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Montana Field Guide

Montana Field Guides

Western Bumble Bee - Bombus occidentalis

Native Species

Global Rank: G3
State Rank: SNR

Agency Status
USFS: Sensitive - Known in Forests (BD, BRT, KOOT)


External Links

General Description
Bees are insects in the order Hymenoptera. Bumble bees belong to the Apidae family. Genetic evidence supports Bombus occidentalis as a valid species of bumble bee (Williams et al. 2014). Two subspecies are described, but only B. o. occidentalis occurs in Montana.

Bumble bees are large, fuzzy, easily recognized bees that go through complete metamorphosis (larvae, pupae and adult stages). Color patterns can vary within species and sexes making identification more difficult and this is true for Bombus occidentalis. Females collect pollen in a broad, concave area on their hind legs called a corbicula or pollen basket. Bombus occidentalis are distinguished from other bumble bees in Wyoming by the white or sometimes tan tip at the end of their body on a usually dark abdomen.

They are primarily eusocial insects, meaning they form a colony with distinct castes: queen (19-21 mm length), worker (all female;13-17 mm length) and drone (males; 14-17 mm length).

Bumble bees are active throughout the growing season and colonies re-establish annually. Newly mated queens overwinter underground and emerge in spring to establish a nest. Queens are visible in the spring while they establish their nest and forage while the first generation develops into adults. When the colony is large enough (usually ~40 to 200 individuals), the queen will remain underground in the nest while worker bees forage. She will produce several generations of workers each summer. In the late summer and early fall, the queen lays eggs that develop into drones (males) and new queens. The new queens and drones leave the nest and mate, and the mated queens overwinter. The remainder of the colony, including the old queen, die at the end of the summer. Overwintered bumble bee queens are one of the first bees that emerge in early spring. New queens and drones are frequently the last bees seen in the fall.

Diagnostic Characteristics
Please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. Queens and workers differ from other Montana Bombus by having a cheek slightly shorter than wide, and white or yellow hairs on abdominal T5 (and usually T4); T1 and T6 are always black, T2 usually black (at least on the posterior half), T3 may be black or yellow. Yellow-banded Bumble Bee, (B. terricola) always has yellow hairs on T2 and T3 (Koch et al. 2012).

For definitions and diagrams of bumble bee morphology please see the Montana State Entomology Collection's Key to Female Bumble Bees in Montana. Medium sized and short-tongued: queens 20-21 mm, workers 9-15 mm. Head short, cheek slightly shorter than broad; oceli on a line between the back (top) edges of the compound eyes (not below the line), hindleg tibia flat on outer surface and lacking long hair, but with long fringe on sides forming a pollen basket. Hair moderately short and even; upper surface of the thorax with at least a large central black spot (often a black band) between the wings. Abdominal T1 and T6 always black, T2 usually black anteriorly, but if predominantly yellow then head and thorax also predominantly yellow. If T2 and T3 entirely black then T4 and T5 completely or extensively white or yellow-orange. Even darkest individuals with white on tail have some grayish hairs on face and dorsal surface of head. Males 12-16 mm, hair color pattern similar to queens and workers, antenna short, flagellum just over 2x longer than the scape (Williams et al. 2014).

Species Range
Resident Year Round

Recorded Montana Distribution

Click the map for additional distribution information.
Distributional Information Provided in Collaboration with the
Montana Entomology Collection at Montana State University


Range Comments
Mountain West from the Pacific Coast to the western Great Plains. Throughout the Cascades, Sierra Nevada, and Rocky Mountains from Alaska to southern California, east in the north to the northwestern Great Plains of Saskatchewan and Montana, and in the south to eastern Wyoming, Colorado, and New Mexico. Most abundant below 3000 m elevation in Colorado but ranged to 4300 m (Macior 1974). Also, in the Black Hills of southwestern South Dakota and pine-forested ridges of adjacent northwestern Nebraska. Some populations, particularly in the Palouse Prairie region of Washington and Idaho and west of the Sierra Nevada and Cascades have declined dramatically since the 1990's or earlier (Cameron et al. 2011, Hatten et al. 2013, and Williams et al. 2014). Western bumble bees are less common in the western part of their range in recent years (Graves et al. 2020) and their habitat is predicted to continue to decline (Janousek et al. 2023).

