The American bumble bee (Bombus pensylvanicus), is so named because it occurs throughout North America, from Quebec south to Florida and into Mexico. The American bumble bee is a generalist feeder that forages on plants in the milkweed (Asclepiadaceae), composite (Asteraceae), forget-me-not (Boraginaceae), honey-suckle (Caprifoliaceae), morning-glory (Convolvulaceae), legume (Fabaceae), mint (Lamiaceae), blazing star (Primulaceae), rose (Rosaceae), snapdragon (Scrophulariaceae), and nightshade (Solanaceae) families.
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (200,000 to >2,500,000 square km (about 80,000 to >1,000,000 square miles)) The range of this species has probably contracted northeastward, and it has declined more widely. Records since 2007 in South Dakota (Johnson, 2009), Illinois (Grixti et al., 2008), Arkansas (Warriner, 2011), and Mississippi (Garner, 2011) suggest the core range now for B. p. pensylvanicus may now be the central USA. Its pre-1998 range was from Quebec across southern Ontario west to South Dakota and throughout Nebraska and south through much of Texas and to southern Florida, and (B. pensylvanicus sonorus) into the mountains of central Mexico. During the 1990s and early 2000s, Schmidt and Jacobson (2005) found populations of B. p. sonorus in each of five southern Arizona mountain ranges they studied. They indicated its presence in the nearby deserts, but not in the more contiguous mountains of northern Arizona. The range of that taxon seems to be from western Texas to southern California and well into Mexico. It is confined to mountains, generally above 1500 meters, in arid regions. The range of B. p. pensylvanicus included nearly all of the eastern U.S. and apparently most of Texas, as well as the foothills and shortgrass prairies just east of the Rockies. In the Midwest, its northern limit was in the southern parts of Minnesota, Wisconsin, and Michigan and South Dakota where it was first collected in 2008. This species has declined in Illinois and Arkansas, but still occurs there, and seems to be found now mostly in the south-central US (e.g. Warriner, 2011). the Xerces society also has a 2012 record for Laurens County, Georgia.
American bumble bee queens are between 21 and 25.5 mm long while workers and drones are smaller, between 14 and 19 mm and 16 and 22 mm, respectively. The queen's head is entirely black with a few gray hairs. The thorax is yellow towards the head and black towards the abdomen; the abdomen is yellow and black. Workers resemble the queen. A drone has a grayish-white face, a black head with some gray, and a thorax with similar coloration to the queen. On a drone, the first part of the abdomen is yellow, and the rest is variable.
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Comments: Adults are generalized nectar and pllen gathers. See Di Trani De La Hoz (2006) for a very good autecological study of subspecies sonorus. Super and Moyer's account would be applicable for most of the USA.
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: > 300
Comments: Very rare in many areas, probably extirpated in parts of the range northward, but apparently has not declined as much in the south-central US.
Life History and Behavior
Ants and many bees and wasps are eusocial - meaning they are socially highly organized. Eusocial insects are reproductively specialized, with a reproductive division of labor often involving sterile members caring for the reproductive members. Other defining features of eusociality are overlapping of generations and cooperative care of the young. All ants are eusocial with morphologically different workers and queens. Some bee and wasp species, including honey bees ( Apis mellifera ), carpenter bees ( Xylocopa spp.), bumble bees ( Bombus spp.), paper wasps ( Polistes spp.), and yellowjackets ( Vespula spp.) also exhibit eusociality.
Interestingly, humans are also defined as eusocial.
These bees are eusocial and have an annual nest. In spring, a queen emerges from hibernation in search of a new nest. Worker bees then develop, followed by drones and new queens towards the end of summer. Drones tend to wait around the entrance to hives for the queens to emerge, competing with each other for access. Mated queens hibernate over winter, while workers, drones, and old queens die towards the end of the summer after living about 14 to 25 weeks.
