Bombus occidentalis - Greene, 1858
Western Bumble Bee
Synonym(s): Bombus (Bombus) occidentalis Greene, 1858 ;Bombus proximus Cresson, 1863 ;Bombus terricola occidentalis (Nylander, 1848)
Taxonomic Status: Accepted
Related ITIS Name(s): Bombus occidentalis Greene, 1858 (TSN 714827)
Unique Identifier: ELEMENT_GLOBAL.2.806561
Element Code: IIHYM24250
Informal Taxonomy: Animals, Invertebrates - Insects - Bumble Bees
 
Kingdom Phylum Class Order Family Genus
Animalia Mandibulata Insecta Hymenoptera Apidae Bombus
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Concept Reference
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Concept Reference: Thorp, R. W., and M. D. Shepherd. 2005. Profile: Subgenus Bombus. In Shepherd, M. D., D. M. Vaughan, and S. H. Black (Eds). Red List of Pollinator Insects of North America. CD-ROM Version 1 (May 2005). Portland, OR: The Xerces Society for Invertebrate Conservation.
Concept Reference Code: N05THO01EHUS
Name Used in Concept Reference: Bombus (Bombus) occidentalis
Taxonomic Comments: Subgenus: Bombus. B. occidentalis was previously considered to be conspecific with B. terricola (e.g. Williams, 2008, Milliron, 1971). A large-scale mitochondrial DNA analysis by Williams et al. (2012) supports B. occidentalis as a species separate from B. terricola. Within it, there are two distinct and apparently disjunct groups: a northern group with longer pile (subspecies mckayi, described as a separate species by Ashmead 1902) and a southern group with shorter pile (subspecies occidentalis, which includes the taxon proximus, described previously as a separate species).
Conservation Status
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NatureServe Status

Global Status: G4
Global Status Last Reviewed: 08Apr2018
Global Status Last Changed: 17Jul2014
Ranking Methodology Used: Ranked by calculator
Rounded Global Status: G4 - Apparently Secure
Reasons: Bombus occidentalis is currently understood to include two subspecies, B. o. mckayi and B. o. occidentalis. Subspecies mckayi has a moderate range, area of occupancy and number of occurrences are stable or slightly decreasing, but subspecies occidentalis has undergone drastic declines. Overall, the species appears to be secure in about 25-30% of its entire range.
Nation: United States
National Status: N2N3 (15Jan2011)
Nation: Canada
National Status: N3N5 (22Jun2017)

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Alaska (S4), Arizona (S2?), California (S1), Colorado (SNR), Idaho (S3), Montana (SNR), Nebraska (SNR), Nevada (SNR), New Mexico (SNR), North Dakota (SNR), Oregon (S1S2), South Dakota (SNR), Utah (SNR), Washington (S2S3), Wyoming (SNR)
Canada Alberta (S3), British Columbia (S2S4), Northwest Territories (S2S3), Saskatchewan (S3S5), Yukon Territory (S3S4)

Other Statuses

Implied Status under the Committee on the Status of Endangered Wildlife in Canada (COSEWIC):T,SC
Comments on COSEWIC: Subspecies occidentalis is designated Threatened and subspecies mckayi is designated Special Concern.
IUCN Red List Category: VU - Vulnerable

NatureServe Global Conservation Status Factors

Range Extent: >2,500,000 square km (greater than 1,000,000 square miles)
Range Extent Comments: Bombus occidentalis is currently understood to include two subspecies, B. o. mckayi and B. o. occidentalis. The full range of the species includes much of western North America, ranging from the tundra region in Alaska and Yukon south along the west coast to southern British Columbia to central California, Arizona and New Mexico and east into southern Saskatchewan and northwestern Great Plains (Williams et al. 2014).

The historical range extent of this species is estimated at 9,448,511 km², and the extent of current (2008-2017) observations is 5,518,376 km². Controlling for larger historical sample size, the range of this species has declined by an estimated 19%.

Area of Occupancy: 501-2,500 4-km2 grid cells
Area of Occupancy Comments: Considering all available historical data, the historical (i.e., observations before 2008) Area of Occupancy for this subspecies was 10,468 km². The recent period (2008-2017) AOO is 2,592 km²; after controlling for the larger historical sample size, this represents an increase 56% in AOO.

Number of Occurrences: > 300
Number of Occurrences Comments: In the recent period (2008-2017), 420 occurrences (i.e., observations separated from others by >5 km) of this subspecies have been reported, a 55% decline over historical occurrences when controlling for larger historical sample size.

Population Size: Unknown
Population Size Comments: It is not possible to estimate this parameter for bumble bee populations.

