The Status of Bombus occidentalis and B. moderatus in Alaska with Special Focus on Nosema bombi Incidence

Four North American bumble bee species in the subgenus Bombus sensu stricto, including Bombus occidentalis (Hymenoptera: Apidae), are experiencing dramatic declines in population abundance, geographic range and genetic diversity. The prevailing hypothesis concerning their decline is the transmission of the intracellular fungal pathogen, Nosema bombi (Microsporida), and other pathogen species from commercially reared bumble bees to wild populations. While N. bombi incidence has been investigated in the contiguous U.S.A. and Canada, virtually nothing is known about the status of Alaskan bumble bees. This study presents the distribution and relative abundance of B. occidentalis and B. moderatus, two co-occurring species in the subgenus Bombus sensu stricto, in Alaska and the prevalence of N. bombi detected in these species. Bombus occidentalis and B. moderatus accounted for 28% and 4% of the 942 bumble bees surveyed, respectively. Bombus occidentalis was the most abundant species collected in the survey and prevalence of N. bombi infections (44% infected) was also highest in this species. The proportion of infected B. moderatus did not differ significantly from other co-occurring bumble bees. Despite the presence of N. bombi infections, both Bombus s. str. species were commonly detected in Alaska with our survey method. Alaskan bumble bee populations may thus provide important insights on the role of pathogens, particularly N. bombi, in bumble bee decline in the contiguous U.S.A.