Bombus affinis 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Hymenoptera Apidae

Scientific Name: Bombus affinis
Species Authority: Cresson, 1863
Common Name(s):
English Rusty Patched Bumble Bee
Taxonomic Source(s): Cresson, E.T. 1863. List of the North American species of Bombus and Apathus. Proceedings of the Entomological Society of Philadelphia 2: 83-116.

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A2b ver 3.1
Year Published: 2015
Date Assessed: 2014-12-29
Assessor(s): Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S., Foltz Jordan, S. & Evans, E.
Reviewer(s): Ascher, J., Jha, S., Williams, P., Lozier, J., Cannings, S., Inouye, D., Yanega, D. & Woodard, H.
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., Richardson, L., Savard, M., Scott, V., Scully, C., Sheffield, C., Sikes, D., Strange, J., Surrette, S., Thomas, C, Thompson, J., 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. & Jepsen, S.

Historically common and broadly distributed in the Upper Midwest and Eastern North America, Bombus affinis has recently experienced serious declines in relative abundance, persistence and distribution across its range. Despite dramatically increased awareness and survey effort for B. affinis, numerous regional studies have reported local extirpations and declines in this species (reviewed in Jepsen et al. 2013), and range-wide studies have found relative abundance declines up to 95% (Cameron et al. 2011a), and range losses of 70 to 87% in recent years (Colla et al. 2012, Cameron et al. 2011a). Consistent with these findings, our analysis found B. affinis has exhibited a 92.54% relative abundance decline over the past decade, suggesting a Critically Endangered Red List category for this species. Note that the Average Decline of 69.36% (based on relative abundance, persistence, and range loss) points toward an Endangered Red List category (very near the cut-off for Critically Endangered). Since the range loss detected in our analysis (45.32%) is much lower than that reported in other studies (87% in Cameron et al. 2011a, considering only the U.S. range; 70% in Colla et al. 2012, considering U.S. and Canada range), we elected to use the Relative Abundance Decline to estimate Past Reduction, rather than Average Decline. This species is listed as Endangered in Canada (COSEWIC 2010) and has been petitioned for Endangered Species Listing in the United States (Jepsen et al. 2013). Based on our recent analysis (Hatfield et al. 2014), along with published reports of bumble bee decline and the assessors' best professional judgement, we recommend this species for the Critically Endangered Red List category at this time.     

Geographic Range [top]

Range Description:Historically, Bombus affinis was broadly distributed in the Northeastern United States and adjacent Canada, in the Eastern Temperate Forest and Boreal Forest regions, north to southern Quebec, Ontario, and Maine; south in a narrow band along the Appalachian Mountains to the northeast corner of Georgia, and west to the margin of the Great Plains in eastern North Dakota, South Dakota, Minnesota, and Iowa (Williams et al. 2014). Known records are at elevations from sea level to c. 6,000 feet (Jepsen et al. 2013). 

In recent years, this species has declined severely across the Northeast, and recent records are mostly from the Midwest and southern Ontario, with very few individuals seen each year (Williams et al. 2014). Our understanding of the current distribution of the rusty patched bumble bee has been greatly informed by a number of research studies as well as a citizen monitoring effort that began in 2008 to specifically target this species (Xerces Society 2014). Since 2000, one or more B. affinis individuals have been reported in the following states and provinces: Connecticut, Illinois, Indiana, Iowa, Maryland, Massachusetts, Minnesota, Ohio, Ontario, Tennessee, Virginia, and Wisconsin  (Jepsen et al. 2013, Xerces Society 2014). The presence of this species is uncertain in the great majority of the states and provinces where it once occurred.

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.
For further information about this species, see 44937399_Bombus_affinis.pdf.
A PDF viewer such as Adobe Reader is required.
Countries occurrence:
Canada (Ontario); United States (Connecticut, Illinois, Indiana, Iowa, Maryland, Massachusetts, Minnesota, Ohio, Tennessee, Virginia, Wisconsin)
Additional data:
Continuing decline in extent of occurrence (EOO):Unknown
Range Map:Click here to open the map viewer and explore range.

Population [top]


A number of studies, both local and range-wide, have shown a significant reduction in the range and relative abundance of this historically common species (reviewed in Jepsen et al. 2013). 

