[1] A recent review of all of its close relatives worldwide appears to have confirmed its status as a separate species.

[3] The first color variation is found from northern California, north to British Columbia, and east to southwest Saskatchewan and Montana.

[3] Bumble bee species identification tends to refer to colorations on the abdominal segments.

Additionally, the hind legs of the females tend to be wider and fatter with a pollen basket often visible.

Although closely related to Bombus terricola, DNA evidence supports that they are a distinct species.

The supposed subspecies each have specific haplotype groups, which is reflected by the differences in hair length between the populations.

[6] A recent study in 2016 suggests that the Western bumble bee population is rebounding, possibly due to evolutionary development of resistance to Nosema.

The female workers also have other roles such as foraging for nectar and pollen and defending the colony against predators and parasites.

Gynes have the potential to become queens, and it is their responsibility to find a space for hibernation during the winter so that they can start a colony again next season.

Like other bumble bees, B. occidentalis nests underground in cavities or random burrows left behind by rodents or other animals.

The queen must then construct a wax structure and collect pollen to create a mass to lay eggs on.

[4] When the first brood of female workers have become adults, they take over the jobs of foraging for nectar and pollen, defending the colony, and feeding larvae.

Bombus occidentalis has been commercially reared to pollinate crops such as alfalfa, avocados, apples, cherries, blackberries, cranberries, and blueberries.

The amount of recruitment a returning forager is able to garner depends on the quality (i.e. concentration) of the nectar (or sucrose) that it has found.

[12] Although bumble bees cannot apparently communicate resource location, it appears that foraging activation can communicate which floral species was particularly rewarding through scent, as the activated nestmates show preference for the odour brought home by the returning forager.

However, these queens will stay within the vicinity of the foreign brood, making short flights around the entrance of the nest before reentering it.

[15] Threats to this species include:[8] Due to their role as pollinators, loss of bumble bee populations can have far-ranging ecological impacts.

[3] Agricultural and urban development has resulted in bumble bee habitat becoming increasingly fragmented.

Upon returning to North America, affected bumble bees interacted and spread the disease to wild populations.

Additionally, these four bumble bee species are closely related and belong to the same subgenus; Bombus sensu stricto.

[8] In 2007, the National Research Council determined that the major cause of decline in native bumble bees appeared to be recently introduced non-native fungal and protozoan parasites, including Nosema bombi and Crithidia bombi.

[8] A petition was submitted by the Xerces society, Defenders of Wildlife, and the Center for Food Safety to the California Fish and Game Commission in October 2018 to list Bombus occidentalis and three others as endangered under the California Endangered Species Act.

[18] A subsequent legal challenge of the CESA's definition of a fish as "a wild fish, mollusk, crustacean, invertebrate, amphibian, or part, spawn, or ovum of any of those animals"[18] was eventually overruled, because the explicit intent was for all invertebrates (therefore including insects) to be qualified for protection under this legal definition.

[20] Furthermore, due to careless regulation between states in America and Europe, Nosema parasitism became prevalent within the B. occidentalis population.

Now they are no longer bred or sold commercially because of the threateningly low number, and B. impatiens have been used in their place.