Western bumble bee (Bombus occidentalis)

Close up of a western bumble bee on a plant.
A western bumble bee (Pacific Northwest Bumble Bee Atlas Gallery)
Category: Arthropods
State status: Candidate
Vulnerability to climate change (More details)


If you see this species, please share your observation using the WDFW wildlife reporting form. Providing detailed information such as a photo and exact coordinates will improve the confidence and value of this observation to WDFW species conservation and management.

The population size of the western bumble bee is low and the trend is declining. Bumble bees have recently become the focus of conservation concern and efforts due to their precipitous population declines and prodigious capabilities as pollinators. In a recent status assessment, International Union of Conservation for Nature identified three Washington species as facing high or extremely high risk of extinction: western bumble bee and Morrison’s bumble bee were ranked Vulnerable, and Suckley cuckoo bumble bee was ranked Critically Endangered.

Description and Range

Physical description

The western bumble bee is classified within the Bombus subgenus. See the Pacific Northwest Bumble Bee Atlas for a detailed description of this bumble bee species.

Ecology and life history

Bombus subgenus species live in small, highly social and interdependent colonies with structured roles: egg-laying females (queens), foraging and nesting females (workers), and males. 

Bumble bees depend on habitats with rich floral resources throughout the nesting season, and many species select specific suites of plants for obtaining nectar and pollen. They also select flowers based on their structure and the bee’s tongue length. Bumble bees require above and below-ground micro-sites for overwintering and nesting, including logs, stumps, and abandoned rodent and ground-nesting bird nests.

Close up of a western bumble bee on light pink milkweed flowers.
A western bumble bee explores milkweed flowers for nectar. born1945 - Creative Commons Zero

Bumble bees are adaptable; they do not require native vegetation. However, intensive agricultural development has been shown to result in regional bumble bee declines. Although habitat loss and insecticide use have played a role in bumble bee declines, their rapid and widespread declines even from apparently high quality habitats support the current prevailing hypothesis that pathogens introduced into the wild from commercial bumble bee facilities are the main factor in declines.

Bumble bee colonies are annual. In late-winter or early-spring, queens, which are the sole survivors from the previous year, emerge from their overwintering sites to feed on floral nectar, collect pollen, and search for suitable nest sites, which are often abandoned rodent holes. Bombus subgenus queens lay eggs in their individual nests and gather nectar and pollen to feed their first brood of workers. In the nest, eggs develop into larvae and then spin cocoons in which they pupate.

Once they emerge from their cocoons, the workers then take over tending and provisioning young, while the queen continues to lay eggs, and typically no longer leaves the nest.

Late in the season, the colony produces males and new queens which mate. Males, workers, and old queens eventually die; only the newly mated queens are capable of surviving through winter.

Bumble bees are key generalist pollinators of native plants and agricultural crops. Through their foraging and collection of nectar and pollen they physically transfer the latter between plants, allowing them to reproduce.

The bumble bees' unique behavior of “buzz pollination”, in which they grab onto and strongly shake an entire flower by vibrating their powerful wing muscles, results in large amounts of pollen being released and produces a more complete fruit set than other pollinators, including honey bees.

Geographic range

The western bumble bee species historically occurred in healthy populations across large geographic areas. This species was historically common in the western United States and Canada: western South Dakota south to northern New Mexico west to northern California and north to southern Alaska.

Recent surveys reveal significant declines in their numbers, distribution, and ranges. Surveys have located only a handful of populations in Washington, primarily in remote subalpine and montane sites. A 28% reduction was estimated in detected range area in a recent study, and western bumble bee was found largely absent from the western portion of its range (including Washington). Over the past decade, relative abundance of western bumble bee populations is estimated to have declined approximately 50%, while Washington has experienced even greater decline. 

Additional surveys are needed to determine the location and number of extant Washington populations of western bumble bee.

For maps of the geographic range and conservation status of the western bumble bee and other information, check out NatureServe Explorer and the International Union for Conservation of Nature Red List.

Climate vulnerability

Sensitivity to climate change


Climate sensitivity of the western bumble bee (WBB) is likely driven by temperature increases, reduced snowpack, earlier snowmelt, later first frost, increased wildfire, drought, and increased frequency of temperature and precipitation extremes. In Washington, WBB is extant in several higher elevation sites where temperature increases, reduced snowpack, earlier snowmelt, and later first frost will likely disrupt the timing of the bumble bee's life cycle, sometimes with lethal effects; result in phenological asynchrony between the bee and key forage plants; and may eventually change the flowering plant species make-up. WBB has also been recently documented in the Palouse Prairie, an ecological system assessed as moderately vulnerable to climate change with a high confidence in the Washington State Wildlife Action Plan (SWAP). Temperatures may also affect the distribution of this species, as it appears to prefer cooler environments. Increasing fire frequencies may help maintain bumble bee foraging habitat by preventing conifer encroachment on meadows with abundant flowers, but will also likely be lethal to bumble bee colonies.