Range-wide declines: A 2007-2009 field survey of more than 16,000 bumble bees from throughout the U.S., compared to collections of more than 73,000 historical bumble bee specimens, revealed that the historic range of B. affinis has contracted by an estimated 87% (Cameron et al. 2011a). This same study concluded that the relative abundance of B. affinis has declined by 95%; the species was only detected at low numbers in three Illinois locations and one Indiana location in the recent survey (Cameron et al. 2011a). A separate analysis of nearly 45,000 eastern bumble bee records from museum collections and contemporary surveys considering both Canada and the U.S. concluded that B. affinis has undergone a greater than a 70% range decline (persisting in less than 30% of re-sampled historically occupied 50 x 50 km grid cells throughout its United States and Canada range). The relative abundance of B. affinis  from 1991-2009 was 87% less than its relative abundance in collections from <1931-2000 (Colla et al. 2012). Similarly, a 2004-2006 study of approximately 9,000 bumble bees from 28 sites where B. affinis historically occurred in southern Ontario, plus 15 sites within the bee’s historic range in eastern North America, found that this species significantly decreased in relative abundance between the 1970-1973 and 2004-2006 survey periods (Colla and Packer 2008). In fact, only a single bee of this species was detected in the recent survey period, despite numerous reports that the species was historically common (Colla and Packer 2008). In the Northeastern United States, a long-term study of relative abundance changes of the entire regional bee fauna (based on >30,000 museum records over a 140-year period), found B. affinis to be one of three bees (out of 438 species) that have exhibited a rapid and recent population collapse (Bartomeus et al. 2013).

Regional declines: In addition to range-wide declines, numerous regional studies have found local extirpations and declines in relative abundance and distribution in this species (reviewed in Jepsen et al. 2013). For example, in a study of 56 sites across Illinois, comparisons between recent surveys and historic records of B. affinis revealed that the distribution of this species has decreased by nearly one third in that state since 2000, with only 67% of its pre-2000 distribution remaining (Grixti et al. 2009). In Indiana, a multi-year survey of more than 880 bumble bees found just 25 B. affinis specimens out of 217 (12%) in 2001, two out of 451 (0.004%) in 2002, and zero out of 553 in 2003 (reviewed in Jepsen et al. 2013). In Minnesota, a survey of 464 bumble bees at Long Lake Regional Park found 98 rusty patched bumble bee individuals in 1994-1995, whereas surveys during the summers of 2007 and 2008 at the same park found no B. affinis among the 593 bumble bees recorded (reviewed in Jepsen et al. 2013). Since 2011, B. affinis has been found in nearby parks, although in very small numbers (10 out of 3,235 bumble bees recorded between 2011 and 2014; E. Evans pers. comm. 2014). In New York, a 2003 survey including over 1,261 bumble bees failed to find any members of this species, despite this species historically considered “moderately abundant” in the state, and well represented in historical collections from the region (Giles and Ascher 2006). A study by Colla and Packer (2008) of two sites in southern Ontario comparing a recent collection of nearly 1,200 bumble bees to a historical collection (Macfarlane 1974) of >3,600 bumble bees from the same locations revealed that the rusty patched bumble bee had been extirpated from both sites, despite the fact that it comprised approximately 14% of the 1970s collection. Similarly, P. Williams reported that the rusty patched bumble bee was formerly abundant in Toronto, Ontario in 1983 but was not seen during regular surveys in the Toronto area from 2003 to 2008 (reviewed in Jepsen et al. 2013). In Maryland, a sample of nearly 1,000 bumble bees on the Patuxent National Wildlife Refuge from 2002 to 2007 found a single B. affinis specimen in 2002, and none since, despite this species being numerous in collections in the 1980s in this area. In the Great Smoky Mountains National Park in North Carolina and Tennessee, where B. affinis was once abundant, this species has not been seen since 2000. In North Carolina, surveys of spring queens consistently found this species from 1995 to 2001, yet between 2002 and 2007, no queens of this species were found, despite the detection of other bumble bee species (reviewed in Jepsen et al. 2013). In Virginia, a recent survey of bee populations at 17 sites detected just one B. affinis among nearly 35,000 bees collected and examined in the study (Smithsonian Science 2014).

