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Bombus pensylvanicus 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Hymenoptera Apidae

Scientific Name: Bombus pensylvanicus (DeGeer, 1773)
Common Name(s):
English American Bumblebee, Sonoran Bumblebee
Synonym(s):
Apis pensylvanica DeGeer, 1773
Bombus americanorum Fabricius, 1804
Bombus sonorus Say, 1837
Taxonomic Source(s): Williams, P.H., Thorp, R.W., Richardson, L.L. and Colla, S.R. 2014. The Bumble bees of North America: An Identification guide. Princeton University Press, Princeton.
Taxonomic Notes:

Recent DNA barcodes support that Bombus sonorus is conspecific with B. pensylvanicus (Williams et al. 2014). A population from the south of Mexico (state of Chiapas) is morphologically and genetically distinct from the rest of North America, and could be a separate undescribed species. Further sampling is needed to clarify the situation.

Assessment Information [top]

Red List Category & Criteria: Vulnerable A2be ver 3.1
Year Published: 2015
Date Assessed: 2014-08-19
Assessor(s): Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S.
Reviewer(s): Ascher, J., Cannings, S., Inouye, D., Jha, S., Lozier, J., Pineda Diez de Bonilla, E.P., Vandame, R.V., Williams, P., Woodard, H. & Yanega, D.
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., Pineda Diez de Bonilla, E.P., Richardson, L., Sagot, P., Savard, M., Scott, V., Scully, C., Sheffield, C., Sikes, D., Strange, J., Surrette, S., Thomas, C, Thompson, J., Vandame, R.V., 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.
Justification:
Bombus pensylvanicus, historically among the broadest ranging bumblebees in North and Central America, has exhibited significant declines in both abundance and range in recent years. Range decline is most severe in northern areas, which have been well sampled, although abundance in those northern areas was probably low historically. The relative abundance trend has been slowly moving downward until recently, when the downward trend has become much sharper. Cameron et al. (2011) estimates a 23% range loss, we also found a range loss of 23% along with a 50% drop in persistence and 88.56% drop in relative abundance for this species. The average decline value of 51.38% over the past decade (based on relative abundance, persistence, and range decline) leads to an Endangered (EN) category, but the value is only just over the 50% threshold and we suspect that the actual population decline may be below this threshold - we have therefore decided that the more realistic assessment is to list the species as Vulnerable (VU). Criterion A2e was used for: "population reduction suspected based upon effects of introduced pathogens or parasites" (Cameron et al. 2011 - this study showed a significantly higher prevalence of individuals infected with N. bombi than stable species - 15.2% of individuals collected were infected). This review includes B. sonorus (Hatfield et al. 2014).

Based on the above calculations and trends, along with published reports of bumblebee decline and the assessors' best professional judgement, we recommend this species for the Vulnerable Red List category at this time.
For further information about this species, see 21215172_Bombus_pensylvanicus.pdf.
A PDF viewer such as Adobe Reader is required.

Geographic Range [top]

Range Description:Bombus pensylvanicus is widespread in the Eastern Temperate Forest and Great Plains regions throughout the eastern and central U.S. and extreme southern Canada, absent from much of the Mountain West, but found in the Desert West and adjacent areas of California and Oregon (Williams et al. 2014). This species also occurs in Mexico, where it is known from the following states: Aguascalientes, Baja California, Baja California Sur, Chiapas, Chihuahua, Coahuila, Distrito Federal, Durango, Guanajuato, Guerrero, Hidalgo, Jalisco, México, Michoacán, Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro, San Luis Potosí, Sinaloa, Sonora, Tamaulipas, Tlaxcala, Veracruz, and Zacatecas (ECOSUR Database 2015).

Countries occurrence:
Native:
Canada (Alberta, Ontario, Québec); Mexico (Aguascalientes, Baja California, Baja California Sur, Chiapas, Chihuahua, Coahuila, Durango, Guanajuato, Guerrero, Hidalgo, Jalisco, México Distrito Federal, México State, Michoacán, Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro, San Luis Potosí, Sinaloa, Sonora, Tamaulipas, Tlaxcala, Veracruz, Zacatecas); United States (Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, West Virginia, Wisconsin, 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]

Population:

Recent DNA barcodes support that Bombus sonorus is conspecific with B. pensylvanicus (Williams et al. 2014). A population from the south of Mexico (state of Chiapas) is morphologically and genetically distinct from the rest of North America, and could be a separate undescribed species. Further sampling is needed to clarify the situation.

