Bombus occidentalis 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Hymenoptera Apidae

Scientific Name: Bombus occidentalis Greene, 1858
Common Name(s):
English Western Bumble Bee
Bombus mckayi Ashmead, 1902
Taxonomic Source(s): Greene, J.W. 1858. II. Descriptions of several new Hymenopterous insects from the north west coast of America. Annals of the Lyceum of Natural History of New York 7: 11-12.
Taxonomic Notes:

DNA evidence (divergence in COI-barcode sequences) supports Bombus occidentalis as a valid species, separate from B. terricola (Williams et al. 2012). COI-barcode divergence also suggests that B. occidentalis can be divided into northern (including Alaska, Yukon Territory, and northern British Columbia) and southern (including the western contiguous United States and southern British Columbia) populations, each of which have distinctive haplotype groups (Williams et al. 2012). These distinct haplotypes correspond with morphological differences; specifically, shorter pile (hair) has been noted in the southern populations and longer pile in the northern populations (Williams et al. 2012; Williams et al. 2014). Taxonomic work is in progress to formally distinguish two subspecies of B. occidentalis (C. Sheffield pers. comm. May 2013 in Jepsen and Foltz Jordan 2013). 

Assessment Information [top]

Red List Category & Criteria: Vulnerable A2be ver 3.1
Year Published: 2015
Date Assessed: 2014-12-29
Assessor(s): Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S.
Reviewer(s): Ascher, J., Jha, S., Williams, P., Lozier, J., Cannings, S., Inouye, D., Yanega, D. & Woodard, H.
Contributor(s): Antweiler, G., Arduser, M., Ascher, J., Bartomeus, N., Beauchemin, A., Beckham, J., Cromartie, J., Day, L., Droege, S., Evans, E., Fiscus, D., Fraser, D., Gadallah, Z., Gall, L., Gardner, J., Gill, D., Golick, D., Heinrich, B., Hinds, P., Hines, H., Irwin, R., Jean, R., Klymko, J., Koch, J., MacPhail, V., Martineau, R., Martins, K., Matteson, K., McFarland, K., Milam, J., Moisan-DeSerres, J., Morrison, F., Ogden, J., Packer, L., Richardson, L., Savard, M., Scott, V., Scully, C., Sheffield, C., Sikes, D., Strange, J., Surrette, S., Thomas, C, Thompson, J., Veit, M., Wetherill, K., Williams, N., Williams, P., Winfree, R., Yanega, D. & Zahendra, S.
Facilitator/Compiler(s): Foltz Jordan, S., Hatfield, R., Colla, S., MacPhail, V. & Jepsen, S.
Historically broadly distributed in western North America, Bombus occidentalis has experienced serious declines in relative abundance, persistence, and range in recent years (Hatfield et al. 2014, Cameron et al. 2011a, Thorp 2008). The average decline value of 40.32% over the past decade (based on relative abundance, persistance, and range declines) suggests a Vulnerable Red List Category for this species (Hatfield et al. 2014). This finding, along with published reports of bumble bee decline and the assessors' best professional judgement, leads us to recommend this species for the Vulnerable Red List category at this time. The use of criterion A2b (where population reduction is suspected) can be justified by changes in relative abundance. Criterion A2e may be justified by the potential effects of pathogens or parasites on B. occidentalis populations (e.g., Cameron et al. 2011a).

Geographic Range [top]

Range Description:

Bombus occidentalis occurs along the West Coast and Mountain West of North America, from Arizona, New Mexico and Mediterranean California, north through the Pacific Northwest and Tundra/Taiga region to Alaska. Eastward, the distribution stretches to the northwestern Great Plains and southern Saskatchewan. Elevations of known sites range from sea level to over 2,000 m asl (Williams et al. 2014). Formerly common throughout much of its range, populations from central California to southern British Columbia and west of the Sierra-Cascade Ranges have declined sharply since the late 1990s (Williams et al. 2014, Thorp 2008), and there has been significant range losses in these regions, particularly from lower elevation sites in California, western Oregon and western Washington (Thorp 2008, Hatfield et al. 2014). 

This species was recently documented from Ontario, based on a record that was confirmed by mitochondrial DNA (BOLD analysis). Since this record is considered an introduction well-outside of the species' native range and the species' status in the area is unclear (Williams et al. 2014), we have listed Ontario as "Presence Uncertain" and "Introduced."

