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Western Bumble Bee
As a short-tongued species, the western bumble bee typically drinks from flowers with shallow tubes. It can, however, “rob” deep-tubed flowers by chewing a hole through the side. This allows the bee to drink the nectar, but robs the flower of pollination.
About This Species
Up until the 1990s, the western bumble bee was one of the most common bumble bees in the West, but over the last 20 years, it has declined steeply. In what remains of its dramatically reduced geographic range, it exists in much lower numbers.
In the 1990s, the western bumble bee was commercially raised for crop pollination. Queens were sent to rearing facilities in Europe where, it’s thought, they were exposed to a microscopic parasite known as Nosema bombi. When returned to the United States for commercial use, this disease likely spread to wild western bumble bee colonies. The commercial use of the western bumble bee has largely ceased due to population decline and concerns about the spread of disease. This once common species has been proposed for listing under the U.S. Endangered Species Act. CPW has designated the western bumble bee as a Tier 1 Species of Greatest Conservation Need in the 2025 State Wildlife Action Plan and will be prioritizing actions to preserve this vulnerable species.
More Information:
Physical Characteristics
The western bumble bee can be difficult to identify because different populations have different color patterns, or color morphs. The color morph typical of the Rocky Mountains has an all black face; a black stripe across the middle of the thorax; yellow on the back of the second and all of the third abdominal segments; and all white fifth and sixth abdominal segments.
Range
Prior to 1998, the western bumble bee was widely distributed across the western United States and Canada. Its numbers have since dramatically declined, and its range is very limited.
Habitat
Based on observations from the Xerces Society Bumble Bee Atlas, the western bumble bee is found mostly in meadows and grasslands. However, it can be found in a wide diversity of habitats spanning 5,000 to 11,000 feet in elevation, including roadsides, shrublands, agricultural fields, urban areas and forests.
Diet
Like all bumble bees, the western bumble bee is a generalist, visiting a vast diversity of flowers depending on what is available on the landscape. The Xerces Bumble Bee Atlas found they frequently visit plants with small flowers, including spirea, lupine and goldenrod.
Reproduction
This species has a single annual cycle, which means it is born, matures, reproduces and dies within one year. Mated queens emerge in spring from their hibernacula, where they spend winter. (It’s thought that queens dig shallow holes in the ground to hibernate in.) After waking from hibernation, the queen seeks a suitable nest site, typically an old rodent burrow or natural cavity. The queen lays the first generation of eggs, forages nectar and pollen for the offspring, and raises them to adulthood. This first generation of daughters take over the responsibilities of foraging and tending the next generation, leaving the queen to continue egg laying. After the first generation of daughters reach their adult stage, the queen never leaves the nest again. The colony grows throughout the season until the fall, when males and the next generation of queens emerge. These young queens forage to build up their fat reserves and mate prior to hibernation, preparing for the following spring. Once they find a suitable hibernation location, these queens settle down underground and enter diapause (insect hibernation) for the winter. They emerge in spring and begin the cycle anew.
Threats to Species
Commercially managed bees (both bumble bees and European honey bees) pose a significant threat to native bumble bees. Managed bees can drive disease spillover, when a disease jumps from one population to another. Ongoing research demonstrates that honey bees can outcompete native bumble bees for resources and disrupt their natural foraging behavior. Pesticides also pose a threat to the western bumble bee, with both lethal and nonlethal consequences; bees can be exposed both directly through contact and indirectly through contaminated pollen, nectar and soil. Habitat loss is another major threat to the western bumble bee, as they require healthy populations of flowers throughout the growing season and access to suitable nest sites. Finally, climate change poses a significant threat to the western bumble bee in many ways, affecting the bees directly and indirectly through the ways their habitats will change.
Additional Information
There are a number of ways you can support this species: 1) plant native flowers incorporating species that bloom across the entire growing season, 2) avoid using pesticides (both herbicides and insecticides), and 3) participate in community science projects like iNaturalist or the Bumble Bee Atlas to help expand our understanding of this species’ distribution and abundance.
If you would like to help CPW understand and protect our state’s bumble bees, you can participate in the Mountain States Bumble Bee Atlas. CPW is partnering with the Xerces Society for Invertebrate Conservation to get the Mountain States Atlas off the ground. Learn more here about how to get involved. If you are not able to participate in the Bumble Bee Atlas formally, you can submit photos of bumble bees you spot as one-off observations to the Bumble Bee Watch.