites. Short flights, made over several weeks, would eventually carry them several kilometres from their origins.

Could artificial rearing, and carrying antennas on their backs, explain this odd behaviour? No, the researchers say. Tracking wild bumbles showed that they behave in the same way as the domesticated ones. So, why do bees do this?

After the long winter sleep, bees need rest and recreation to regain their strength and develop their ovaries. There are advantages, also, to locating colonies well away from the natal ones; young queens can distribute themselves more evenly throughout the countryside. Creatures which disperse, like the Portuguese navigators of old, discover new areas to exploit and avoid competition for dwindling resources at home.

Establishing colonies in virgin territory might offer pioneers better breeding prospects. Genetic analyses show that bee colonies can be established far from a queen’s origins. Bumblebees imported to Chile moved up to 200km in a year.

Going elsewhere also helps avoid ‘consanguinity’; a wanderer is less likely to mate with a close relative. Young male mammals avoid incest by moving away from where they were born, while their sisters tend to remain closer to home. Birds adopt a similar strategy, although the roles are usually reversed: females disperse, while their brothers stick around.

Male swallows return to the same barn year after year; their sisters and daughters set up home some distance away. A queen bee will have been fertilised prior to hibernation, but dispersal fosters genetic diversity among her later descendants. Our bees are in trouble and need help.

These findings, the researchers claim, “suggest practices that may be valuable for conservation efforts ... friendly corridors between conserved landscape catches could be beneficial. It may also be helpful to leave vegetation, such as leaf litter and long grass, undisturbed until late in the spring, giving queen bumblebees safe places to rest”.

James Makinson, et al. Harmonic radar tracking reveals random dispersal pattern of bumblebee queens after hibernation. Scientific Reports. 2019.