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.

For further information about this species, see 44937492_Bombus_occidentalis.pdf.
A PDF viewer such as Adobe Reader is required.
Countries occurrence:
Canada (Alberta, British Columbia, Northwest Territories, Saskatchewan, Yukon); United States (Alaska, Arizona, California, Colorado, Idaho, Montana, Nebraska, Nevada, New Mexico, Oregon, South Dakota, Utah, Washington, Wyoming)
Additional data:
Continuing decline in extent of occurrence (EOO):Unknown
Range Map:Click here to open the map viewer and explore range.

Population [top]


Multiple studies indicate that B. occidentalis has declined, both range-wide and locally, especially in the southern and western portions of its range. Ongoing surveys targeting the IUCN Critically Endangered Bombus franklini in southern Oregon and northern California have revealed alarming declines not only in B. franklini but also in B. occidentalis (Thorp 2008, R. Thorp pers. comm. 2014). Both species comprised 11-12% of the records at the start of the study in 1998, but, in parallel, have exhibited precipitous declines in relative abundance in the years since (Thorp 2008). Specifically, Bombus franklini was not found in 2004 or 2005, one individual was seen in 2006, and the species has not been detected any year since. Bombus occidentalis was not detected during any surveys 2003 through 2007, and has been seen only in very low numbers (one to seven per year) each year from 2008 to 2014 (R. Thorp pers. comm. 2014). 

In a study comparing current and historical distributions of eight bumble bee species in the United States using museum records and nationwide survey data, Cameron et al. (2011a) found that B. occidentalis has undergone a 28% range decline between recent (2007-2009) and historic (1900-1999) time periods. Despite B. occidentalis historically having among the broadest geographic ranges of any bumble bee species in North America, the 2007-2009 surveys detected the species only throughout the Intermountain West and Rocky Mountains; it was largely absent from the western portion of its range. The relative abundance of this species also significantly declined between the time periods examined, in both of the regions examined (Pacific West and Intermountain West) (Cameron et al. 2011a). It has experienced serious declines across most of its range in Oregon and Washington, while still occurring in relatively high abundance in northeastern Oregon (Rao et al. 2011). The species also remains common in some of the northern parts of its range, such as Alaska and the Yukon (Koch and Strange 2012, S. Cannings pers. comm. 2014). 

We evaluated this species’ spatial distribution range-wide over time using a measure of change in the extent of occurrence (EOO; see Figure 2 in the Supplementary Material) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014). We also assessed changes in the species’ relative abundance (see Figure 1 in the Supplementary Material), which we consider to be an index of abundance relevant to the taxon, as specified by the IUCN Red List Categories and Criteria (IUCN 2012). For all three calculations we divided the database into historical (1805-2001, N=128,572) and current (2002-2012, N=74,682) records. This timeframe was chosen to meet the IUCN criteria stipulation that species decline must have been observed over the longer of three generations or 10 years. Average decline for this species was calculated by averaging the change in abundance, persistence, and EOO. We used these measures of change between the recent and historic time periods to estimate the population trend that has occurred in the past 10 years, and to inform the application of an IUCN category. This analysis yielded the following results (see also the graph in Supplementary Material of relative abundance and map of change in EOO over time):       

  • Current range size relative to historic range: 77.96%
  • Persistence in current range relative to historic occupancy: 72.56%
  • Current relative abundance relative to historic values: 28.51%
  • Average decline: 40.32% 

This analysis indicates that across its entire range, this species has declined over 20% in both range and persistence, with particularly high (>70%) declines in relative abundance over the time period examined. Examination of long-term trends reveals that the species' relative abundance in the current decade is lower than any other decade. In the past decade, this species' relative abundance has declined from 0.035 to 0.018 (c. 50%; see Figure 1 in the Supplementary Material). If this species' long term relative abundance trend continues at the same rate, it will cross the x axis in 60-70 years (Hatfield et al. 2014). These results are consistent with findings from other studies, discussed above (e.g., Cameron et al. 2011, Thorp 2008). Overall, the species has declined dramatically in parts of its historic range, sometimes to the point of local extirpation. Where it remains detectable, we infer that it occurs at lower abundances than historically. Note that the EOO, Persistence and Relative Abundance measures are almost identical to that of B. terricola, a closely related species also with a Vulnerable IUCN Red List category.

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.

