Status under active review. USFWS has proposed listing Bombus occidentalis as federally endangered. As of this profile’s review date, the final listing determination has not been confirmed. Check USFWS ECOS for the current determination. This profile should be updated immediately upon final listing decision — review quarterly.
Overview
In the late 1990s, something went very wrong with the western bumble bee. A species that had been one of the most abundant and widely distributed bumble bees in North America — found from Alaska to California, across the Rocky Mountain West, common enough to be the species most people pictured when they thought of a bumble bee — simply disappeared from most of its range within a few years. The collapse was catastrophic and, by the time anyone was paying close enough attention to understand what was happening, largely complete.
The cause, established through subsequent research, was a pathogen. Commercial greenhouse operations in the early 1990s reared bumble bees for crop pollination services by removing wild-caught bumblebees, breeding them in captivity, and then releasing commercially raised bees back into the environment. The captive rearing operations allowed the microsporidian fungal parasite Nosema bombi to amplify to levels not seen in wild populations, and the commercially raised bees carrying high pathogen loads were released into the landscape where they mingled with wild western bumble bees and transmitted the disease. Wild populations had no resistance to the elevated pathogen load. They crashed.
The western bumble bee is now proposed for federal endangered listing, a determination that reflects a species reduced to a fraction of its historical abundance with no evidence of natural recovery from the pathogen-driven collapse of the 1990s. In Colorado, the species is still detected in mountain meadow habitats — the high-elevation subalpine and montane zones where patchier survey coverage may partly explain the records — but at densities that bear no resemblance to historical abundance.
Natural History
The western bumble bee (Bombus occidentalis) is a medium to large bumble bee — queens up to 22 millimeters in length, workers smaller. The characteristic coloration is yellow on the thorax (back), black on the abdomen with a white tail — though color pattern variation exists among populations and can complicate field identification. Like all social bumblebees, colonies are annual: a mated queen overwinters and founds a new colony each spring; workers are raised through summer; new reproductive males and queens emerge in late summer and mate; the colony dies with the first hard frost, and only mated queens survive to the following spring.
The western bumble bee historically occupied a broad elevation range — from near sea level to above 3,000 meters — and a broad habitat range within that elevation band: prairies, meadows, agricultural areas, forest edges, and the diverse wildflower-rich habitats of the mountain West. This generalist habitat use reflected the species’ role as a dominant bumble bee in western North American ecosystems.
As the primary host of the Suckley’s cuckoo bumble bee (Bombus suckleyi), the western bumble bee’s fate directly determines the fate of the cuckoo species. The cuckoo bee can only reproduce by parasitizing western bumble bee colonies. As western bumble bee populations collapse, the cuckoo bee’s ability to find host colonies collapses with them. The two species’ conservation stories are inseparable.
Habitat in Colorado
Historical records place the western bumble bee across Colorado from the eastern plains through the montane and subalpine zones — wherever there was sufficient diversity and abundance of flowering plants to support a colony through the growing season. Mountain meadows and the wildflower-rich subalpine zone were strongholds.
Current Colorado records cluster in higher-elevation habitats where the species is still detectable, though at greatly reduced density compared to historical records. The most consistent recent sightings come from subalpine meadow complexes in the San Juan, Sawatch, and other major ranges.
Survey effort for bumble bees in Colorado has been limited relative to the need. Absence from a location does not mean the species is absent; it may mean no one looked systematically. Citizen science surveys through Bumble Bee Watch have been the most consistent source of recent distribution data for the species in Colorado.
Threats
Disease — specifically the pathogen Nosema bombi — is the proximate driver of the species’ collapse and the ongoing constraint on recovery. The pathogen causes reduced colony growth, impaired worker foraging, and increased queen and worker mortality. Wild populations have not shown evidence of recovering natural resistance.
Commercial bumble bee industry pathogen spillover was the mechanism of the initial collapse and remains a potential source of continued pathogen pressure as commercial greenhouse bumble bee operations continue in and near agricultural landscapes where wild bees forage.
Habitat loss reduces the diverse, season-long floral resources that bumble bee colonies require to develop and maintain colony health. Agricultural intensification, invasive plant dominance in rangeland, and development have all reduced the diversity and abundance of native flowering plants in areas that historically supported western bumble bee colonies.
Pesticide use — particularly systemic neonicotinoid insecticides — impairs bumble bee navigation, foraging efficiency, and immune function at sublethal doses, reducing colony health and potentially increasing susceptibility to pathogens.
Climate change is shifting the phenological timing of both flowers and bumble bee colonies and is projected to reduce suitable habitat at lower elevations while pushing remaining populations to higher, cooler areas with limited room to move.
Recovery & Conservation
USFWS has proposed listing the western bumble bee as federally endangered. As of this profile’s review date, the final rule has not been confirmed and this profile uses federal_status: "proposed" accordingly. The proposed listing reflects the species’ severe population decline, ongoing threats, and lack of natural recovery after three decades.
Recovery for a species affected by a persistent pathogen is more complex than for species whose decline is driven by habitat loss alone. Eliminating pathogen exposure from commercial bumble bee operations would require regulatory changes to the rearing industry. Building resistance in wild populations is not achievable through conventional conservation management.
Habitat conservation — maintaining diverse, abundant native flowering plant communities in occupied and potential habitat — supports colony health and may increase resilience to pathogens by improving general colony condition. CPW’s Working Lands for Wildlife program addresses bumble bee habitat on agricultural land in Colorado.
What You Can Do
- Submit observations to Bumble Bee Watch (bumblebeewatch.org) and iNaturalist. Any bumble bee photograph from Colorado contributes to distribution data. For western bumble bee identifications, photograph from multiple angles — face, thorax, and abdomen — to document the coloration pattern. Expert review is available through both platforms.
- Plant native, diverse flowering plants that provide forage from early spring through fall. Single-season plantings are far less valuable than diverse native plantings with sequential bloom.
- Avoid neonicotinoid insecticides — imidacloprid, clothianidin, thiamethoxam — in gardens and on managed land. These systemic compounds persist in soil and nectar and affect bumble bee colony health at sublethal concentrations.
- Check USFWS ECOS quarterly to determine whether the proposed endangered listing has been finalized.
Sources
Species status, population data, and natural history drawn from:
- USFWS Proposed Rule for Bombus occidentalis (primary source for listing status)
- USFWS Species Status Assessment, Western Bumble Bee
- NatureServe Explorer: Bombus occidentalis (G2)
- Cameron et al. (2011): Patterns of widespread decline in North American bumble bees
- Xerces Society: Western Bumble Bee species profile
- Colorado Parks and Wildlife: Bumble Bee Conservation Program
Last reviewed: January 2024