Bombus occidentalis Greene 1858

The western bumble bee (Bombus occidentalis) is named after its historic range; it was once common in 14 western US states and four Canadian provinces. Since the late 1990's this bee has undergone a dramatic decline in the western part of its range and populations are currently found only in the northern and eastern portions of its range. The rapid decline is believed to be caused by an introduced pathogen, possibly Nosema bombi, which was likely picked up by queens sent to European rearing facilities and then sent back to the United States in the early 1990's. It is hypothesized that after the return of these newly infected bees, the disease then spread to wild populations of western bumble bees, Franklin's bumble bees (B. franklini), rusty patched bumble bees (B. affinis), and yellowbanded bumble bees (B. terricola). A decline in each of these species was noticed in the late 1990's. Additional threats to bumble bees include other pests and diseases, habitat destruction, pesticides, invasive species, natural pests or predators, and climate change.

The western bumble bee has several color variations. One variant found from northern California, north to British Columbia and east to southwest Saskatchewan and to Montana has yellow hair on the front part of its thorax. The abdominal segments have black hair, with the lower abdomen being whitish colored. The head is entirely black. Another color variant occurs along the central coast in California and differs from the first in that it has yellow hairs on the sides of the abdomen and reddish brown hair lower on the abdomen. The third common variant occurs in the Rocky Mountains to Alaska and differs from the first in that it has yellow hairs behind the wings and in the middle of the abdomen. In all variants, males and females have similar coloring except that males have pale yellowish hair on the front of the face and the top of the head, with a few black hairs on the sides.

The western bumble bee is an important pollinator of both crops and wildflowers, and has been reared commercially for pollination of greenhouse tomatoes and field berry crops in the western United States. It also pollinates other crops such as alfalfa, avocado, apple, cranberry, blackberry, blueberry, and cherry. Additionally, the western bumble bee pollinates many wildflowers, such as aster (Aster spp.), cosmos (Cosmos spp.), mountain lupine (Lupinus monticola), willow (Salix spp.), goldenrod (Solidago spp.), and clover (Trifolium spp.).