U.S. Sees Massive Drop in Bumble Bees

Bumble bees are vitally important pollinators of wild plants and agricultural crops worldwide. Fragmentary observations, however, have suggested population declines in several North American species. Despite rising concern over these observations in the United States, highlighted in a recent National Academy of Sciences report, a national assessment of the geographic scope and possible causal factors of bumble bee decline is lacking.

The Proceedings of the National Academy of Sciences reports the results of a three-year interdisciplinary study of changing distributions, population genetic structure, and levels of pathogen infection in bumble bee populations across the United States. The study compared current and historical distributions of eight species, compiling a database of  more than 73,000 museum records for comparison with data from intensive nationwide surveys of  more than 16,000 specimens. The research shows that the relative abundances of four species have declined by up to 96 percent and that their surveyed geographic ranges have contracted by 23 to 87 percent, some within the last 20 years. The study shows that declining populations have significantly higher infection levels of the microsporidian pathogen Nosema bombi and lower genetic diversity compared with co-occurring populations of the stable (nondeclining) species. Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in North America, although cause and effect remain uncertain.