In general, bumblebees are likely sensitive to climate-driven changes in nesting, foraging, and overwintering habitat, but detailed information is currently lacking. Shifts in temperature, precipitation, and snowpack may affect bumblebee distribution and life history, potentially forcing them into unfavorable habitats, to emerge at non-optimal times (i.e., mismatch with vegetation), and/or affecting energy demands during overwintering periods. These climate-driven changes may also affect habitat quality and availability. One of the primary concerns for bumblebee species is a shift in the abundance, distribution, and/or phenological synchrony of key forage flowering vegetation, as pollen and nectar availability influences reproduction and overwintering success of queens.

Confidence: Moderate

Exposure to climate change


  • Increased temperatures
  • Reduced snowpack
  • Earlier snowmelt
  • Later first frost
  • Altered fire regimes
  • Increased frequency of temperature and precipitation extremes
  • Drought
Confidence: Moderate

How to

Check out Habitat At Home to learn ways to attract pollinators in your own backyard, such as planting pesticide-free and native plants for butterflies and bees on your landscape or in a container garden; and see how to a craft a bee hotel! Habitat at Home, formerly known as the Backyard Wildlife Sanctuary Program, is the Washington Department of Fish and Wildlife's effort to encourage Washingtonians to connect with nature where they live.


This species is identified as a Species of Greatest Conservation Need (SGCN) under the State Wildlife Action Plan (SWAP). SGCN-classified species include both those with and without legal protection status under the Federal or State Endangered Species programs, as well as game species with low populations. The WDFW SWAP is part of a nationwide effort by all 50 states and five U.S. territories to develop conservation action plans for fish, wildlife and their natural habitats—identifying opportunities for species' recovery before they are imperiled and more limited.

Conservation Threats and Actions Needed

  • Agricultural and aquaculture side effects
    • Threat: Importation of bumble bees for use in pollination of commercial crops introduces pathogens into the wild.
    • Action Needed: Review of federal/state policies that allow translocation and establishment of commercially-reared bumble bees in North America.
  • Resource information collection needs
    • Threat: Lack of data on current status and distribution.
    • Action Needed: Determine distribution and population status.

The habitats of this species must also be protected from insecticides.

See the Climate vulnerability section for information about the threats posed by climate change to this species. Climate vulnerability is a way to assess the degree to which a habitat or species is susceptible to, and unable to cope with adverse impacts of climate change.

Our Conservation Efforts

The Western States Bumble Bee Atlas strives for conservation across four western states
The Washington Department of Fish and Wildlife is taking part in an initiative that unites Oregon, Washington, Idaho, and California in a conservation effort dedicated to understanding and protecting bumble bees. The Western States Bumble Bee Atlas will establish a regionwide network to gather data to provide a better understanding of bumble bee distributions and habitat needs. This project builds on work done since 2018 by the Pacific Northwest Bumble Bee Atlas, a collaboration between state agencies and conservation organizations in Oregon, Washington, and Idaho. Fueled by volunteer community scientists, the atlas gathers data throughout the three-state area that helps shape conservation work. The project is funded by the U.S. Fish and Wildlife Service’s Wildlife and Sportfish Restoration program through a competitive State Wildlife Grant, the U.S. Forest Service, the Bureau of Land Management, and the Oregon Conservation and Recreation Fund. Now, California is joining this effort, extending the geographic area covered to almost 15% of the contiguous United States. Nearly 60% of North America’s bumble bee species are known to occur in this region, underscoring the importance of this initiative. 



Bumble Bee Watch. 2014. Available: http://bumblebeewatch.org/ Accessed November 3, 2014.

Cameron, S., J. Lozier, J. Strange, J. Koch, N. Cordes, L. Solter, and T. Griswold. 2011. Patterns of widespread decline in North American bumble bees. Proceedings of the National Academy of Sciences 108:662–667.

Hatfield, R., S. Colla, S. Jepsen, L. Richardson, and R. Thorp. 2014. IUCN assessments for North American Bombus spp. for the North American IUCN bumble bee specialist group. The Xerces Society for Invertebrate Conservation. Portland, OR.

Schweitzer, D., N. Capuano, B. Young, and S. Colla. 2012. Conservation and management of North American bumble bees. NatureServe, Arlington, Virginia, and USDA Forest Service, Washington, D.C.

Thorp, R., D. Horning, Jr, and L. Dunning. 1983. Bumble bees and cuckoo bumble bees of California (Hymenoptera: Apidae). Bulletin of the California Insect Survey: Vol. 23. University of CA Press. Berkley and Los Angeles.

Williams, P. H., S. R. Colla, and Z. Xie. 2009. Bumblebee vulnerability: common correlates of winners and losers across three continents. Conservation Biology 23(4):931-940.

Wilson, J., L. Wilson, L. Loftis, and T. Griswold. 2010. The montane bee fauna of north central Washington, USA, with floral associations. W North American Naturalist 70(2):198-207.

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