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Field Notes

Covering the natural wonders of the Pacific Northwest

June 3, 2011 at 10:00 AM

Chilly spring confounds bumblebees

Fat bumblebees have been buzzing my backyard despite the drizzly cold, which made me wonder: Has our unseasonable weather had any effect on these “warm-blooded” insects?

The answer is yes, according to University of Washington biologist Sean O’Donnell, who has studied local bumblebee colonies and keeps an eye on what they’re up to.

This spring, O’Donnell is seeing a weird juxtaposition: Newly emerged queens foraging alongside mature workers from colonies that got an earlier start.

“There appears to be a very, very long period when queens are emerging from their hibernation sites and looking for new nests,” O’Donnell said.

Usually by now, most queens have long awakened from their winter slumber inside a rotting log or under an eve. Most have already scouted out an abandoned mouse burrow or other ground cavity and laid eggs that have hatched into worker bees. But this year, some queens are still rubbing the sleep out of their multifaceted eyes.

These tardy queens are the biggest, fattest bumblebees you’re likely to spot now, hovering above the ground like miniature helicopters in search of a nest site. The smaller bumblebees visiting flowers now are most likely the first batch of workers hatched from new nests.

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Bumblebees set up housekeeping in a chickadee nest in Bonney Lake (photo by Vicki Biltz)

As the colonies grow and are able to provide more pollen and nectar to nourish larvae, workers hatched later in the season will be bigger. Once a queen has cranked out sufficient workers to cover the task of food-gathering, she will never again leave the nest.

One of O’Donnell’s studies focused on the ingenious ways bumblebees regulate the temperature in their nests. When it’s too cold, certain workers vibrate their muscles to generate body heat and press their abdomens against the wax cells that hold larvae. When it’s too hot, another class of workers fans stands on their tip toes and fans the nest with their wings.

Bumblebees will sometimes take up residence in bird boxes, as Vicki Biltz, of Bonney Lake discovered. She first noticed bumblebees in her Bonney Lake yard in April. Now, the colony is well-ensconced in one of her chickadee houses. And she’s enjoying the show.

“It’s kind of exciting to watch them go in and out,” said Biltz, who shared pictures of her new companions.

A a few bumblebee tidbits too cool not to share:

  • Colony sizes range from a few dozen to 100 workers
  • Hair “baskets” on their hind legs allow workers to collect masses of pollen
  • Nectar is sucked into a “non-digestive” stomach, called a crop, then regurgitated to feed the young.
  • Larvae are protected in their sealed cells; Workers poke a hole for feeding, then close it up.

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A worker bumblebee visits a flower (photo by Erika Schultz)

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