NEW YORK (GenomeWeb) – A new genetic study in the Proceedings of the National Academy of Sciences supports the notion that transmission of the fungal pathogen Nosema bombi from commercial to wild bumble bee colonies may have played a part in the decline of wild bumble bee populations in North America.
"Our results support the hypothesis that Nosema is a key player in US bumble bee declines," first author Sydney Cameron, an entomology researcher at the University of Illinois at Urbana-Champaign, said in a statement. "They also indicate a temporal connection between historical infections in wild bumble bee populations and the late 1990s Nosema-induced collapse of commercial production of Bombus occidentalis [bumble bees] in North America."
Cameron and colleagues from the University of Illinois, the University of Alabama, and the University of California, Davis, used PCR-based DNA testing to retrace the presence and prevalence of N. bombi in museum specimens from five Bombus bumble bee species stretching back to 1979, prior to a suspected introduction of N. bombi to North America in the 1990s.
While DNA from the fungal pathogen turned up only infrequently in bee samples collected in the 1980s, the team saw a significant jump in N. bombi prevalence over the 1990s, corresponding with a rise in commercial pollination processes, including colony rearing of North American bumble bee species in Europe and their subsequent return to North America.
One bumble bee species, B. occidentalis, fell out of favor for commercial colony production not long after due to heavy N. bombi infestation by 1997, the researchers explained, though the source of the pathogens and their precise role in bumble bee declines, if any, have been difficult to pin down.
"[E]valuating Nosema frequencies in host populations over time, before and subsequent to the population declines, could reveal whether N. bombi prevalence has increased in declining populations in association with commercial production for pollination," the study authors noted.
The team used PCR to screen nearly 2,050 bumble bee samples collected between 1979 and 2011, focusing on five species found in North America that have been impacted by declining populations: B. affinis, B. franklini, B. occidentalis, B. terricola, and B. pensylvanicus.
Though DNA from N. bombi turned up occasionally in samples from the 1980s, the results pointed to a steep increase in the frequency of N. bombi infections in all but one of the North American wild bumble bee species during the early to mid-1990s, consistent with potential contamination of wild bumble bees with N. bombi from infected commercial colonies.
Following their initial search for the presence or absence of the fungus in bumble bees, the researchers used targeted small subunit ribosomal RNA gene sequencing and multilocus amplicon sequencing to look at genetic relationships and genetic diversity, respectively, in N. bombi.
Using a subset of the North American samples, as well as B. terrestris samples collected in Europe from 1988 to 1991, they saw genetic similarities, both in North American and western European N. bombi samples and in samples found in North America before and after the fungus became more common in the 1990s. Pathogens on both sides of the Atlantic showed modest levels of genetic diversity.
Though the findings so far argue against the introduction of particularly pathogenic strains from Europe, the team noted that the apparent spread of N. bombi pathogens between bumble bees from commercial pollination enterprises and wild bumble bees suggests that more rigid processes are needed to prevent contamination of wild populations.