Molecular Biology and Genetics
Statistics of barcoding coverage: Bombus pensylvanicus
Public Records: 0
Specimens with Barcodes: 3
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: NU - Unrankable
Rounded National Status Rank: NU - Unrankable
NatureServe Conservation Status
Rounded Global Status Rank: G3 - Vulnerable
Reasons: Until about the late 1990s this was probably the most common bumblebee in eastern and central United States, and it was often called "The American Bumblebee". Now, even where it still occurs, such as in Arkansas (Warriner, 2011) and Illinois (Grixti et al., 2008), that is generally no longer the case. Even more than for most declining bumble bees, the status, current trend, severity of threats, and prognosis for B. pensylvanicus are very unclear. It is not even clear that the causes of the decline are similar to those causing large-scale crashes and extirpations of subgenus Bombus species, although they probably are. While apparently widespread (reportedly from Vermont to Mexico), the decline of this species, is almost certainly not as rapid compared to most species of subgenus Bombus. Still, there is no basis to suggest whether this species will be widespread but less common, stabilize (and perhaps even remain common) in a greatly reduced range, be reduced to scattered refugia, or become completely extinct in the next few decades. Within a few decades the conservation status rank could plausibly be anything from GX (extinct) to G4 (apparently secure). Decline is already underway and not speculative but since it is declining a bit more slowly compared to species in subgenus Bombus, there may be more time for B. pensylvanicus to evolve some resistance to whatever is causing the decline--especially if the main cause of decline is exotic pathogens or parasites to which some bumblebees are relatively resistant. On the other hand, B. pensylvanicus does not penetrate widely into very cold regions, which seem to be the current refugia for two more seriously declining species, B. occidentalis and B. terricola. There is no really appropriate rank for severely declining, but once very widespread, species, especially when extinction within a few decades is a plausible outcome but so is recovery. Information on the status of B. p. sonorus since 2000 is especially limited. The NatureServe Rank Calculator version 3.1 as of August 2012 produces a rank of G1G3. However this species is not on the brink of extinction, at least not yet, and it is possibly secure westward. An important consideration in future assessments will be whether it continues turning up regularly in the central states such as South Dakota, Illinois, Arkansas and Mississippi, as it did in the late 2000s.
Intrinsic Vulnerability: Moderately vulnerable
Comments: Queens of this species begin activity later in the season than most others (Grixti et al., 2009), a trait that is shown by Williams et al. (2009) to predispose bumblebees to declines in various parts of the world.
Environmental Specificity: Broad. Generalist or community with all key requirements common.
Global Short Term Trend: Decline of 50 to >90%
Comments: Dr. Sydney Cameron's website (2009) notes that this species has been widely reported as declining, including B. p. sonorus. Essentially all of this decline has been after 1998. It is suspected to be declining in Vermont (Richardson, 2008) and Minnesota (DeVore, 2009), and was not found at all by Colla and Packer (2008) in southern Ontario, which represents a statistically significant decline there (see also Williams et al., 2009). Grixti et al. (2008) document a large decline in relative abundance and distribution in Illinois during 2000-2007 compared with 1950-1999. However, the species still occurred there in moderate numbers. They observed 160 in 2007. Among the other 15 species of bumblebees known to occur in Illinois, they saw none of four species, and over 1000 for two others. On the other hand, all three South Dakota specimens found in Johnson's (2009) compilation of specimen data were collected in 2008. The species was not represented in collections from the 1920s to 1970s there. Smith et al. (2012) report that it was the most common bumblebee in the Mississippi blackbelt prairies from 1999 to 2001, but that must now be considered historic status. Warriner (2011) found it to still be widespread in adjacent Arkansas, but no longer the most common bumblebee as reported by earlier authors, and notably he did not find its obligate nest parasite at all. Notably Smith did. More recently, Garner (2011) reported some from Rankin County, Mississippi in August 2008. The extent and severity of the decline is uncertain, but very substantial and it is extirpated, or very nearly so, in Ontario and has become quite rare to extirpated in the Northeast and Middle Atlantic States. It was not noted as declining in the Smokies by Super and Moyer (2003), but that now needs to be considered a historic, not current, status assessment.
Global Long Term Trend: Decline of 50-90%
Comments: Same as short term trend. Decline started by or soon after 1998. Species was more or less stable before then.
Degree of Threat: Very high - high
Comments: The severity and exact nature of the threats are uncertain, and the selection of "moderate" for severity may soon prove too conservative. The very recent declines or suspected declines reported from Vermont to Mexico suggest that some of the same factors that are causing the much more extreme, or at least more rapid, declines of species in subgenus Bombus, such as some, but perhaps not all, of the same pathogens (see Otterstatter and Thomson, 2008; Federman, 2009). Habitat changes and perhaps pesticides could also be factors in some places, but the species is persisting as of 2007 in Illinois in close to "worst case" habitat conditions (Grixti et al., 2008), and their data indicate the species did not decline much there during the mid 20th century as would be expected if habitat loss were the main factor. Furthermore there has not been any habitat change so pervasive as to cause a sudden decline apparently from Vermont to Illinois and into Mexico since 1998. Pesticides would be expected to cause local extirpations only, and of all bumblebees in the area, not just declines or extirpations of two subgenera. Unlike most declining or disappearing bumblebees in the US this one does have a late phenology (Grixti et al., 2009) which Williams et al. (2009) document as a risk factor predisposing such species to declines at least on other continents.