We evaluated this species’ range-wide spatial distribution over time using a measure of change in the extent of occurrence (EOO; see Figure 2 in the Supplementary Material) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014). We also assessed changes in the species’ relative abundance (see Figure 1 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. We used these measures of change between the recent and historic time periods to estimate the population trend that has occurred in the past 10 years, and to inform the application of an IUCN category. This analysis yielded the following results (see also the graph in Supplementary Material of relative abundance and map of change in EOO over time):

  • Current range size relative to historic range: 54.68%
  • Persistence in current range relative to historic occupancy: 29.77%
  • Current relative abundance relative to historic values: 7.46%
  • Average decline: 69.36%

The relative abundance graph of this species over time illustrates that the relative abundance of this species in the past decade is lower than any other decade (see Figure 1 in Supplementary Material). The most severe decline has been relatively recent – since the early 1990s – despite active searches throughout its historic range, described above. Note that the range loss detected in our analysis (45.32%) is much lower than that reported in other studies (87% in Cameron et al. 2011a, considering only the U.S. range; 70% in Colla et al. 2012, considering U.S. and Canada range).

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.

For further information about this species, see 44937399_Bombus_affinis.pdf.
A PDF viewer such as Adobe Reader is required.
Current Population Trend:Decreasing
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:Bombus affinis typically occurs close to or within woodlands (Williams et al. 2014). It has been documented in grasslands, marshes, agricultural landscapes and more recently from residential parks and gardens (Colla and Packer 2008, Colla and Dumesh 2010, Williams et al. 2014, reviewed in Jepsen et al. 2013). 

Bumble bees, 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. Although little is known about the overwintering habits of Bombus affinis, queens of other species frequently dig a few centimetres into soft, disturbed soil and form an oval shaped chamber in which she will spend the duration of the winter. Compost in gardens or mole hills may provide suitable sites for queens to overwinter (Goulson 2010). Queens emerge from hibernation in the early spring and immediately start foraging for pollen and nectar and begin to search for a nest site. Bombus affinis is one of the earliest species to emerge in North America, with queens typically emerging from hibernation as early as March or April (Milliron 1971, Colla and Dumesh 2010, Williams et al. 2014, reviewed in Jepsen et al. 2013).

In general, bumble bee 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. Nests of B. affinis have occasionally been observed above ground, but are usually one to four feet below ground in abandoned rodent nests or other cavities (reviewed in Jepsen et al. 2013). Thus, nesting sites may be limited by the abundance of rodents.

After establishing a nest, the bumble bee queen does all of the foraging and care for the colony until the first workers emerge and assist with these duties. Colonies of B. affinis are considered large compared to other species of bumble bees, producing up to 1,000 workers throughout the season (Macfarlane et al. 1994). New queens and males are produced during the later stages of colony development, generally from mid-July or August to September in this species (Plath 1922, Milliron 1971, Macfarlane et al. 1994). As B. affinis emerges early, but does not produce reproductives until late in the summer, this species may be particularly susceptible to stressors (Jepsen et al. 2013).

To meet its nutritional needs, B. affinis – like other bumble bees - requires a constant supply of flowers that bloom throughout the duration of the colony life cycle (approximately April to September). Nectar is a critical source of carbohydrates for bumble bees, and pollen provides essential protein. The amount of pollen available to bumble bee colonies during the growing season directly affects the number of queens that can be produced (Burns 2004). This, in turn impacts future bumble bee population levels. Bombus affinis probably needs floral resources to be located in relative close proximity to its nest sites, as studies of other bumble bee species indicate that they routinely forage within less than one kilometre of their nests (reviewed in Jepsen et al. 2013).