Historically, Bombus pensylvanicus was among the broadest ranging bumblebees in North America (Cameron et al. 2011). However, numerous studies indicate that this species has recently declined, both locally and regionally, especially in the northern parts of its range. For example, a study examining changes in bumblebee community composition throughout Illinois found that although B. pensylvanicus comprised 28.1% of all bumblebees sampled between 1900 and 1949, it represented only 4.4% of the relative abundance in collections from 1950-2007, including extensive surveys performed at 56 sites across the state in 2007. In addition to abundance declines, this study found B. pensylvanicus has exhibited significant range declines in Illinois, and is no longer found in the northern 1/3 of the state (Grixti et al. 2009). In New York State, Giles and Ascher (2006) failed to find this species in a survey of 1200 bumblebees in the Black Rock Forest, despite evidence it was historically common in the region. In a study in Guelph, Ontario, a comparison of relative abundances of bumblebees collected from 1971-1973 and from 2004-2006 found B. pensylvanicus to be completely absent in surveys from 2004-2006, despite previously having a relative abundance of almost 5% in similar surveys carried out in the same region from 1971-1973 (Colla and Packer 2008).

Looking more broadly, a recent long-term study examining historical changes in 187 species of bees (both Bombus and non-Bombus) in the northeastern United States found that B. pensylvanicus, along with two other Bombus species, has exhibited statistically significant, rapid, and recent population collapse across its northeastern U.S. range (Bartomeus et al. 2013). Similarly, in another study comparing current and historical distributions of eight bumblebee species in the United States using nationwide survey data and museum records, Cameron et al. (2011) found that the relative abundances of B. pensylvanicus has sharply declined in recent years, from >45% to ~10% relative abundance in the Global East, and from >20% to <5% in the Northern/Coastal East, in comparison with the co-distributed B. impatiens, B. bimaculatus, B. terricola, and B. affinis (see Figure 2 in Cameron et al. 2011). In this study, intensive survey efforts of >16,000 bees in 40 states (2007-2009) failed to find this species across large parts of its historical northern and eastern range, resulting in a 23% range reduction of this species in the recent time period (Cameron et al. 2011). It remains common in parts of its range, largely in the southern Midwest and the southern U.S., especially Texas, Oklahoma, Missouri, and Alabama, although comparisons of survey data with museum collections of multiple species suggest that its relative abundance has declined in recent years in these regions (Cameron et al. 2011). In another comprehensive study of bumblebee decline across both the United States and Canada, Colla et al. (2012) found B. pensylvanicus to be one of the most declining species of the 21 species examined, persisting in only 34% of re-sampled historically occupied 50 x 50 km grid cells throughout its United States and Canada range (Colla et al. 2012). This study highlights B. pensylvanicus as one of the most sharply declining species in North America (Colla et al. 2012).

We evaluated this species’ spatial distribution over time in North America (north of Mexico) using a measure of change in the extent of occurrence (EOO) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014) (Figure 1 in the Supplementary Material). We also assessed changes in the species’ relative abundance (Figure 2 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. This analysis yielded the following results (see Supplementary Material for map of change in EOO over time (Figure 1) and graph of change in relative abundance (Figure 2)):

  • Current range size relative to historic range: 81.18%
  • Persistence in current range relative to historic occupancy: 53.24%
  • Current relative abundance relative to historic values: 11.44%
  • Average decline: 51.38% 


The analysis indicates a decline in both distribution and relative abundance in North America, north of Mexico. The decline in EOO is most severe in northern areas, which have been well sampled, although the species may have been historically uncommon in the northern part of its range. Its relative abundance trend has been slowly moving downward until recently when the downward trend has become much sharper (but is not statistically significant).

Mexican summary:  
The ECOSUR database (2015) includes 599 recent (2012-2013) records of this species from numerous Mexican states (Aguascalientes, Baja California, Baja California Sur, Chiapas, Chihuahua, Coahuila, Distrito Federal, Durango, Guanajuato, Guerrero, Hidalgo, Jalisco, México, Michoacán, Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro, San Luis Potosí, Sinaloa, Sonora, Tamaulipas, Tlaxcala, Veracruz, Zacatecas). In addition, there are 2127 Mexican records of this species from a number of collections with unavailable record information (R. Vandame pers. comm. 2014).

For further information about this species, see 21215172_Bombus_pensylvanicus.pdf.
A PDF viewer such as Adobe Reader is required.
Current Population Trend:Decreasing
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:

Bumblebees, as a whole, are eusocial insects that live in colonies composed of a queen, workers, and reproductives (males and new queens). Colonies are annual and only the new, mated queens overwinter. These queens emerge from hibernation in the early spring and immediately start foraging for pollen and nectar and begin to search for a nest site. Nests are often located underground in abandoned rodent nests, or above ground in tufts of grass, old bird nests, rock piles, or cavities in dead trees. Initially, the queen does all of the foraging and care for the colony until the first workers emerge and assist with these duties. Bumblebees collect both nectar and pollen of the plants that they pollinate. In general, bumblebees forage from a diversity of plants, although bumblebee species in a given area can vary greatly in their plant preferences, largely due to differences in tongue length. Bumblebees are well-known to engage in “buzz pollination,” a very effective foraging technique in which they sonicate the flowers to vibrate the pollen loose from the anthers.

