Patterns of widespread decline in North American bumble bees
- aDepartment of Entomology and Institute for Genomic Biology, University of Illinois, Urbana, IL 61801;
- bUnited States Department of Agriculture-Agricultural Research Service Pollinating Insects Research Unit, Utah State University, Logan, UT 84322;
- cDepartment of Biology, Utah State University, Logan, UT 84321; and
- dIllinois Natural History Survey, Institute of Natural Resource Sustainability, University of Illinois, Champaign, IL 61820
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Edited* by Gene E. Robinson, University of Illinois, Urbana, IL, and approved November 24, 2010 (received for review October 3, 2010)

Abstract
Bumble bees (Bombus) 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. Here, we report results of a 3-y interdisciplinary study of changing distributions, population genetic structure, and levels of pathogen infection in bumble bee populations across the United States. We compare current and historical distributions of eight species, compiling a database of >73,000 museum records for comparison with data from intensive nationwide surveys of >16,000 specimens. We show that the relative abundances of four species have declined by up to 96% and that their surveyed geographic ranges have contracted by 23–87%, some within the last 20 y. We also show 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.
Footnotes
- 1To whom correspondence should be addressed. E-mail: scameron{at}life.illinois.edu.
Author contributions: S.A.C., J.D.L., J.P.S., L.F.S., and T.L.G. designed research; S.A.C., J.D.L., J.P.S., J.B.K., N.C., L.F.S., and T.L.G. performed research; J.D.L., J.P.S., J.B.K., and N.C. analyzed data; and S.A.C. and J.D.L. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database (accession nos. HM142724–HM142729 and HM173334–HM173341).
↵*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1014743108/-/DCSupplemental.