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Morrison's Bumble Bee
Bombus morrisoni
NatureServe conservation status
Global (G-rank): G3
State (S-rank): S3
External links
Phenology
B. morrisoni, 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. The flight period for queens begins in early March and peaks in late May/early June; workers begin to emerge shortly after and peak in July/August (Williams et al., 2014; Xerces Society, 2022). 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). Though the exact timing of this shift will vary, it is generally believed to occur in late August for B. morrisoni (Xerces Society, 2022). At the end of the colony cycle, new queens mate and enter diapause to begin the cycle again the following spring.
Species range
This species is historically found throughout the intermountain west and desert southwest, ranging from southern British Columbia south across the Sierra Nevada mountain range and east to New Mexico and parts of West Texas (Killingsworth et al., 2023; Williams et al., 2014). It is especially seen in highlands in the desert southwest stretch of its range (Williams et al., 2014; Xerces Society, 2022). It is uncommon throughout much of its historic range, and seems to be in decline; a 2023 analysis found that the relative abundance of this bee has been reduced by 74% from historic values, with most of the decline occurring after the year 2000 (Killingsworth et al., 2023). B. morrisoni has also seen a range reduction of nearly 20%, and has only a 27.49% persistence in its current range compared to its historic occupancy (Hatfield et al., 2015). Sampling is limited in parts of this species’ range, which makes it difficult to make a complete assessment of its status. However, the Four Corners area has been well sampled in recent years, an effort which has yielded only limited observations of B. morrisoni. However, other areas of Utah appear to show high numbers of this species.
Habitat
Like other bumblebees, B. morrisoni 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). It has been suggested that the availability of nest sites for subterranean-nesting bumble bees may depend on rodent abundance (and by extension, number of abandoned rodent burrows) for this reason (Goulson, 2010). Only one nest description has been published for this species; it describes a hive under chopped straw on the plywood floor of a shed, indicating this species may occasionally nest among manmade structures (Koch & Cane, 2022).
The most important habitat requirement for B. morrisoni 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). In this species, this habitat requirement manifests as a need for abundant nectar and pollen resources March-October within several kilometers of nesting habitat (Xerces Society, 2022). B. morrisoni is a generalist that can be found on many different flowers, though they possess a relatively short tongue length that lends itself to foraging flowers with short corollas. This species has been observed from open dry scrub, shrubland, grassland, and dry coniferous forests (Williams et al., 2014); as previously mentioned, they are particularly associated with highland sites in the desert southwest.
Food habits
As described above, B. morrisoni 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. morrisoni, 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
Bumblebees possess a number of traits that make them effective pollinators. Unlike many other bees, bumblebees 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. Bumblebees are such effective pollinators that several wild flowers depend exclusively on bumblebees for pollination (Defenders of Wildlife, 2015). Known host plants of B. morrisoni include Solanum eleagnifolium, Asclepias, Astragalus, Chrysothamnus, Cirsium, Cleome, Ericameria, Helianthus, Lupinus, Melilotus, and Senecio (RICP, n.d.). A 1983 analysis from California found this species primarily associated with plants from the Asteraceae, Fabaceae, Ranunculaceae, and Capparaceae families, though this may not represent bee preference but rather the abundance of these flowers on the landscape (Thorp et al., 1983; Xerces Society, 2022).
Threats or limiting factors
One of the most obvious drivers of decline in B. morrisoni across its range is climate change. Increased temperature and drought, increased variability in temperature and precipitation, early snow melt, and late frost events may lead to changes in pathogen pressure, the availability of floral habitat, and the availability of nesting sites suitable for B. morrisoni (Xerces Society, 2022). Phenological mismatch may also increase; that is, the timing of flower blooms and bee life cycles may no longer correspond to one another, leaving fewer food sources available for bumble bees during periods critical for hive persistence and reproduction. In the Rocky Mountains, a new mid-summer period of low floral resources has been linked to climate change; this provides an example of phenological mismatch that may disrupt the life cycle of bumble bee colonies (Alridge et al., 2011). Research also predicts range losses for B. morrisoni specifically, partly as a result of the aridity of its current habitat (Guzman et al., 2021; Sirois-Delisle and Kerr, 2018; Xerces Society, 2022). B. morrisoni is considered to exist at the edge of bumble bee tolerance in its current habitat, and will likely lose the southern extent of its range as it grows hotter and drier. Persisting populations may be forced to higher elevations which will lead to fragmentation and genetic isolation.
Agriculture, cropland conversion, and livestock grazing also pose threats to the conservation of B. morrisoni. 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 (Killingsworth et al., 2023; 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. morrisoni. Other anthropogenic threats to this species include fire suppression that changes the makeup of plant communities, urban development that alters and fragments habitats, and energy development/mining on public lands (Killingsworth et al., 2023).
While the effects of pathogens and parasite loads have not been well studied in this species, there is indirect evidence to suggest that pathogens spread by commercial bee populations may be contributing to the decline of B. morrisoni. 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 among some declining bumblebee species (Cameron et al., 2011; Cameron et al., 2016). Introduced honey bees also pose a threat to B. morrisoni 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.