Bombus affinis is a short-tongued species (Williams et al. 2014), typically found on open flowers and those with short corollas rather than flowers with deep corollas. Colla and Dumesh (2010) suggest that B. affinis is likely dependent upon woodland spring ephemeral flowers, since this bumble bee emerges early in the year and is associated with woodland habitats. This species is reported from a wide variety of wild plants including Aesculus, Agastache, Dalea, Eupatorium, Helianthus, Impatiens, Lonicera, Monarda, Prunus, Solidago, and Vaccinum (Williams et al. 2014). See Jepsen et al. (2013) for a list of additional plant species used by B. affinis

Generation Length (years):1

Threats [top]

Major Threat(s):

The primary threats attributed to the severe decline of Bombus affinis include pathogen spill-over from commercial to wild bees; habitat loss due to agriculture and development; pesticide use; and climate change (reviewed in Jepsen et al. 2013). Reduced genetic diversity, which can be a result of declining, isolated subpopulations caused by any of the aforementioned factors, likely also threatens this species (reviewed in Jepsen et al. 2013). 

The spillover of the microsporidian parasite Nosema bombi from commercial to wild bumble bees has been hypothesized as a cause of the sudden, rapid decline of B. affinis and three other closely related North American bumble bees – B. franklini, B. occidentalis, and B. terricola (Thorp and Shepherd 2005, Evans et al. 2008, Colla and Packer 2008, Cameron et al. 2011a, Jepsen et al. 2013). This hypothesis is supported by the timing, speed and severity of the population declines of B. affinis and its close relatives. The major decline of species in the subgenus Bombus was first documented in B. occidentalis, as Nosema nearly wiped out commercial hives, leading to the cessation of commercial production of this species. Wild populations crashed simultaneously and the closely related B. franklini has also declined; it has not been found since 2006 despite extensive surveys. Cameron et al. (2011a) found a significantly higher prevalence of N. bombi in declining North American bumble bee species (B. occidentalis and B. pensylvanicus). Bombus affinis was also examined in this study, but the sample size was so low that the data were excluded from the statistical analyses. However, the authors note that the available data show that this species followed the same infection trend of the other declining species, with infected individuals collected at four of five sites, and infections detected in seven of the 14 individuals collected. Additional pathogens of significance to B. affinis include the protozoans Crithidia and Apicystis bombi, the mite Locustacarus buchneri, the nematode Sphaerularia bombi, and RNA viruses (see Jepsen et al. 2013 for details).

Habitat loss and degradation due to agriculture and development are also likely to have attributed to B. affinis decline, by limiting access to sufficient food, nesting sites, and overwintering sites (Jepsen et al. 2013). Agricultural intensification is primarily blamed for the decline of bumble bees in Europe (Goulson et al. 2008), and may also pose a significant threat to bumble bees in the United States. Bombus affinis historically occupied the grasslands of the Upper Midwest and Northeast, which have largely been lost or fragmented by agricultural conversion and urban development, or transformed by fire suppression, invasive species and livestock grazing. Increases in farm size and changes in technology and operating efficiency have led to many practices that are detrimental to bumble bees, including loss of hedgerows, weed cover and legume pastures. The widespread application of the herbicide glyphosate in conjunction with increased planting of genetically modified crops that are tolerant to glyphosate has reduced the availability of wildflowers in agricultural field margins (Pleasants and Oberhauser 2012, Morandin and Winston 2005). The decline of B. affinis and other bumble bees in Illinois from 1940-1960 coincides with a period of major agricultural intensification in the Midwest (Grixti et al. 2009).

Pesticides are used widely in agricultural, urban and even natural areas across B. affinis’ range, including many known to have both lethal and sublethal toxic effects on bumble bees (see Jepsen et al. 2013). Foraging bumble bees can be poisoned by pesticides when they absorb toxins directly through their exoskeleton, drink contaminated nectar, gather contaminated pollen or when larvae consume contaminated pollen. As bumble bees nest in the ground, they may be uniquely susceptible to pesticides used on lawns or turf. Any application of pesticides can threaten bumble bees, but pesticide drift from aerial spraying can be particularly harmful. Neonicotinoids, an increasingly ubiquitous class of systemic insecticides used in corn and soy production, along with numerous other crops and ornamental plants, pose a unique threat to B. affinis. Colla and Packer (2008) suggested that neonicotinoids may be one of the factors responsible for the decline of this species, since the use of this class of insecticides began in the U.S. in the early 1990s, shortly before the decline of this bee was noticed. Numerous studies have found that field-realistic exposure to neonicotinoids can have direct lethal impacts to bees (Mommaerts et al 2010, reviewed in Hopwood et al. 2012), as well as a variety of sublethal impacts, including reduced colony growth and queen production (Whitehorn et al. 2012), reduced brood production (Laycock et al. 2013), reduced drone production (Mommaerts et al. 2010), impaired foraging behavior (Gill et al. 2012, Gill and Raine 2014, Morandin and Winston 2003), longer foraging times (Mommaerts et al. 2010) and reduced food storage (Al-Jabr 1999). Additional insecticides and herbicides of significant threat to B. affinis are reviewed in Jepsen et al. (2013).

