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American Bumble Bee
Bombus pensylvanicus
NatureServe conservation status
Global (G-rank): G3G4
State (S-rank): S1
External links
Phenology
B. pensylvanicus, 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 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. pensylvanicus (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
Though this species is known from Utah, it is actually absent throughout much of the Mountain West and is more widespread in the Great Plains and along the east coast (Williams et al., 2014). However, it is found in western deserts and adjacent areas of California and Oregon, giving it one of the largest geographic ranges of any North American bumble bee. It was also the most commonly observed species of bumble bees in the United States: prior to 2002, B. pensylvanicus comprised 10.6% of all bumble bee observations (CBD & BPALS, 2021). However, B. pensylvanicus has seen dramatic declines across its range. A 2015 IUCN assessment found this species currently exists at only 11% of its previous relative abundance (Hatfield et al., 2015). Furthermore, its range has been reduced by 23%, as key habitat has been lost in northern parts of its range as well as the southwestern United States. In a study of 21 species, B. pensylvanicus only persists at 34% of historically occupied sites; this study suggests that it is facing one of the most dramatic declines of any bumble bee in North America (Colla et al., 2012).
There is only one historic observation of B. pensylvanicus from Utah; its current presence in the state is known largely from community science initiatives (CBD & BPALS, 2021). Current observations actually seem to suggest an increase in relative abundance of this species in Utah, but this is likely an effect of varying sampling effort between historic surveys and current monitoring, rather than a genuine increase in the abundance of B. pensylvanicus.
A possible subspecies, B. sonorus, is sympatric with B. pensylvanicus from California to Central Texas and south to southern Mexico (Beckham et al., 2024). This taxa has a contentious taxonomic history, and may be elevated to species level. Currently, however, it is largely treated as conspecific with B. pensylvanicus by most sources.
Habitat
Like other bumblebees, B. pensylvanicus requires suitable nesting sites, access to diverse floral resources during the forage season, and sites that are suitable for overwintering queens. Unlike some other species of bumble bee, B. pensylvanicus nests primarily above-ground among long grass. Williams et al. (2014) ties this tendency to the increased aggression the bees exhibit compared to other bumblebees, suggesting B. pensylvanicus has adapted to protect its exposed nests. Though most burrows are above ground, this species does occasionally nest in abandoned underground rodent burrows (Xerces Society, 2022).
The most important habitat requirement for B. pensylvanicus 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. pensylvanicus is a generalist that can be found on many different flowers. This species possesses a long tongue, and can therefore forage flowers with long corollas; it is often described as using plants from the families Fabaceae and Asteraceae for forage (Xerces Society, 2022). While habitat characterization for this species has not been established range-wide, local-scale studies have found B. pensylvanicus associated with open habitats, including disturbed ones such as farmlands; its wide range means it is present in grasslands, shrublands, semi-arid deserts, and woodlands (Williams et al., 2014; Xerces Society, 2022).
Food habits
As described above, B. pensylvanicus forages flowers with long corollas. However, they also engage in a behavior known as “nectar robbing”, 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. pensylvanicus, like other bumblebee species, are generalists with numerous traits that facilitate feeding on floral host plants. Morphological differences such as the longer 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 wildflowers depend exclusively on bumblebees for pollination (Defenders of Wildlife, 2015). Known host plants of B. pensylvanicus include Astragulus, Chrysothamnus, Cirsium, Cornus, Dalea, Echinacea, Gossypium, Helianthus, Kallstroemia, Liatris, Linaria, Mentzelia, Silphium, Solanum, Trifolium, Vicia, and Viguiera (RICP, n.d.).
Certain parasitic bees such as B. variabilis use B. pensylvanicus as a host, killing the queens of developing nests in order to provision their own offspring. B. variabilis has only been documented parasitizing B. pensylvanicus. Because they are obligate social parasites, B. variabilis are dependent on the availability of host colonies in order to survive. Occurrence records for this parasitic bumble bee are concentrated in the Great Plains; it has never been observed in Utah (Williams et al., 2014).
Threats or limiting factors
There is indirect evidence to suggest that pathogens spread by commercial bee populations may be contributing to the decline of B. pensylvanicus. 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 higher levels among B. pensylvanicus than other stable species (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.
One of the most obvious drivers of decline in B. pensylvanicus 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. pensylvanicus (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).
Because of the wide range of this species, threats to it are numerous. In Utah, urban development is a major threat; most of the observations of this species are surrounding St. George, an area being rapidly developed (Irvin et al., 2023). Development reduces suitable nesting and floral habitat, leads to an increased exposure of pesticides, and leads to fragmentation and genetic isolation that hurts colony fitness and long-term population stability. Other anthropogenic habitat modifications threatening this species include mineral extraction, OHV recreation, cropland conversion and grazing. 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. pensylvanicus.
