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January 23, 2023

BOZEMAN, Mont. — The western bumble bee was once common in western North America, but increasing temperatures, drought, and pesticide use have contributed to a 57% decline in the occurrence of this species in its historical range, according to a new U.S. Geological Survey-led study.  

Western bumble bee on tufted vetch
The native western bumble bee (Bombus occidentalis) on tufted vetch (Vicia cracca) in Saanichton, British Columbia. This species has been in decline in recent decades, but used to be widespread and locally common in the western U.S. (Photo credit: Jeremy Gatten, LGL Limited, used with permission)

Using data from 1998-2020, scientists determined that increasing summer temperatures and drought partly drove declines of the native western bumble bee in recent decades, with rising temperatures being particularly important. The decline in pollinators is a cause for concern because most flowering plants depend on pollinators such as the western bumble bee to promote reproduction. Pollinators are also essential to our agriculture industry and economy and provide fruits, seeds and nuts that both humans and wildlife rely on. To further complicate matters for the western bumble bee, climate change continues to make rising temperatures and drought more common in the western states. 

“There has been an ongoing global decline in pollinators, including in North America,” said Will Janousek, USGS scientist and co-lead author of the study. “The decline in the once common western bumble bee shows that common, widespread species are not excluded from this trend and our study showed that climate change is an important reason for the decline of this native bee species.”    

The research team found another reason for the reduced distribution of the once common western bumble bee in a pesticide use dataset spanning 2008-2014: a group of insecticides called neonicotinoids, which are commonly used in agriculture. In areas where neonicotinoids were applied, the western bumble bee was less likely to occur and as the rate of neonicotinoid application increased, the bumble bee’s presence declined further.  

Western bumble bee on aster flower
The once common western bumble bee (Bombus occidentalis) on aster (Symphyotrichum sp.) in the Wallowa Mountains of Northeastern Oregon. This species of native bee is now in decline. (Photo credit: Rich Hatfield, the Xerces Society, used with permission)

The scientists also projected the future status of the western bumble bee in 16 regions of the western United States in the 2050s under different future scenarios, considering increasing levels of future climate stressors, changing forest and shrub cover, and other factors.  

“Even considering the most optimistic scenario, western bumble bee populations are expected to continue to decline in the near future in nearly half of the regions across the bumble bee’s range,” said Tabitha Graves, USGS scientist and co-lead author on the study. “Considering the more severe, but probably more likely scenarios, western bumble bee populations are expected to decline an additional 51% to 97% from 2020 levels depending on the region.” 

This study was a collaborative effort between the USGS, U.S. Fish and Wildlife Service, U.S. Department of Agriculture-Agriculture Research Service, Dickinson College, Canadian Wildlife Service, Montana State University, Xerces Society for Invertebrate Conservation, University of Colorado Boulder, The Ohio State University, and the University of Wyoming. It is published in the journal Proceedings of the National Academy of Sciences. For more information on bumble bee research in the West, please visit the USGS Northern Rocky Mountain Science Center website

USGS scientist Tabitha Graves collects western bumble bee samples in eastern Montana.
USGS scientist Tabitha Graves collects western bumble bee samples in eastern Montana. 

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The USGS provides science for a changing world. Learn more at or follow us on Twitter @USGS, Instagram @USGS, or Facebook @USGeologicalSurvey

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