Found in open grassy areas, prairie, urban parks and gardens, sagebrush steppe, mountain meadows to alpine tundra (Macior 1974, Bauer 1983, Bowers 1985, Tommasi et al. 2004, Cook et al. 2011, Hatten et al. 2013, and Williams et al. 2014). Also found in commercial highbush blueberry and cranberry fields in southern British Columbia (Ratti et al. 2008). Nests in southern Alberta constructed in woods, open meadows, and most often in the woods-meadows ecotone (Richards 1978). In Wyoming, they have observed them in urban and forested areas, but seldom in sagebrush steppe ecosystem (Bell 2019). They are well suited to higher elevations due to their ability to generate body heat by vibrating their flight muscles and their insulating fur.
Predicted Suitable Habitat Model

This species has a Predicted Suitable Habitat Model available.

To learn how these Models were created see

Food Habits
A generalist forager. A short tongue requires it to often rob nectar from flowers it visits (Pyke et al. 2012). Flowers visited include Columbia Monkshood, (Aconitum columbianum), Allium, Arnica, Astragalus, Balsamorhiza, Brassica,Calypso, Castilleja, Ceanothus, Centaurea, Tweedy Snowlover, (Chionophila tweedyi), Chrysothamnus, Cirsium, Clematis, Corydalis, Delphinium, Dianthus, Dodecatheon, Epilobium, Erigeron , Eriogonum, Erysimum,
Frasera, Geranium, Grindelia, Haplopappus
, Cow-parsnip, (Heracleum), Ipomopsis, Iris, Lathyrus, Ligusticum, Linaria, Lotus, Lupinus, Malus, Medicago,
Melilotus, Mentha, Mertensia
, Nama, (Nama densum), Soft-hairy False Gromwell, (Onosmodium mole), Oregano, (Origanum), Orthocarpus, Oxytropis, Pedicularis, Penstemon, Phacelia, Polygonum, Potentilla, Prunus, Raphanus, Rhododendron, Ribes, Rosa, Rubus, Salix, Salvia, Sedum, Senecio, Sisyrinchium, Solidago, Symphoricarpos, Tanacetum, Taraxacum, Thermopsis, Trifolium, Vaccinium, Vicia and Corn, (Zea) (Hobbs 1968, Beattie et al. 1973, Macior 1974, Ackerman 1981, Bauer 1983, Thorp et al. 1983, Mayer et al. 2000, Rao and Stephen 2007, Ratti et al. 2008, Wilson et al. 2010, Colla and Ratti 2010, Koch and Strange 2012, Koch et al. 2012, Pyke et al. 2012, Williams et al. 2014, and Miller-Struttmann and Galen 2014). May be involved in pollinating the Fairy Slipper, (Calypso bulbosa) in our region (Ackerman 1981). Visits commercial highbush blueberry and cranberry (Vaccinium) fields in British Columbia (Ratti et al. 2008, and Colla and Ratti 2010), and prefers to visit flowers of alfalfa over sweetclover where both are available (Hobbs 1968).

Reproductive Characteristics
This species nests underground. In southern Alberta, nest initiation spans mid-May to mid-June (Richards 1978). First broods average 8-9 eggs, larvae, and pupae, second and third broods about half that size (Hobbs 1968). One California colony in July contained five young queens and more than 80 workers (Plath 1934). Males patrol regular circuits in search of queens (Williams et al. 2014). Nests and broods destroyed by parasitic bees, including Suckley Cuckoo Bumble Bee, (Bombus suckleyi). One Alberta study reported Bombus suckleyi depredated 80% of 15 B. occidentalis nests (Hobbs 1968).

On March 16, 2016, the U.S. Fish and Wildlife Service published a notice in the Federal Register indicating that, "Based on our review of the petition and sources cited in the petition, we find that the petition presents substantial scientific or commercial information indicating that listing the western bumble bee (Bombus occidentalis) may be warranted". Completion of status review is expected in 2021. Additional information on the species' management can be found on the U.S. Fish and Wildlife Service's Species Account.

