Bombus (Bombus) occidentalis

The western bumble bee (Bombus occidentalis) is named after its historic range; it was once common in 14 western US states and four Canadian provinces. Since the late 1990's this bee has undergone a dramatic decline in the western part of its range and populations are currently found only in the northern and eastern portions of its range. The rapid decline is believed to be caused by an introduced pathogen, possibly Nosema bombi, which was likely picked up by queens sent to European rearing facilities and then sent back to the United States in the early 1990's. It is hypothesized that after the return of these newly infected bees, the disease then spread to wild populations of western bumble bees, Franklin's bumble bees (B. franklini), rusty patched bumble bees (B. affinis), and yellowbanded bumble bees (B. terricola). A decline in each of these species was noticed in the late 1990's. Additional threats to bumble bees include other pests and diseases, habitat destruction, pesticides, invasive species, natural pests or predators, and climate change.

Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) The current range is partially uncertain. This bee ranged along the west coast of North America from Alaska to central California and east to northern Nevada, Arizona and New Mexico (Thorp 2005, Evans et al. 2008). Milliron (1971) indicated that the species ranged as far east as the Dakotas and Nebraska, and Schmidt and Jacobson (2005) found it in the Sky Islands of the southern Arizona mountains in the 1990s and early 2000s. Johnson (2009) also lists specimen records from three counties in southwestern South Dakota collected in the 1920s and 1930s, as well as one from far to the east in Brooking County in 1929. In Canada this bee was documented in British Columbia, Alberta, and Yukon. While this bee was said to be persisting mostly eastward by Thorp (2005), Cranshaw (2010) reports it has now declined in Colorado which is very near its eastern limit. Farther west, the Oregon Natural Heritage Program has records from several places in 2006-2008, mostly singletons (in one case one among 2000 bumblebee observations), but 49 were were found in a prairie in northeastern Oregon during 2007-2008, for about 5% of bumblebees in June, and around 2% later in the season Rao et al. (2011). See Rao and Stephen (2007) for more information on the Polk County records. According to Sarina Jepsen of the Xerces Society (email to Nicole Capuano, September 2009) there were documented records for 2009 in Jefferson County, Washington; near Mount Hood in Oregon; and Kootenay Lake, British Columbia. Otherwise, records in at least southern British Columbia appear to cease after 2005 (COSEWIC, 2009, Rao and Stephen, 2007), but the species was still 2.2% in a 2007 bumblebee sample in Alberta (Evan et. al., 2009) compared to 8.4% in 1998. Syd Cannings (pers. comm. to Nicole Capuano, September 2009) reports this species is still fairly common and locally abundant in the Yukon Territories and Koch (2011) found the species to be still common in central Alaska.

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

The western bumble bee has several color variations. One variant found from northern California, north to British Columbia and east to southwest Saskatchewan and to Montana has yellow hair on the front part of its thorax. The abdominal segments have black hair, with the lower abdomen being whitish colored. The head is entirely black. Another color variant occurs along the central coast in California and differs from the first in that it has yellow hairs on the sides of the abdomen and reddish brown hair lower on the abdomen. The third common variant occurs in the Rocky Mountains to Alaska and differs from the first in that it has yellow hairs behind the wings and in the middle of the abdomen. In all variants, males and females have similar coloring except that males have pale yellowish hair on the front of the face and the top of the head, with a few black hairs on the sides.

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: 21 to >300

Comments: Formerly millions, no idea now.

Unknown

Comments: Formerly at least millions. Three colonies were found in Washington state in 2006 (Rao and Stepen, 2007).

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 4
Specimens with Barcodes: 8
Species With Barcodes: 1

The following is a representative barcode sequence, the centroid of all available sequences for this species.

There are 27 barcode sequences available from BOLD and GenBank.  Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.  See the BOLD taxonomy browser for more complete information about this specimen and other sequences.

Download FASTA File

Rounded Global Status Rank: G2 - Imperiled

Reasons: There was a previous conservation status rank of G4, but that was not based on current information, and actually until the mid 1990s this very common western bumblebee probably would have been best ranked G5. B. occidentalis was among the two most abundant bumblebees in most of western North America. While there were perhaps millions of populations in 1998, and the range and area of occupancy were huge, there is no basis for assessing how many populations still exist, whether any of them are potentially viable, or what the current range is. In less than 15 years this has gone from the second most common bumblebee in the western US to undetectable in substantial areas and rare elsewhere except in the far north and perhaps highest elevations. The decline of this subgenus is on-going and continent-wide. Recently (Cranshaw, 2010) it is reported to have declined even as far east as Colorado. Although Evans et al. (2008) indicate that this species still occurs regularly in the mountains in Colorado and Arizona this information could now be outdated. The species has not yet declined in the Yukon (S. Cannings) or apparently in interior Alaska (Koch, 2011). Although there were a few observations in Oregon and one in British Columbia in 2007-2009, Rao and Stephen ( 2010) indicate that this once common species no longer occurs in coastal and valley regions of Oregon, and it has also disappeared in the San Francisco area (McFrederick and Lebuhn, 2006), not far from where North American declines of subgenus Bombus apparently started from commercial colonies of this species. Rao and Stephen's (2007) reports in the Willamette Valley are from 2006, and with this subgenus records that old should not be assumed to indicate extant populations five years later. This species has declined, and in many places disappeared, from the Pacific Coast as far east as Colorado. It is persisting (at least for now) at the northern end of its range in the Yukon and Alaska, and perhaps still is at high elevations farther south. The decline is probably due mostly to introduced pathogens (e.g. Nosema bombi) and is likely to become range-wide in the 2010s, but there still is some uncertainty, and Koch's 92011) findings need to be explained, although no widely accepted alternative hypothesis has been proposed. Optimistically it is possible that cold climates such as at high elevations or in the far north will provide refugia, but there is little, if any, supporting evidence. The Xerces Society considers this species in steep decline and COSEWIC considers it of conservation concern in Canada. The decline appears to have spread considerably from 2005-2010. Sokolova et al. (2010) suggest there may also be very closely related native North American Nosema, and Koch (2011) finds B. occidentalis to be common, but heavily parasitized by a Nosema in Alaska--which could prove to be native. In a plausible worst case scenario some or all endemic North American species of subgenus Bombus could be extinct within a decade or two, and B. franklini may be already.

