On The Trails: Bumblebee queens

Willow catkins provide important food for bumblebees in early spring. (Photo by Bob Armstrong)

By Mary F. Willson

When a new bumblebee queen emerges from her natal nest in late summer or early fall, she makes a so-called ‘nuptial’ flight. During that flight, she cruises around and mates multiple times with males that she meets.

After successful matings, she stores the sperm in a special sac. Then she makes a nest — perhaps in an old mouse nest in a big grass clump, underground in a vole nest, in a tree hole, or under a mat of moss. She often rearranges a bit of the nesting material to make a more convenient size for her and a brood. She settles down for the winter in a state of diapause (reduced metabolism and dormancy or torpor) and usually does not rouse, except perhaps if some unusually warm weather occurs.

Come spring, she wakes up and produces eggs. Some of these she fertilizes with the sperm stored from her nuptial flight — these will become workers or possibly future queens. Her unfertilized eggs will become males. The larvae are often half-siblings, many of them with different fathers.

With the eggs ready to lay, she goes foraging. When the eggs hatch, in a few days, those larvae will need food right away. If flowers are available, she can load up on nectar and pollen, which she will carry back to her nest.  There she builds a clump of pollen and constructs a number of waxy cups on top of it, and then lays an egg in each cup, storing more food in any vacant ones. She may periodically sit on the brood clump, shivering her wings to generate a bit of heat to facilitate larval growth. 

Bumblebees in their nest. (Rob Cruickshank / CC BY 2.0 DEED)

Now the timing is critical — because spring weather is notoriously variable, how well does her timing match with that of early flowers? There must be some years when the timings just don’t match very well. Some of the most important early flowers are willows, whose catkins offer nectar and pollen. Garden crocuses can be another good source.

She feeds the larvae on nectar and pollen in a tasty mixture. Some of it she regurgitates directly into the larval mouths. Some may be stored in a clump near each larva, which can then feed directly on it. The nectar provides immediate energy and the pollen provides protein needed for growth. Larvae grow for 10 to 14 days, pupate, and transform into full adult bees. After four or five weeks of development, the first mature larvae emerge as adult workers and help feed their nest-mates. Sometime during larval growth, the queen decides which fertilized eggs are to become future queens and gives them extra feedings. (How does she decide which ones get the extra food?) When the new queens mature and emerge from their nests, the cycle begins all over again. And the old queen typically dies when summer is over.

Those new queens find nest sites and overwinter. But many dangers threaten them. Nests on or near the surface of the ground may be destroyed by fires, or mowing, or large mammals moving about (for example). Underground nests are at risk of flooding from big rains or melting snow and ice.

But a recent study has shown that wintering bumblebees can survive being submerged for several days. They begin diapause, with a lowered metabolism as a normal part of overwintering; but if submerged, their metabolism drops even more and is partially anaerobic. And, surprisingly, they can somehow extract some oxygen from the water.

This research was done with Bombus impatiens, native to eastern North America. This species has been introduced to British Columbia and other places to enhance the pollination of some commercial crops and the species has spread from the points of introduction.  

There are over 200 species of bumblebees (genus Bombus), mostly in the northern hemisphere. They are likely to differ in some respects, of course. It would be interesting to learn if the species in Alaska are capable of surviving submergence too.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.