Bourdon à tache rousse

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).

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).