Bombus pensylvanicus is found in open farmland and fields throughout its range (Williams et al. 2014), although it was common in urban areas in central Midwest (MO, KS, AR, IL, etc.) during the 1980s and early 1990s (S. Cameron and J. Lozior pers.comms. 2012). It nests mostly on the surface of the ground, among long grass, but occasionally underground (Williams et al. 2014). It is one of the more aggressive bumblebee species, probably as an adaptation to protect the more exposed aboveground nests (Williams et al. 2014).  Males congregate outside nest entrances in search of mates (Williams et al. 2014). Example food plants include Astragalus, CirsiumCornus, Dalea, Echinacea, Helianthus, Kallstoemia, Liatris, Mentzelia, Silphium, Solanum, Trifolium, and Vicia. This species is host to Bombus variabilis (Williams et al. 2014).


Systems:Terrestrial
Generation Length (years):1
Movement patterns:Not a Migrant

Use and Trade [top]

Use and Trade: No trade for this taxon

Threats [top]

Major Threat(s):

This species occurs across a large range primarily across the continental USA and parts of southern Canada and Mexico. It is unlikely that one threat explains declines throughout this species' range. 

There is indirect evidence that pathogen spillover may be a threat. Cameron et al. (2011) found that declining B. pensylvanicus had significantly higher levels of Nosema bombi and lower genetic diversity than stable species. Additionally, Gillespie (2010) found B. fervidus and B. pensylvanicus to be among the most uncommon species in Massachusetts but those individuals present had significantly higher levels of Nosema bombi (but not other parasites) compared to the common species. Szabo et al. (2012) found a weak but significant correlation linking the decline of this species (and one other) to regions with high vegetable greenhouse densities. The same study did not find a significant correlation between corn production density, pesticide use or human population density and Bombus pensylvanicus decline. 

Cameron et al. (2011) found Bombus pensylvanicus (US individuals) had low genetic diversity and high parasitic loads compared to non-declining co-occurring species. Reduced genetic diversity can be particularly concerning for members of the Family Hymenoptera, because of their unique method of sex-determination, haplodiploidy, and since genetic diversity already tends to be low in this group due to the colonial life cycle (i.e., large numbers of bumblebees found locally may represent only one or a few queens) (Goulson 2010, Hatfield et al. 2012, but see Cameron et al. 2011 and Lozier et al. 2011).  

Grixti et al. (2009) found declines in Bombus pensylvanicus throughout the state of Illinois and linked bumblebee diversity declines to change in agricultural practices in the mid 1900s. Pesticides, and especially the most widely used class of pesticides, neonicotinoids, can have adverse effects on beneficial insects (see review by Hopwood et al. 2012).   

Natural system modifications - Open grassland habitats and other open old fields are likely to be the most suitable habitat for this species in its range (Colla and Dumesh 2010). These many grassland habitat types are of conservation concern and exist only in small remnants. Since this species prefers above ground nesting in natural grasslands or agricultural fields, it is particularly susceptible to habitat loss and management activities (Hatfield et al. 2012).  

Natural wildfires and prescribed burning may benefit bees by creating open forage in otherwise unsuitable habitat. As such, the suppression of natural fires can result in habitat loss for bees and other grassland species, particularly in forested regions. In light of this, prescribed burning is frequently used as a conservation management tool to restore natural ecosystems (e.g. grasslands), increase biodiversity (particularly plant species), and control invasive species (e.g. Brockway et al. 2002; Hatch et al. 2002). However, depending on fire intensity, duration, season, frequency, and patchiness, prescribed fire may result in population loss for pollinators, particularly at sites where few individuals of a species exist (e.g. Swengel 1996). As such, both fire suppression and fire itself may threaten this species in some areas.



Conservation Actions [top]

Conservation Actions:

A 120-year long study in Illinois found this species to be the most connected pollinator species in the region and its decline resulted in loss of ecosystem function (Burkle et al. 2013).  Therefore conservation actions are important and may include:

  • Determining the effects of pathogens on populations of B. pensylvanicus and if possible eradication efforts should be implemented.
  • Restoration, protection and creation of natural grassland habitat throughout its range
  • Avoiding the use of pesticides adjacent to forage, overwintering and nesting habitat.
  • Promoting the use of nitrogen-fixing fallow in farming management plans. 


Any conservation actions that benefit B. pensylvanicus will also benefit B. variabilis, which is a social parasite of this species (Williams et al. 2014), and has also experienced dramatic population declines (Grixti et al. 2009; Hatfield et al. 2014).

Additional specific conservation and research needs for this species have not been identified. Research needs for North American bumblebees (as a whole) are summarized in Cameron et al. (2011b), the final report for the 2010 North American Bumble Bee Species Conservation Planning Workshop.


More details on the population trends and status of this species in Mexico are needed. 


Citation: Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S. 2015. Bombus pensylvanicus. The IUCN Red List of Threatened Species 2015: e.T21215172A21215281. . Downloaded on 17 August 2018.
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