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Western Bumble Bee
Bombus occidentalis
NatureServe conservation status
Global (G-rank): G3
State (S-rank): S1S2
External links
Phenology
Bombus occidentalis, like most bumblebees, has an annual life cycle in which most colonies live for only one year. Reproductive queens overwinter before emerging in spring to search for a suitable nest site to incubate their eggs. Thorp et al. (1983) describe the flight period of B. occidentalis queens in California occurring from early February to late November, with workers and males beginning to emerge in April and continuing to be seen through early November. In Cache County, queens emerge in March, with workers and males appearing in early May (Koch et al., 2012).Though the actual active season and timing of life stages varies by latitude and elevation, peak detection rates of B. occidentalis usually occur in July (Cameron and Sad 2020; Graves et al. 2020; Hatfield et al. 2021; Williams et al., 2014; Winfree & Cariveau, 2011). Sometime during the summer, colonies begin to prioritize producing new queens and males over workers. The timing of this shift is poorly understood, though it is possibly related to colony size and age of the colony’s reproductive queen (Williams et al., 2014). At the end of the colony cycle, new queens mate and enter diapause to begin the cycle again the following spring.
Species range
The once widespread B. occidentalis now faces population decline across much of its range. According to Williams et al. (2014), B. occidentalis historically occurs broadly spread across the Mountain West south to AZ and NM, along the West Coast North from central CA to British Columbia, and east through southern Alberta and western SD. However, a 2011 analysis by Cameron et al. found a 28% reduction in range between historic time periods and 2007-2009. Separate unpublished analysis suggests a more dramatic range reduction of nearly 62% (Xerces Society, 2022). Surveys during this recent time period have shown the absence of B. occidentalis in the western extent of its range, with most observations occurring in the Intermountain West and Rocky Mountains. Even in these areas, the population is in decline; A 2015 assessment by the IUCN (Hatfield et al., 2015) found that the relative abundance of the species had declined significantly, >70% over a ten year period. This finding represents the lowest relative abundance of the species compared to any previously studied decades. Observation data suggests that most populations of B. occidentalis in the west are restricted to high elevation sites (Xerces Society, 2022), underscoring the importance of sites like these in the conservation of this species.
Habitat
Like other bumblebees, B. occidentalis requires suitable nesting sites, access to diverse floral resources during the forage season, and sites that are suitable for overwintering queens. Relatively little is known about overwintering sites compared to the other habitat needs of the species, in part because these sites are difficult to locate. Overwintering bumblebee queens have been reported to use the existing burrows of other animals, or dig their own burrows in loose substrate (Williams et al. 2014). These underground burrows tend to be in sheltered, shaded areas without dense vegetation, and in areas with tree litter, moss, or short grasses (Xerces, 2022). Though queens may occasionally excavate their own overwintering sites, bumblebees typically do not dig nest sites for the colony, instead relying on existing cavities and animal dens (Williams et al., 2014). Evans et al. (2008) suggest that the availability of nest sites for B. occidentalis may depend on rodent abundance (and by extension, number of abandoned rodent burrows) for this reason. Although it is less common to see above-ground nests, the species has been recorded nesting among manmade structures, as well as relatively open and flat grassland areas (Xerces, 2022).
The most important habitat requirement for B. occidentalis is the availability of suitable floral resources during the active forage season, as colonies subsist on pollen and nectar. Along with a variety of other factors, future bumblebee population size is determined by the amount of pollen available to foraging bees, as pollen provides the protein and nutrition required to produce new reproducing queens (Burns, 2004). B. occidentalis is a generalist that can be found on many different flowers, though they possess a relatively short tongue length that lends itself to foraging small flowers such as knapweeds and thistles (Hatfield et al. 2021).
