Bombus occidentalis

Greene, 1858

Western bumble bee, white-bottomed bee

Bombus occidentalis, the western , is a medium to large bumble bee native to western North America. Once among the most common bumble bees in its range, it has experienced severe declines since the late 1990s, with losses exceeding 40% in some areas and near-disappearance from parts of California, Oregon, and Washington. The species was listed as endangered under the California Act in 2019. It is a forager and effective of crops including tomatoes, peppers, and berries through buzz pollination.

Bombus occidentalis by (c) USGS Bee Inventory and Monitoring Lab, some rights reserved (CC BY). Used under a CC-BY license.

Bombus (Bombus) occidentalis by the Smithsonian. Used under a CC0 license.

Pronunciation

How to pronounce Bombus occidentalis: /ˈbɒmbəs ˌɒksɪˈdɛntælɪs/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Distinguished from similar by abdominal coloration patterns, particularly the variable presence of yellow, black, whitish, and reddish-brown hair on segments T1-T6. The 'white-bottomed' refers to the pale markings on the lower abdominal segments in the northern color variant. Males are distinguished from females by counting abdominal segments (seven vs. six) or antennal segments (13 vs. 12), and by the absence of on hind legs. evidence confirms species status separate from the closely related Bombus terricola.

Images

Appearance

exhibit three distinct color variations across their range. The first variation, found from northern California to British Columbia and east to Montana, features yellow hair on the , black hair on the basal fourth abdominal segment with whitish lower margins on the fourth and fifth segments, sparse whitish to black hairs on the sixth abdominal segment, and an entirely black . The second variation, found along the central California coast, has yellow hair on the sides of the second abdominal segment and all of the third, with reddish-brown hair on the fifth segment. The third variation, from the Rocky Mountains to Alaska, shows yellow hair on the thorax behind the wings and on the rear of the second and all of the third abdominal segments. Queens and workers have six abdominal segments; males have seven. Females possess 12-segmented and on widened hind legs, while males have 13-segmented antennae, no stinger, and lack pollen baskets.

Habitat

Nests underground in pre-existing cavities or burrows, particularly those abandoned by rodents or other animals. Forages in diverse from Mediterranean California to tundra regions of Alaska, including alpine meadows, montane forests, and agricultural landscapes. Requires suitable nest sites with adequate insulation and proximity to floral resources.

Distribution

Western North America from Mediterranean California north to Alaska, and east to the Rocky Mountains. Historically widespread across northern California, Oregon, Washington, Idaho, Montana, Wyoming, Utah, Colorado, northern Arizona, New Mexico, western Nebraska, western North and South Dakota, and British Columbia. Range has contracted approximately 20% since the 1990s, with severe declines and near-disappearance from southern British Columbia to central California.

Seasonality

Queens emerge in early spring to establish colonies. appear after the first develops. period extends from early February to late November in some areas. Males appear in late summer and early fall. Colony cycle ends with winter hibernation of new queens ().

Diet

forager collecting nectar and pollen from diverse flowering plants. Nectar provides ; pollen provides protein. store nectar in wax pots and pack pollen into corbiculae () on hind legs. Has been observed foraging on manzanita, lupines, penstemons, clovers, sages, wild lilacs, wild buckwheats, and various crops.

Life Cycle

eusocial colony cycle initiated by solitary in early spring. Queen finds nest site, constructs wax structures, and provisions first . First emerge and assume foraging, defense, and larval feeding duties. Queen then specializes in -laying. Colony may reach approximately 1,600 workers. In fall, reproductive males and new queens () are produced. Old queen, workers, and males die with winter onset; gynes hibernate to initiate colonies the following spring.

Behavior

Performs buzz pollination: grasps pollen-producing flower structures in and vibrates wing musculature to dislodge pollen from anthers. Exhibits foraging activation: successful foragers stimulate nestmates to forage, with recruitment strength correlated to nectar quality. Returning foragers may communicate rewarding floral through scent cues. Short-tongued individuals sometimes practice nectar robbing by cutting floral tissue with toothed mandibles to access nectar without contacting reproductive structures. Queens recognize own nest and through colony-specific chemical cues; upon encountering foreign brood, queens inspect more extensively and may adopt rather than destroy unrelated colonies. deposit colony-specific odor marks at nest entrances using labial gland secretions, learned during first orientation , to facilitate homing and prevent nest usurpation errors.

Ecological Role

Important of wild flowering plants and agricultural crops, particularly those requiring buzz pollination such as tomatoes, peppers, cranberries, and blueberries. foraging enables pollination across diverse plant . Serves as for parasitic cuckoo bumble bees (Psithyrus).

Human Relevance

Formerly among the most commercially reared for greenhouse crop pollination, including alfalfa, avocados, apples, cherries, blackberries, cranberries, and blueberries. Commercial rearing ceased due to declines and concerns. Listed as endangered under the California Act in 2019. Population declines attributed to introduced (particularly Nosema bombi) from commercial bumble bee trade, loss and fragmentation, , plants, and climate change.

Similar Taxa

Bombus terricolaClosely related formerly considered ; distinguished by evidence and subtle morphological differences
Bombus vosnesenskiiYellow-faced bumble bee with overlapping range; B. occidentalis distinguished by variable abdominal coloration patterns and 'white-bottomed' appearance in northern variants
Bombus melanopygusBlack-tailed bumble bee with overlapping range; distinguished by consistent black tail versus variable color patterns in B. occidentalis

More Details

Population Decline and Conservation Status

declined approximately 40% between 1998 and 2008, with range contraction of about 20%. Decline linked to commercial rearing practices in the 1990s, when B. occidentalis and B. impatiens were shipped to European facilities and returned, potentially introducing including Nosema bombi. Four closely related in subgenus Bombus sensu stricto (B. occidentalis, B. franklini, B. affinis, B. terricola) have all declined since the 1990s. Climate change poses continuing threat; B. occidentalis is a cold-adapted species showing declines as temperatures rise.

Brain Plasticity Research

B. occidentalis has been used as a model organism for studying neural plasticity because its division of labor is based on body size rather than age, allowing researchers to dissociate age and experience effects. Foraging experience, not age, correlates with increased mushroom body size, particularly the medial calyx.