Number of Occurrences with Good Viability/Integrity: Unknown

Overall Threat Impact: High
Overall Threat Impact Comments: The following is largely adapted from Hatfield et al 2015 and largely apply to the subspecies B. o. occidentalis; threats for subspecies B. o. mckayi, which is not suffering the same declines are not well-understood.

Pathogen spillover: Populations of this declining species have been associated with higher levels of the microsporidian Nosema bombi and reduced genetic diversity relative to populations of co-occurring stable species (Cameron et al. 2011a, Cordes et al. 2012, Lozier et al. 2011). The major decline of the subgenus Bombus was first documented in B. occidentalis, as Nosema nearly wiped out commercial hives, leading to the cessation of commercial production of this species (Flanders et al. 2003). A landscape-scale analysis found that greater usage of the fungicide chlorothalonil was a strong predictor of pathogen (Nosema bombi) prevalence in four species of bumble bees (Bombus occidentalis, B. pensylvanicus, B. affinis and B. terricola) that are known to be experiencing range contractions (McArt et al. 2017).

By screening museum specimens (including B. occidentalis), Cameron et al. (2016) show that N. bombi prevalence increased significantly in declining species in the early to mid-1990s, coincident with N. bombi outbreaks in North American commercial stocks. There is no evidence that exotic Nosema strains were introduced from Europe. Regardless of geographic origins, the temporal connection between N. bombi epizootics in commercial Bombus stocks and increases in wild populations suggests a substantial risk of pathogen transmission with domestication. Confirming a direct causal link between N. bombi and North American bumble bee decline will require further research. (Cameron et al. 2016).

In addition to disease, this species is faced with numerous other stressors including habitat loss and alteration due to agricultural intensification, urban development, conifer encroachment (resulting from fire suppression), grazing, logging and climate change (reviewed in Jepsen and Foltz Jordan 2013, Evans et al. 2008). Modifications to bumble bee habitat from over grazing by livestock can be particularly harmful to bumble bees by removing floral resources, especially during the mid-summer period when flowers may already be scarce (reviewed in Hatfield et al. 2012). In addition, livestock may trample nesting and overwintering sites, or disrupt rodent populations, which can indirectly harm bumble bees. Indirect effects of logging (such as increased siltation in runoff) and recreation (such as off-road vehicle use) also have the potential to alter meadow ecosystems and disrupt B. occidentalis habitat. Additional habitat alterations, such as conifer encroachment resulting from fire suppression (Panzer 2002, Schultz and Crone 1998, Roland and Matter 2007), fire, agricultural intensification (Williams 1986, Carvell et al. 2006, Diekötter et al. 2006, Kosior et al. 2007, Goulson et al. 2008a), urban development (Jha and Kremen 2012, Bhattacharya et al. 2003), and climate change (Memmott et al. 2007, Thomson 2010, Cameron et al. 2011b) may also threaten B. occidentalis.

Insecticides, which are designed to kill insects directly, and herbicides, which can remove floral resources, both pose serious threats to bumble bees. Of particular concern are neonicotinoids, a class of systemic insecticides whose toxins are extraordinarily persistent, are expressed in the nectar and pollen of plants (and therefore are actively collected by bumble bees), and exert both lethal and sublethal effects on bumble bees (Colla and Packer 2008, Whitehorn et al. 2012, Gill et al. 2012, Gill and Raine 2014, Laycock et al. 2014, reviewed in Hopwood et al. 2012).

Since B. occidentalis has recently undergone a dramatic decline in range and relative abundance, reduced genetic diversity and other genetic factors make this species especially vulnerable to extinction (reviewed in Zayed 2009), and may lead to increased pathogen susceptibility (Altizer et al. 2003, Whitehorn et al. 2009). Recent research indicates that populations of B. occidentalis have lower genetic diversity compared to populations of co-occurring stable species (Cameron et al. 2011a, Lozier et al. 2011).

Short-term Trend: Unknown
Short-term Trend Comments: The available collections and observational data do not allow us to estimate short-term (2008-2017) trends for this species.

Long-term Trend: Decline of 70-90%
Long-term Trend Comments: Current relative abundance of the overall species B. occidentalis is 78% lower than a historical high in the mid-20th century, and 72% lower than its historical average relative abundance.