If pesticides are an important factor in bumblebee declines, systemic ones such as neonicotinoids that might reach lethal concentrations in nectar or "guttation drops" (Girolami et al., 2009) would be the most likely suspects. As those authors point out guttation drops are formed by many plants, especially grasses, are consumed by honeybees, and contain neonicotinoid concentrations well above the lethal dose for honeybees. However, Abbott et al. (2008) document that lethal doses were not approached in pollen with two unrelated bees in field conditions and this probably applies to nectar. Girolami et al. (2009) directly demonstrate that consumption of guttation drops from corn grown from neonicotinoid-treated seeds causes death of the bees within minutes. They also review information on concentrations in nectar and pollen which are generally below lethal levels (for honeybees at least) but are still suspected by some to be impacting bees, including contributing to "colony collapse disorder". Widespread use of these systemic toxins to control grubs, sucking insects, and other pests may explain seasonal (spring) declines of honeybees in the region of Italy where this research was conducted. Queen bumblebees consuming guttation drops in spring could be quickly killed or behaviorally impaired before they complete their nest, later in the season exposed workers would be affected. It is apparently unknown to what extent bumblebees drink guttation drops.
Global Protection: Unknown whether any occurrences are appropriately protected and managed
Comments: Many colonies and even substantial populations occurred, or recently occurred, in protected places. But given the uncertainties regarding the decline, and the strong possibility of exotic pathogens being a major factor, it cannot be concluded with any confidence that such populations really are in any meaningful way protected.
Relevance to Humans and Ecosystems
Studies have found the American bumble bee to be a significant pollinator of several plants, including the federally endangered Alabama leather flower (Clematis socialis). These bees visit the Alabama leather flower plants with great frequency resulting in a high single-visit seed set. In the Chihuahuan Desert, the American bumble bee is one of the main pollinators of lechuguilla (Agave lechuguilla) - an indicator species for the Chihuahuan Desert. American bumble bee queens and workers are important pollinators of wavyleaf or pale purple coneflower (Echinacea simulata) and males are the major pollen carriers of tall or rough blazing star (Liatris aspera). Morning glory (Ipomoea purpurea and Ipomoea hederacea) is commonly pollinated by the American bumble bee, as is the yellow fringeless orchid (Platanthera integra). The American bumble bee is also known to forage on tall ironweed ( Veronia gigantea ), red clover (Trifolium pretense), Queen Anne's lace (Daucus carota), and sunflower (Helianthus spp.)
The American bumblebee (Bombus pensylvanicus) is a species of bumblebee found across North America in open fields, but its numbers are declining. It is similar in colour and range to the yellow bumblebee (Bombus fervidus).
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Names and Taxonomy
Comments: Subgenus: Thoracobombus
Based the discussion of apparently intermediate phenotypes in populations in northern Mexico and southwest Texas (Williams 2008), and limited DNA evidence being consistent with such a designation (Cameron et al. 2007), B. sonorus is treated as conspecific with B. pensylvanicus despite minor differences in genitalia of at least some males. It could be treated as a primarily Mexican subspecies as was done by Di Trani de la Hoz (2006). The ecology and phenology of this taxon in Mexico seems similar to that of populations much farther north. However, Schmidt and Jacobson (2005) report that in the Arizona mountains this species nests lower on the slopes and in the desert, unlike any other bumblebees there. They treat it as a separate species and as a foraging visitor only to the "Sky Islands" where other bumblebees nest, these observations and more so the absence of this taxon in cooler northern Arizona tend to suggest it may be a different species. More information and probably more specimens from Texas and Mexico will be needed to resolve the taxonomy for certain and it is possible these are two species that hybridize occasionally in a small area of contact. For now it is included within B. pensylvanicus here.
Described in 1773 by De Geer, who originally placed it in genus Apis.
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