The blaeberry bumble bee, the hardy highlander

Hibernation is a covert preparation for a more overt action. — Ralph Ellison

by Athayde Tonhasca


A blaeberry bumble bee (Bombus monticola) ©Iain McGowan

This bumble bee queen (a mated female) is busy digging herself a cosy, safe chamber in which to spend the winter, at about 950 m above sea level on a Scottish hill. A mammal burrow or a crevice between rocks would have been suitable as well. After mating and gorging on pollen and nectar to build up her fat reserves, she’s preparing for the approaching winter.

The blaeberry bumble bee, also known as the bilberry or mountain bumble bee, is a species of higher latitudes or higher altitudes of Western Europe. In Scotland, it is at home in the uplands and moorlands, where it visits a variety of flowering plants and collects pollen mainly from bilberries, clovers and bramble.

Insects are poikilotherm animals (i.e., their body temperature depends on ambient temperature), but species of cold climates such as the blaeberry bumble bee are masters of thermoregulation, the ability to control their body temperatures. Some species do that by just moving to sunny spots or by shivering to heat up. Bumble bees (and some moths as well) go further by flapping their wings at a tremendous speed. This generates a lot of energy, so that the bumble bee’s body quickly warms up to about 30°C, which is the minimum temperature for flight. Their stocky bodies covered in dark hairs also help to store heat.

But the life of a hot, nimble pollinator is almost over for this bee. Her colony has crumbled away, and all worker bees have died or will die soon. Now it’s time to get ready for a new stage of her life. Once she’s settled in her shelter, her metabolism will slow down to a trickle, and she will enter a state of torpor. To further protect herself from freezing, she will produce glycerol, a natural antifreeze that prevents the formation of ice crystals inside the cells of her body.

If all goes well for our queen, she will awake and emerge in spring. Most of her energy reserves will be spent, so she will need to restore herself quickly by drinking lots of nectar. She will then search for another nest site to start a colony of her own.

Some pollinator species have shifted northwards or towards higher elevations in response to climate change, but most bumble bee species have failed to follow this pattern. We don’t know why, but this failure to adjust to new environmental conditions suggests that bumble bees are susceptible to local extinctions. These bees are important pollinators of many plants, particularly in temperate and high-elevation regions, but this ecological service has an uncertain future in a warmer planet.