  • Literature Cited AboveLegend:   View Online Publication
    • Ackerman, J.D. 1981. Pollination biology of Calypso bulbosa var. occidentalis (Orchidaceae): a food-deception system. Madroño 28(3): 101-110.
    • Bauer, P.J. 1983. Bumblebee pollination relationships on the Beartooth Plateau tundra of Southern Montana. American Journal of Botany. 70(1): 134-144.
    • Beattie, A.J., D.E. Breedlove, and P.R. Ehrlich. 1973. The ecology of the pollinators and predators of Frasera speciosa. Ecology 54: 81-91.
    • Bell, C. 2019. Sampling methods and distribution modeling of bees: the status of the western bumble bee (Bombus occidentalis) in Wyoming. M.Sc. Thesis. Laramie, WY: University of Wyoming. 51 p.
    • Bowers, M.A. 1985. Bumble bee colonization, extinction, and reproduction in subalpine meadows in northeastern Utah. Ecology 66(3): 914-927.
    • Cameron, S.A., J.D. Lozier, J.P. Strange, J.B. Koch, N. Cordes, L.F. Solter, and T.L. Griswold. 2011. Patterns of widespread decline in North American bumble bees. Proceedings of the National Academy of Sciences 108(2): 662-667.
    • Colla, S.R. and C.M. Ratti. 2010. Evidence for the decline of the western bumble bee (Bombus occidentalis Greene) in British Columbia. Pan-Pacific Entomologist 86(2): 32-34.
    • Cook, S.P., S.A. Birch, F.W. Merickel, C.C. Lowe, and D. Page-Dumroese. 2011. Bumble bee (Hymenoptera: Apidae) community structure on two sagebrush steppe sites in southern Idaho. Pan-Pacific Entomologist 87(3): 161-171.
    • Graves, T.A., W.M. Janousek, S.M. Gaulke, A.C. Nicholas, D.A. Keinath, and C.M. Bell. 2020. Western bumble bee: declines in the continental United States and range-wide information gaps. Ecosphere 11(6):e03141.
    • Hatten, T.D., C. Looney, J.P. Stange, and N.A. Bosque-Pérez. 2013. Bumble bee fauna of Palouse Prairie: survey of native bee pollinators in a fragmented ecosystem. Journal of Insect Science 13(26): 1-19.
    • Hobbs, G.A. 1968. Ecology of species of Bombus (Hymenoptera: Apidae) in southern Alberta. VII. Subgenus Bombus. Canadian Entomologist 100(2): 156-164.
    • Janousek, W.M., et al. 2023. Recent and future declines of a historically widespread pollinator linked to climate, land cover, and pesticides. PNAS 120(5):1-9.
    • Koch, J., J. Strange, and P. Williams. 2012. Bumble bees of the western United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 143 p.
    • Koch, J.B. and J.P. Strange. 2012. The status of Bombus occidentalis and B. moderatus in Alaska with special focus on Nosema bombi incidence. Northwest Science 86:212-220.
    • Macior, L.M. 1974. Pollination ecology of the Front Range of the Colorado Rocky Mountains. Melanderia 15: 1-59.
    • Mayer, D.F., E.R. Miliczky, B.F. Finnigan, and C.A. Johnson. 2000. The bee fauna (Hymenoptera: Apoidea) of southeastern Washington. Journal of the Entomological Society of British Columbia 97: 25-31.
    • Miller-Struttmann, N.E. and C. Galen. 2014. High-altitude multi-taskers: bumble bee food plant use broadens along an altitudinal productivity gradient. Oecologia 176:1033-1045.
    • Montana State University. 2023. Bumble bees of Montana. Montana Entomology Collection. Bozeman, MT. Accessed 20 November 2023.
    • Plath, O.E. 1934. Bumblebees and their ways. New York, NY: Macmillan Company. 201 p.
    • Pyke, G.H., D.W. Inouye, and J.D. Thomson. 2012. Local geographic distributions of bumble bees near Crested Butte, Colorado: competition and community structure revisited. Environmental Entomology 41(6): 1332-1349.