For further information about this species, see 44937492_Bombus_occidentalis.pdf.
A PDF viewer such as Adobe Reader is required.
Current Population Trend:Decreasing
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:

Bumble bees are primitively eusocial insects that live in colonies made up of one queen, female workers and, near the end of the season, reproductive members of the colony (new queens, or gynes, and males). New colonies are initiated by solitary queens, generally in the early spring. This process includes locating a suitable nest site; collecting pollen and nectar from flowers; building a wax structure to store nectar; forming a mass of pollen to lay eggs on; and building a wax structure to enclose the eggs and pollen. Bombus occidentalis, like most other species of bumble bees, typically nests underground in abandoned rodent burrows or other cavities (Hobbs 1968, MacFarlane et al. 1994, Plath 1922, Thorp et al. 1983). Most reports of B. occidentalis nests are from underground cavities such as old squirrel or other animal nests and in open west-southwest slopes bordered by trees, although a few nests have been reported from above-ground locations such as in logs among railroad ties (Hobbs 1968, MacFarlane et al. 1994, Plath 1922, Thorp et al. 1983). Availability of nests sites for B. occidentalis may depend on rodent abundance (Evans et al. 2008). Nest tunnels have been reported to be up to 2.1 m long for this species and the nests may be lined with grass or bird feathers (MacFarlane et al. 1994).

Once the colony has been initiated by the queen and the first brood of female workers have grown, pupated and emerged as adults, the female workers take over all duties of foraging for pollen and nectar, colony defense and feeding larvae. The queen’s only responsibility at this point is to lay eggs. Bombus occidentalis colonies can contain as many as 1,685 workers and produce up to 360 new queens; this colony size is considered large relative to many other species of bumble bees (MacFarlane et al. 1994). According to Thorp et al. (1983), the flight period for B. occidentalis queens in California is from early February to late November, peaking in late June and late September. The flight period for workers and males in California is from early April to early November; worker abundance peaks in early August, and male abundance peaks in early September (Thorp et al. 1983). In the late summer or fall, depending on the bumble bee species and elevation, colonies produce reproductive individuals (males and gynes), which leave the colony and mate. Males of this species patrol in circuits in search of mates (Williams et al. 2014). As winter approaches, the old queen, workers, and males die, while the gynes continue to forage for nectar, then find a suitable location (hibernacula), usually burrowed a few centimeters underground, in which to spend the winter. Very little is known about the hibernacula, or overwintering sites, utilized by B. occidentalis, although Hobbs (1968) reported B. occidentalis hibernacula that were two inches deep in a “steep west slope of the mound of earth.” The closely related B. terrestris reportedly hibernates beneath trees (Hobbs 1968). The newly mated queens store sperm until they initiate a colony the following spring.

Bumble bees, including B. occidentalis, are generalist foragers and have been reported visiting a wide variety of flowering plants. Bumble bees require plants that bloom and provide adequate nectar and pollen throughout the colony’s life cycle, which is from early February to late November for B. occidentalis (although the actual dates likely vary by elevation). The amount of pollen available to foragers directly affects the number of new queens that a bumble bee colony can produce, and since queens are the only type of bumble bees that can form new colonies, pollen availability directly affects the future bumble bee population size (Burns 2004). Early spring and late fall are often periods with lower floral resources; the presence of flowering plants at these critical times is essential. Bombus occidentalis has a short tongue, and thus is best suited to forage at open flowers with short corollas. In addition to foraging at the open face of flowers, B. occidentalis is also known to engage in a behavior called ‘nectar robbing’ in which a hole is chewed in the base of flowers with long corollas to obtain nectar without actually facilitating plant pollination (Williams et al. 2014). Rangewide, example food plants of Bombus occidentalis include Ceanothus, Centaurea, Chrysothamnus, Cirsium, Geranium, Grindellia, Lupinus, Melilotus, Monardella, Rubus, Solidago, and Trifolium (Williams et al. 2014). In Oregon and Washington, the plant genera most commonly associated with B. occidentalis observations or collections include: Trifolium (20 observations), Cirsium (12), Rubus (7), Solidago (6), Epilobium (5), Spiraea (5), Tilia (5), Cicuta (4), Heracleum (3), and Plantago (3) (database supporting Williams et al. 2014). Similarly, in an analysis largely based on records from California, Thorp et al. (1983) reports that B. occidentalis records are primarily associated with plants in the Leguminosae (=Fabaceae), Compositae (=Asteraceae), Rhamnaceae, and Rosaceae families. Note that these floral associations do not necessarily represent B. occidentalis’ preference for these plants over other flowering plants, but rather may represent the abundance of these flowers in the landscape.
The habitat for this species is described as open grassy areas, urban parks and gardens, chaparral and shrub areas, and mountain meadows  (Williams et al. 2014). This species is host to the cuckoo bee Bombus suckleyi, based on a confirmed breeding record. It is likely that this species is also a host to B. bohemicus (=ashtoni), B. insularis, and B. flavidus (=fernaldi) (Williams et al. 2014).
Generation Length (years):1