Based on relative abundance, subspecies B. o. mckayi is apparently stable or slightly decreasing (Richardson 2018). In the other ~75% of the range, B. occidentalis occidentalis shows a decrease of > 80% in relative abundance. The decline has occurred over much of its range, especially from southern British Columbia to California. Cranshaw (2010) reports the species in decline in Colorado, essentially its southeastern limit which suggests the decline of this subspecies has probably become range-wide. See COSEWIC (2009, 2014) for Canada, also Thorp (2005), Evans et al. (2008) for more general information. The species has declined substantially since Thorp's (2005) assessment. Xerces now considers the species in steep decline. Exotic pathogens, especially Crithidia bombi and Nosema bombi, are the apparent cause of the sudden recent decline. However, unlike two other species of this subgenus, B. occidentalis and the very closely related eastern B. terricola, are still being found occasionally in substantial portions of their ranges. See Evans et al. (2008) for documentation of decline in relative abundance where this bee still does occur.

Intrinsic Vulnerability: Highly vulnerable

Environmental Specificity: Broad. Generalist or community with all key requirements common.

Other NatureServe Conservation Status Information

Distribution
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Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) Bombus occidentalis is currently understood to include two subspecies, B. o. mckayi and B. o. occidentalis. The full range of the species includes much of western North America, ranging from the tundra region in Alaska and Yukon south along the west coast to southern British Columbia to central California, Arizona and New Mexico and east into southern Saskatchewan and northwestern Great Plains (Williams et al. 2014).

The historical range extent of this species is estimated at 9,448,511 km², and the extent of current (2008-2017) observations is 5,518,376 km². Controlling for larger historical sample size, the range of this species has declined by an estimated 19%.

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map
Endemism: occurs (regularly, as a native taxon) in multiple nations

U.S. & Canada State/Province Distribution
United States AK, AZ, CA, CO, ID, MT, ND, NE, NM, NV, OR, SD, UT, WA, WY
Canada AB, BC, NT, SK, YT

Range Map
No map available.