Food habits
As described above, B. occidentalis is a generalist, foraging small, open faced flowers. However, they also engage in a behavior known as “nectar robbing” of flowers with long corollas, a behavior named for the way nectar is obtained without facilitating pollination (Williams et al. 2014). In these cases, the bee chews a hole at the base of the flower to obtain nectar directly. B. occidentalis, like other bumblebee species, are generalists with numerous traits that facilitate feeding on floral host plants. Morphological differences such as the shorter tongue length helps reduce potential competition with other pollinators by enforcing different foraging niches, while behaviors such as nectar robbing open a shortcut to harvesting nectar resources not usually available to the bees (Williams et al. 2014).
Ecology
Bumble bees possess a number of traits that make them effective pollinators. Unlike many other bees, bumble bees vibrate their flight muscles to “buzz” flowers while pollinating, a behavior which helps shake pollen from flower anthers; furthermore, their hairy bodies provide a surface that pollen naturally sticks well to as bees travel between flowers (Williams et al., 2014). Individual bumblebee workers often visit just one species of plant at a time, which minimizes transfer of pollen between unreceptive species. Bumble bees are such effective pollinators that several wild flowers depend exclusively on bumble bees for pollination (Defenders of Wildlife, 2015).
Known host plants of B. occidentalis include Ceanothus, Centaurea, Chrysothamnus, Cirsium, Eriogonum, Geranium, Grindellia, Lupinus, Melilotus, Monardella, Rubus, Solidago, Trifolium (RICP, n.d). Thorp et al. (1983)’s analysis of records from California showed association primarily with plants in the families Fabaceae, Asteraceae, Rhamnaceae, and Rosaceae, though it is worth noting that these do not necessarily represent B. occidentalis preference and may simply represent the abundance of these plants as floral resources. B. occidentalis is also an important pollinator of agricultural crops; so much so that it has been reared commercially in the past for pollination of greenhouse tomatoes, avocados, and almonds (Defenders of Wildlife, 2015; Hatfield et al., 2015).
Certain parasitic bees such as B. suckleyi use B. occidentalis as a host, killing the queens of developing nests in order to provision their own offspring. Because they are obligate social parasites, B. suckleyi are dependent on the availability of host colonies in order to survive, and are often observed in low abundance at the edges of their host species range (USFWS, 2024). In addition to B. suckleyi, it is likely that B. occidentalis also plays host to B. bohemicus, B. insularis, and B. flavidus (Williams et al., 2014).
Threats or limiting factors
One of the main drivers of population decline in this species is disease transmission between wild and commercially managed bees. It has recently been shown that the commercial rearing of bumble bees was likely responsible for the spread of Vairimorpha bombi, a fungal pathogen, across North America; this pathogen is now found in high levels within the wild B. occidentalis population (Cameron et al., 2011; Cameron et al., 2016). Introduced honey bees also pose a threat to B. occidentalis and other native bees: in addition to carrying diseases that can transmit to native pollinators, honey bees can competitively exclude native bees and force them to forage plants that provide less pollen and nectar.
Besides disease, habitat modification and loss due to agriculture and livestock grazing are major conservation challenges to B. occidentalis in Utah (Irvin, 2024). Grazing in particular can be harmful to bumblebees, as floral resources that bees rely upon are directly removed. Livestock may also trample nesting and overwintering sites, and reduce the number of suitable nesting locations available to bumblebees (Xerces Society, 2022). In the long term, poorly managed grazing changes the makeup of plant communities on the landscape, with more unpalatable invasive species surviving and spreading, which in turn provide less benefit to pollinators such as B. occidentalis.
Finally, climate change poses a major threat to B. occidentalis. Analysis of the impacts of climate change in other areas of the United States have indicated that habitat suitability, particularly at high elevation sites such as those B. occidentalis is found at, will be reduced as climate change progresses (Kock et al., 2019). Rising temperatures and more frequent/intense droughts will change the availability of moisture, and may impact the timing and structure of plant communities and the pollinators that rely on them; for bumble bees, this means potential changes in life history and community interactions (Xerces Society, 2022).
