Bombus suckleyi

Subgenus: Psithyrus.

This species shows a statistically significant decline in relative abundance since 1900 and there are no recent records in many parts of its range. This decline mirrors that of its principal known host, Bombus occidentalis occidentalis, as well as the closely related northern and eastern host, B. terricola. There is no actual evidence that B. suckleyi is stable anywhere, but there is uncertainty about its status in the northern parts of the range. IUCN assigned it a status of Critically Endangered.

Historically, this species was widespread in the mountains of western North America, ranging from southern Alaska south to northern California, Utah, Colorado, Nebraska, and South Dakota southeast across southern Manitoba, with disjunct eastern populations in Ontario, Quebec, Newfoundland, and Nova Scotia (Williams et al. 2014). The validity of these eastern North America records has been verified by sequence data (Cory Sheffield, unpublished), but the status of the species in the east is poorly understood.

The historical range extent of this species is estimated at 12,874,542 km², and the extent of current (2008-2017) observations is 10,734,201 km². Controlling for larger historical sample size, the range of this species has declined by an estimated 16%.

In the recent period (2008-2017), 16 occurrences (i.e., observations separated from others by >5 km) of this species have been reported, a 94% decline over historical occurrences when controlling for larger historical sample size. However, much of the species' northern range remains unsampled or undersampled.

This is a parasitic bee species and the main threat appears to be the steep declines of at least two of its host species, B. occidentalis and B. terricola (Hatfield et al. 2015). In the northwestern parts of this species' range (e.g. Alaska, Yukon, and northern British Columbia), however, its hosts Bombus occidentalis mckayi and B. terricola have not apparently declined (COSEWIC 2014, 2015). Little is known of the present status of B. terricola across northern Canada.

Additional direct threats that may be impacting this species include pesticide use, habitat loss, pathogens from managed pollinators, competition with non-native bees, and climate change (reviewed in Goulson 2010b, Williams et al. 2009, Williams and Osborne 2009, Fürst et al. 2014, Cameron et al. 2011a, Hatfield et al. 2012, Kerr et al. 2015).

Cordes et al. (2012) found this species exhibited high prevalence of the microsporidium Nosema bombi (25%), although the sample size was low (four individuals).