U.S. Distribution by County Help
State County Name (FIPS Code)
CA Alameda (06001)*, Alpine (06003)*, Butte (06007)*, Calaveras (06009)*, Contra Costa (06013), Del Norte (06015)*, El Dorado (06017)*, Fresno (06019)*, Humboldt (06023), Imperial (06025), Lake (06033)*, Lassen (06035), Marin (06041)*, Mariposa (06043)*, Mendocino (06045)*, Modoc (06049), Monterey (06053)*, Napa (06055)*, Nevada (06057)*, Placer (06061)*, Plumas (06063), Sacramento (06067), San Bernardino (06071)*, San Francisco (06075), San Joaquin (06077)*, San Luis Obispo (06079)*, San Mateo (06081), Santa Clara (06085)*, Santa Cruz (06087), Shasta (06089), Sierra (06091), Siskiyou (06093)*, Solano (06095), Sonoma (06097), Tehama (06103)*, Trinity (06105)*, Tulare (06107)*, Tuolumne (06109)*, Yolo (06113)*, Yuba (06115)*
ID Bonner (16017), Clearwater (16035), Latah (16057), Lewis (16061), Shoshone (16079)
OR Baker (41001), Clackamas (41005), Crook (41013)*, Deschutes (41017)*, Douglas (41019)*, Grant (41023)*, Hood River (41027), Jackson (41029), Jefferson (41031)*, Josephine (41033), Klamath (41035), Lake (41037), Lane (41039), Lincoln (41041)*, Linn (41043)*, Marion (41047)*, Multnomah (41051)*, Polk (41053), Union (41061), Wallowa (41063), Wasco (41065)*, Wheeler (41069)*
WY Sheridan (56033)*, Teton (56039)*
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
10 Upper Tongue (10090101)+*
16 Lake Tahoe (16050101)+*, Truckee (16050102)+*, Upper Carson (16050201)+*
17 Upper Kootenai (17010101)+, Pend Oreille Lake (17010214)+, Coeur D'alene Lake (17010303)+, St. Joe (17010304)+, Snake headwaters (17040101)+*, Brownlee Reservoir (17050201)+*, Burnt (17050202)+*, Powder (17050203)+, Hells Canyon (17060101)+*, Imnaha (17060102)+, Upper Grande Ronde (17060104)+, Wallowa (17060105)+*, Lower Grande Ronde (17060106)+*, Lower Salmon (17060209)+, Upper North Fork Clearwater (17060307)+, Middle Columbia-Hood (17070105)+*, North Fork John Day (17070202)+*, Lower John Day (17070204)+*, Upper Deschutes (17070301)+*, Lower Crooked (17070305)+*, Lower Deschutes (17070306)+, Lower Columbia-Sandy (17080001)+, Middle Fork Willamette (17090001)+*, Upper Willamette (17090003)+, Mckenzie (17090004)+, North Santiam (17090005)+*, South Santiam (17090006)+*, Clackamas (17090011)+*, Alsea (17100205)+*, North Umpqua (17100301)+*, Upper Rogue (17100307)+, Middle Rogue (17100308)+, Applegate (17100309)+, Lower Rogue (17100310)+*, Illinois (17100311)+, Lake Abert (17120006)+*, Warner Lakes (17120007)+*
18 Smith (18010101)+*, Mad-Redwood (18010102)+, Upper Eel (18010103)+*, Lower Eel (18010105)+*, South Fork Eel (18010106)+*, Mattole (18010107)+*, Big-Navarro-Garcia (18010108)+*, Gualala-Salmon (18010109)+, Russian (18010110)+, Williamson (18010201)+*, Sprague (18010202)+*, Upper Klamath Lake (18010203)+, Lost (18010204)+*, Butte (18010205)+*, Upper Klamath (18010206)+, Shasta (18010207)+*, Scott (18010208)+*, Lower Klamath (18010209)+*, Salmon (18010210)+*, Trinity (18010211)+*, South Fork Trinity (18010212)+, Goose Lake (18020001)+, Upper Pit (18020002)+, Lower Pit (18020003)+, Mccloud (18020004)+*, Sacramento headwaters (18020005)+*, Upper Cache (18020116)+*, North Fork Feather (18020121)+, East Branch North Fork Feather (18020122)+, Middle Fork Feather (18020123)+*, Upper Yuba (18020125)+, North Fork American (18020128)+*, South Fork American (18020129)+*, Battle Creek (18020153)+*, Clear Creek-Sacramento River (18020154)+*, Big Chico Creek-Sacramento River (18020157)+*, Butte Creek (18020158)+*, Upper Putah (18020162)+*, Lower Sacramento (18020163)+, Upper Kaweah (18030007)+*, Upper King (18030010)+*, Tulare-Buena Vista Lakes (18030012)+*, San Joaquin Delta (18040003)+*, Upper Merced (18040008)+*, Upper Tuolumne (18040009)+*, Upper Stanislaus (18040010)+*, Upper Calaveras (18040011)+*, Rock Creek-French Camp Slough (18040051)+*, Suisun Bay (18050001)+*, San Pablo Bay (18050002)+, Coyote (18050003)+*, San Francisco Bay (18050004)+, Tomales-Drake Bays (18050005)+*, San Francisco Coastal South (18050006)+*, San Lorenzo-Soquel (18060001)+, Pajaro (18060002)+*, Salinas (18060005)+*, Central Coastal (18060006)+*, Alisal-Elkhorn Sloughs (18060011)+*, Carmel (18060012)+*, Surprise Valley (18080001)+, Honey-Eagle Lakes (18080003)+, Antelope-Fremont Valleys (18090206)+*, Coyote-Cuddeback Lakes (18090207)+*, Mojave (18090208)+*, Salton Sea (18100204)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
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Reproduction Comments: Usually nests underground; males patrol in circuits in search of mates. It is a known host to Bombus suckleyi and possibly other species (Williams et al. 2014).
Habitat Type: Terrestrial
Non-Migrant: Y
Locally Migrant: N
Long Distance Migrant: N
Palustrine Habitat(s): Riparian
Terrestrial Habitat(s): Cropland/hedgerow, Forest - Conifer, Forest - Hardwood, Forest - Mixed, Grassland/herbaceous, Suburban/orchard, Urban/edificarian
Habitat Comments: Rangewide, habitats for this species include open coniferous, deciduous and mixed-wood forests, wet and dry meadows, montane meadows and prairie grasslands, meadows bordering riparian zones, and along roadsides in taiga adjacent to wooded areas, urban parks, gardens and agricultural areas, subalpine habitats and more isolated natural areas (COSEWIC 2014b).
Food Comments: A short-tongued species; food plants include Ceanothus, Centaurea, Chrysothamnus, Cirsium, Geranium, Grindellia, Lupinus, Melilotus, Monardella, Rubus, Solidago, and Trifolium (Williams et al. 2014).
Colonial Breeder: Y
Economic Attributes Not yet assessed
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Management Summary Not yet assessed
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Population/Occurrence Delineation Not yet assessed
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Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
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Authors/Contributors
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NatureServe Conservation Status Factors Edition Date: 08Apr2018
NatureServe Conservation Status Factors Author: Schweitzer, D.F. and Capuano, N.A. (2014); Richardson, L.L. (2018)

Zoological data developed by NatureServe and its network of natural heritage programs (see Local Programs) and other contributors and cooperators (see Sources).

References
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