Use and Trade [top]

Use and Trade:

In the past, B. occidentalis was commercially reared for pollination of greenhouse tomatoes and other crops in North America. Commercial rearing of this species began in 1992, with two origins. Some colonies were produced in rearing facilities in California by a company which imported the technology for rearing bumble bees and applied it to rearing B. occidentalis locally in California. This same year, a distributor for a competitor which did not have rearing facilities in North America at the time was granted permits by the US Department of Agriculture – Animal and Plant Health Inspection Service (USDA-APHIS) for a three year window of time (1992-1994) to export queens of B. occidentalis and B. impatiens to European rearing facilities for colony production. Following rearing, these colonies were then shipped back to the U.S. for commercial pollination. In 1997, commercial producers began experiencing problems with disease (Nosema) in the production of B. occidentalis (Flanders et al. 2003). Eventually, the availability of B. occidentalis became critically low, and western crop producers who had become dependent on pollination provided by this species began requesting that APHIS allow the shipment and use of B. impatiens in western states (Flanders et al. 2003). In 1998, the USDA-APHIS issued permits to allow B. impatiens to be used in the western U.S. However, within a few years, the USDA-APHIS stopped regulating the interstate movement of bumble bees altogether, citing their lack of regulatory authority. Bombus occidentalis is no longer bred and sold commercially and B. impatiens is used widely in the western U.S. (Xerces Society et al. 2010).

Threats [top]

Major Threat(s):

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). Wild populations crashed simultaneously and the closely related B. franklini has been pushed to the brink of extinction. However, Koch and Strange (2012) found high levels of infestation by Nosema in interior Alaska where this bumble bee was still quite common.

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, Diekotter et al. 2006, Kosior et al. 2007, Goulson et al. 2008), 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. 2013, 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).

Conservation Actions [top]

Conservation Actions:

Research Needs: Taxonomic work is in progress to formally distinguish two subspecies of B. occidentalis, the northern (including Alaska and northern British Columbia) and southern (including the western contiguous United States and southern British Columbia) populations, each of which have distinctive haplotype groups that correspond with morphological differences (reviewed in Jepsen and Foltz Jordan 2013). This work to describe two subspecies of B. occidentalis will have important conservation implications, since the northern subspecies, occurring primarily in Alaska and parts of British Columbia, appears to be abundant and stable, whereas the southern subspecies has undergone a dramatic decline. Once the subspecies are described, the southern subspecies will be considered more at risk than it was previously.  

Since the prevailing theory on the decline and localized extirpation of B. occidentalis suggests pathogen transmission of Nosema bombi across wild populations, further work is needed to assess possible genetic differences in the strain of N. bombi associated with Alaskan B. occidentalis (Koch and Strange 2012), and/or genetic differences in the Alaskan B. occidentalis that may reduce N. bombi susceptibility.  In the Alaskan study, B. occidentalis was arguably the most abundant bumble bee, despite it having the highest prevalence of N. bombi (Koch and Strange 2012). 

Survey Needs: Once very common in the western United States and western Canada, B. occidentalis has recently undergone a dramatic decline in abundance and distribution, and is no longer present across much of the historic range. In order to better understand the causes and extent of this species’ decline, as well as the conservation needs of remaining populations, additional comprehensive surveys of this species at historic and potential sites are needed throughout its range.

Management Needs:  All known and potential sites of this species should be protected from practices, such as livestock grazing, and threats, such as conifer encroachment, that can interfere with the habitat requirements of this species (availability of nectar and pollen throughout the colony season and availability of underground nest sites and hibernacula). 


Citation: Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S. 2015. Bombus occidentalis. The IUCN Red List of Threatened Species 2015: e.T44937492A46440201. . Downloaded on 25 September 2018.
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