    • Rao, S. and W.P. Stephen. 2007. Bombus (Bombus) occidentalis (Hymenoptera: Apidae): In decline or recovery. Pan-Pacific Entomoligist 83(4): 360-362.
    • Ratti, C.M., H.A. Higo, T.L. Griswold, and M.L. Winston. 2008. Bumble bees influence berry size in comercial Vaccinium spp. cultivation in British Columbia. Canadian Entomologist 140(3): 348-363.
    • Richards, K.W. 1978. Nest site selection by bumble bees (Hymenoptera: Apidae) in southern Alberta. Canadian Entomologist 110(3): 301-318.
    • Thorp, R.W., D.S. Horning, and L.L. Dunning. 1983. Bumble bees and cuckoo bumble bees of California (Hymenoptera: Apidae). Bulletin of the California Insect Survey 23:1-79.
    • Tommasi, D.A., A. Miro, H.A. Higo, and M.L. Winston. 2004. Bee diversity and abundance in an urban setting. Canadian Entomologist 136(6): 851-869.
    • Williams, P., R. Thorp, L. Richardson, and S. Colla. 2014. Bumble Bees of North America. Princeton, NJ: Princeton University Press. 208 p.
    • Wilson, J.S., L.E. Wilson, L.D. Loftis, and T. Griswold. 2010. The montane bee fauna of north central Washington, USA, with floral associations. Western North American Naturalist 70(2): 198-207.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Burkle L.A., M.P. Simanonok, J.S. Durney, J.A. Myers, and R.T. Belote. 2019. Wildfires influence abundance, diversity, and intraspecific and interspecific trait variation of native bees and flowering plants across burned and unburned landscapes. Frontiers in Ecology and Evolution 7(252):1-14.
    • Cullen, M.G., L.J. Thompson, J.C. Carolan, J.C. Stout, and D.A. Stanley. 2019. Fungicides, herbicides and bees: a systematic review of existing research and methods. PLoS ONE 14(12): e0225743.
    • Delphia, C.M., Griswold, T., Reese, E.G., O'Neill, K.M., and Burkle, L.A. 2019. Checklist of bees (Hymenoptera: Apoidea) from small, diversified vegetable farms in south-western Montana. Biodiversity Data Journal: e30062
    • Dolan, A.C. 2016. Insects associated with Montana's huckleberry (Ericaceae: Vaccinium globulare) plants and the bumble bees (Hymenoptera: Apidae) of Montana. M.Sc. Thesis. Bozeman, MT: Montana State University. 160 p.
    • Dolan, A.C., C.M. Delphia, K.M. O'Neill, and M.A. Ivie. 2017. Bumble Bees (Hymenoptera: Apidae) of Montana. Annals of the Entomological Society of America. 110(2): 129-144.
    • Fultz, J.E. 2005. Effects of shelterwood management on flower-visiting insects and their floral resources. M.Sc. Thesis. Bozeman, MT: Montana State University. 163 p.
    • Goulson, D., E. Nicholls, C. Botias, and E.L. Rotheray. 2015. Bee declines driven by combined stress from parasites, pesticides, and lack of flowers. Science 347(6229):1435-1436.
    • Kearns, C.A. and J.D. Thomson. 2001. The Natural History of Bumble Bees. Boulder, CO. University Press of Colorado.
    • Park, M.G., E.J. Blitzer, J. Gibbs, J.E. Losey, and B.N. Danforth. 2015. Negative effects of pesticides on wild bee communities can be buffered by landscape context. Proc. R. Soc. B 282:20150299.
    • Reese, E.G., L.A. Burkle, C.M. Delphia, and T. Griswold. 2018. A list of bees from three locations in the Northern Rockies Ecoregion (NRE) of western Montana. Biodiversity Data Journal 6: e27161.
    • Simanonok, M.P. and L.A. Burkle. 2019. Nesting success of wood-cavity-nesting bees declines with increasing time since wildfire. Ecology and Evolution 9:12436-12445.
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Western Bumble Bee — Bombus occidentalis.  Montana Field Guide.  .  Retrieved on , from