Western bumblebee

How to recognise it

A medium bumblebee, largely black with a yellow band on the thorax and a distinctly white tail covering the last abdominal segments. Colour form varies considerably across its range.

• Black with a yellow thoracic band and a white tail.
• Western North America, from Alaska south through the Rockies and Pacific states.
• Now scarce or absent from much of the Pacific coast where it was formerly abundant.
• Short-tongued, and a frequent nectar robber.

Body length 10–20 mm (typically 15 mm).

Biology

Social organisation

Eusocial, colony dies each autumn

A true colony with a queen and workers, but only for one season: everyone dies in autumn except mated new queens, who overwinter alone. Bumblebees.

Colony size at peak

50–300 individuals

Colony lifespan

One season.

Winter

Mated new queens overwinter alone in loose soil or litter.

Castes

Queen, Workers, Males, New queens

Nesting

Burrows in the ground; Existing cavities and tubes

Nests underground, usually in abandoned rodent burrows, and occasionally at the surface under debris.

Carries pollen

Corbicula (pollen basket) on the hind leg

On the wing

March to October

Lifecycle

1. 1EmergenceQueens emerge in spring, earlier at lower elevations.March to May.
2. 2Colony growthBuilds through spring and summer on a broad range of forage.
3. 3ReproductionReproductives from midsummer into autumn.

Where it lives and what it does

Habitat

Meadow, prairie, montane grassland, forest edge, farmland and urban areas across western North America.

Distribution

Formerly from Alaska to central California and east through the Rockies. Now largely absent from the Pacific coast portion of that range and reduced elsewhere.

Behaviour

A short-tongued generalist that was, until recently, one of the most abundant and useful wild pollinators in western North America. Its decline was rapid rather than gradual, which points away from habitat loss alone as an explanation.

Pollination role

Historically a major pollinator of cranberry, blueberry, clover, alfalfa and western montane flora. Its functional loss represents a substantial reduction in wild pollination capacity across the region.

High, and now absent from much of the range where it mattered.

What it forages on

A broad short-tongued generalist across western North American flora.

Honey

Keeps only a few days of stores

A few days' worth in wax pots.

Relationship with people

How people interact with it

Reared commercially for glasshouse pollination in the 1990s until the rearing operations collapsed from disease. The wild decline followed closely, and pathogen spillover — particularly of Nosema bombi — from those operations is among the leading explanations.

Conservation

Assessed as vulnerable or endangered across parts of its range and the subject of petitions for federal listing in the United States. The commercial rearing episode makes it one of the strongest cases anywhere for restricting the movement of reared bumblebee colonies.