Climate change may also pose a significant threat to the continued survival of the rusty patched bumble bee. Climatic changes that are expected to have the most significant effects on bumble bee populations include: increased temperature and precipitation, increased drought, increased variability in temperature and precipitation extremes, early snow melt and late frost events. These changes may lead to increased pathogen pressure, decreased resource availability (both floral resources and hibernacula) and a decrease in nesting habitat availability due to changes in rodent abundance or distribution (Cameron et al. 2011b). Changes in the distributions of plants visited by bumble bees have been correlated with a changing climate (Forrest et al. 2010, Inouye 2008), which can cause phenological asynchrony between bumble bees and the plants they use (Memmott et al. 2007, Thomson 2010, Kudo et al. 2004). Early spring is a critical time for bumble bees since that is the time when the foundresses emerge from hibernation and initiate nests. After the fourth-warmest winter on record for the U.S. (2012), a rusty patched bumble bee queen emerged from hibernation in Wisconsin in March (Jepsen et al. 2013). Prior to this observation, the earliest recorded queens of this species from any region were recorded as emerging in April. Since bumble bees are generalist foragers, they do not require synchrony with a specific plant, but asynchrony can lead to diminished resource availability at times that are critical to bumble bee colony success. For example, as the climate in the Rocky Mountains has become warmer and drier in the past 30 years, researchers have observed a mid-season period of low floral resources, a change which can negatively impact pollinators (Aldridge et al. 2011).

Reduced genetic diversity, which could be a result of declining, isolated subpopulations caused by any of the aforementioned factors, likely also threatens this species. Isolated patches of habitat may not be sufficient to support bumble bee populations (Hatfield and LeBuhn 2007, Öckinger and Smith 2007), and populations of bumble bees existing in fragmented habitats can also face problems with inbreeding depression (reviewed in Jepsen et al. 2013). Cameron et al. (2011a) found that several declining bumble bee species are associated with low genetic diversity. Reduced genetic diversity can be particularly concerning for bumble bees, since their method of sex-determination can be disrupted by inbreeding, and since genetic diversity already tends to be low in this group due to the colonial life cycle (i.e., large numbers of bumble bees found locally may represent only one or a few queens) (Packer and Owen 2001, Zayed and Packer 2005, Goulson 2010, Hatfield et al. 2012, but see Cameron et al. 2011a and Lozier et al. 2011). 

For additional details on threats to Bombus affinis and extinction risk, see the recent Endangered Species Act Petition for this species (Jepsen et al. 2013).

Conservation Actions [top]

Conservation Actions:

Survey Needs: Once very common in eastern North America, B. affinis has recently undergone a dramatic decline in abundance and distribution, and is no longer present across much of its historic range. In order to better understand the causes and extent of this species’ decline, as well as the conservation needs of remaining subpopulations, additional comprehensive surveys of this species at historic and potential sites are needed throughout its range.

Management Needs: All known and potential sites of this species should be protected from pesticides, habitat alteration, grazing, and other threats that can interfere with the habitat requirements of this species (availability of nectar and pollen throughout the colony season, underground nest sites, and hibernacula). Note that any conservation efforts that benefit B. affinis are also likely to benefit to benefit B. bohemicus (formerly considered B. ashtoni), a social parasite of B. affinis and B. terricola that is also in serious decline in the North American part of its range (e.g. Bartomeus et al. 2013).

Citation: Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S., Foltz Jordan, S. & Evans, E. 2015. Bombus affinis. The IUCN Red List of Threatened Species 2015: e.T44937399A46440196. . Downloaded on 23 January 2017.
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