Since there is no conservation status rank that addresses a rapidly declining species with so many unknowns, GU would be an easily defensible rank, especially considering uncertainty about scope of threats. NatureServe's Rank Calculator produced a rank of G2G3, which is a reasonable snapshot in time as of about 2010-2011, but under a plausible worst case scenario of rangewide impacts from exotic pathogens in the near future even G2 would understate the degree of imperilment. This rank is very likely to need revision within five years (2016). It is likely that 10-20 years from now one of GH, GX or G4 will apply, in the optimistic scenario Bombus occidentalis would probably be confined to a much more boreal and montane range than it was originally.

Intrinsic Vulnerability: Highly vulnerable

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

Canada

Rounded National Status Rank: NU - Unrankable

United States

Rounded National Status Rank: N2 - Imperiled

Global Short Term Trend: Decline of 70 to >90%

Comments: Like the rest of the subgenus, this species has declined about 70-100% since the late 1990s in many places, especially from British Columbia to California. Cranshaw (2010) report the species in decline in Colorado, essentially its eastern limit. A 70% decline in the US population would probably be conservative, and the decline will continue. See COSEWIC (2009) 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 in some places. In this case, especially Yukon and central Alaska in the far north, in southern Alberta (but apparently declining), and in the mountains in Colorado and Arizona, but also rarely in scattered places in Oregon. See Evans et al. (2008) for documentation of decline in relative abundance where this bee still does occur.

Global Long Term Trend: Decline of 50-90%

Comments: Decline started soon after 1998.

Degree of Threat: Very high - high

Comments: Threats are extreme in warmer western parts of the range, very high in the rest of the US range and southern Canada, but might, or might not, prove to be much less in the far north and in higher parts of the Rockies. Like other severely declining bumblebees, the main cause is thought to be pathogen spillover of a particularly virulent, probably imported, strain of the microsporidian (Nosema bombi) and an imported protozoan parasite (Crithidia bombi) from domesticated bumblebees (this species and Bombus impatiens) that were reared in Europe and returned to the U.S. for greenhouse pollination (e.g. Committee on Status of Pollinators, 2007, Colla and Packer, 2008, Evans et al., 2008; Federman, 2009 and references reviewed in all). The major decline of the subgenus Bombus was first documented in this species, specifically as Nosema nearly wiped out commercial hives, leading to the cessation of commercial production of this species. Wild populations crashed simultaneously and the closely related B. franklini was pushed to the brink of extinction. While effects of the N. bombi are sublethal and sometimes quite mild for B. impatiens and some other bumblebees, impacts to subgenus Bombus appear to be often severe. However, Koch (2011) found high levels of parastitism in B. occidentalis by what is considered to be the same species of Nosema in interior Alaksa where this bumble bee was still quite common. Whether these Nosema are identical with those implicated in declines of this subgenus farther south is not clear. The mite Locustacarum buckneri and the honeybee deformed wing virus may also be contributing to decline. There are probably other localized threats, at least in some places, such as land use changes and other forms of habitat loss, changes in nectar flora etc. A potentially serious threat might be novel pesticides especially new persistent neonicotinoids (Colla and Packer, 2008), but more evidence is needed, and this seems inconsistent with findings that some bumblebee species are stable or increasing. The consensus seems to be that the above pathogens and parasites, perhaps especially an introduced Nosema strain, are probably the main causes of the very recent drastic decline. However, Sokolova et al. (2010) suggest there may also be very closely related native North American Nosema, which would also be among the more likely explanations for Koch's (2011) findings in Alaska. In a plausible worst case scenario some or all North American species of subgenus Bombus could be extinct within a decade or two, and one (B. franklini) may be already. There may ultimately be no refugia from introduced pathogens, although this is not certain especially in extreme climates like the Yukon, interior Alaska, and at high elevations in places like Colorado. As Brown and Paxton (2009) point out, other factors that could be mitigated should not be dismissed. Still there has been no habitat or insecticide use change so pervasive in the past ten years as to possibly account for the drastic coast to coast decline of four of the five North America species of this subgenus, which very strongly points to the pathogens, but the observation that B. occidentalis is heavily parasitized by a Nosema where it is still common in Alaska needs to be followed up. Most of the species ranged widely enough, e.g. this one from Alaska to Nevada and South Dakota, that climate change is a plausible explanation for declines only at the edges of their ranges.

Global Protection: Unknown whether any occurrences are appropriately protected and managed

Comments: Unknown and irrelevant since protection is not now possible.

The western bumble bee is an important pollinator of both crops and wildflowers, and has been reared commercially for pollination of greenhouse tomatoes and field berry crops in the western United States. It also pollinates other crops such as alfalfa, avocado, apple, cranberry, blackberry, blueberry, and cherry. Additionally, the western bumble bee pollinates many wildflowers, such as aster (Aster spp.), cosmos (Cosmos spp.), mountain lupine (Lupinus monticola), willow (Salix spp.), goldenrod (Solidago spp.), and clover (Trifolium spp.).