Management Recommendations for Washington's Priority Habitats and Species: Western Bumble Bee

General Range and Washington Distribution

The western bumble bee (Bombus occidentalis) was once broadly distributed throughout western North America, with its range extending from southern British Columbia to central California, east to Saskatchewan and the northwestern Great Plains, and south to northern Arizona and northern New Mexico (Williams et al. 2014, COSEWIC 2014, Sheffield et al. 2016, Williams 2021). The subspecies of western bumble bee, B. occidentalis mckayi, was recently declared a distinct species, different from B. occidentalis (Williams 2021). These taxa (hereafter B. occidentalis and B. mckayi) are geographically separated within the historical range, with B. mckayi dominant north of 55⁰ latitude and B. occidentalis dominant south of 55⁰ latitude (Rhode 2022).

In Washington, the western bumble bee was once found throughout much of the state, with the potential exception of low elevation portions of the Columbia Basin. Rangewide analysis suggests that the western bumble bee has undergone a range decline of 28 to 53% between recent and historic time periods (Hatfield et al. 2018, Cameron et al. 2011a). Although suitable habitat for foraging, nesting, and overwintering is broadly distributed across the state, this species is now more restricted to high elevations east of the Cascade Crest, although some suitable habitat remains west of the Cascades and in lower elevations throughout the state. The western bumble bee is rarely found in the western coastal portions of Washington where it was once common, except for the area around Sequim in the northeastern Olympic Peninsula (Cameron et al. 2011a, Graves et al 2020).

Rationale

Bumble bees (Bombus spp.) occur throughout much of the world, providing important ecosystem services by pollinating wild and cultivated plants. Bumble bees are particularly important pollinators in a variety of ecosystems and landscapes, from rural to urban, and agricultural to natural, because of a combination of attributes unique to this genus (Goulson et al. 2008). These attributes include their role as mainly generalist foragers, visiting a wide range of plants to collect resources (nectar and pollen), their capacity to survive in cold climates and fly in inclement weather, and their ability to pollinate certain groups of plants more effectively than other pollinators by “buzz” pollinating, a process that involves the vibration of flight muscles at the correct frequency to release pollen (Goulson et al. 2010).

The western bumble bee was once one of the most common bumble bees in the Pacific Northwest, accounting for more than 15% of all observations (Williams et al. 2014, Rhoades et al. 2016). Unfortunately, populations of many species of bumble bees around the world, including the western bumble bee, are decreasing in range and abundance (Kerr et al. 2015, Cameron and Sadd 2020, Hatfield et al. 2021a, Graves et al. 2020). Graves et al. (2020) documented a 93% decline in the western bumble bee’s probability of occupancy across the western U.S. in the past 21 years. The relative abundance of the western bumble bee has declined by 84% when comparing records from 2002 through 2012 to pre-2002 records (Hatfield et al. 2018). Western bumble bee populations crashed in the 1990s, likely due to a combination of several factors (Graves et al. 2020). While the causes of these declines are not fully understood, likely contributing factors include habitat loss and fragmentation, exposure to pesticides and pathogens, climate change, and competition with non-native species (Graves et al. 2020).

The western bumble bee became a Washington State Candidate Species in 2021 in response to recent declines. The species is also under consideration for a federal listing by the U.S. Fish and Wildlife Service under the Endangered Species Act (USFWS 2022).

Protecting the remaining suitable habitat for western bumble bee in Washington is an important strategy for maintaining the species’ genetic diversity, particularly as climate change causes more substantial impacts and conditions continue to deteriorate across portions of the species’ range.