|The range of Bombus occidentalis. (Dashed line indicates former range.)|
The western bumblebee, Bombus occidentalis, is one of around 30 bumblebee species present in the western United States and western Canada. A recent review of all of its close relatives worldwide appears to have confirmed its status as a separate species.
Western bumblebee workers have three main color variations.
- The first color variation is found from northern California, north to British Columbia, and east to southwest Saskatchewan and Montana.
- It has yellow hair on front part of thorax
- The first through the basal section of the fourth abdominal segments have black hair.
- The lower edge of the fourth and fifth abdominal segments are whitish.
- The sixth abdominal segment often has sparse, whitish hairs, but may still appear black.
- The hair on its head is entirely black.
- The second color variation is found along the central coast in California. It has:
- Yellow hair on the sides of the second abdominal segment and all of the third abdominal segment
- Reddish brown hair on fifth abdominal segment
- The third color variation is found from the Rocky Mountains to Alaska. It has yellow hair on the thorax behind the wings and on the rear of the second and all of the third abdominal segments.
All insects have three main body parts; the head, thorax, and abdomen. Bumblebee species identification tends to refer to colorations on the abdominal segments. The abdominal segments are numbered from T1 to T6 (T7 if male) starting from the abdominal segment closest to the thorax and then working ventrally.
A few ways are used to determine the sex of the western bumblebee. The males (drones) have seven abdominal segments, while the females (queens and workers) have only six. The drones' antennae have 13 segments, while the females have only 12. Drones have no stingers. Additionally, the hind legs of the females tend to be wider and fatter with a pollen basket often visible. Drones have thinner hind legs that do not have pollen baskets. Another clue to sexual identity among B. occidentalis species is when they are being observed. Queens are the first to appear in the spring and then the workers appear. All females can then be seen throughout the summer and into early fall. The drones only appear in the late summer and early fall.
Western bumblebees are generalist foragers. Because they do not depend on any one flower type, they are considered to be excellent pollinators. Bumblebees are also able to fly in cooler temperatures and lower light levels than many other bees. Additionally, bumblebees perform "buzz pollination". This behavior is displayed when a bumblebee grabs the pollen-producing structure of the flower in her jaws and vibrates her wing musculature, causing vibrations that dislodge pollen that would have otherwise remained trapped in the flower's anthers. Tomatoes, peppers, and cranberries are some of the plants that require this type of pollination. For these reasons, bumblebees are considered to be more effective pollinators than honey bees. Bombus occidentalis has been commercially reared to pollinate crops such as alfalfa, avocados, apples, cherries, blackberries, cranberries, and blueberries.
Workers collect nectar and store it in their stomachs. Nectar is regurgitated at the nest. Pollen is collected and put into "pollen baskets" located on the hind legs. Nectar provides carbohydrates while pollen provides protein.
Threats to this species include:
- Spread of pests and diseases by the commercial bumblebee industry
- Other pests and diseases
- Habitat destruction or alteration that may degrade, destroy, alter, fragment, and reduce their food supply or nest sites
- Pesticides and insecticides (ground bumblebees are particularly susceptible)
- Invasive plant species that may directly compete with native nectar and pollen plants
- Natural pest or predator population cycles
Due to their role as pollinators, loss of bumblebee populations can have far-ranging ecological impacts. B. occidentalis once had a wide range that included northern California, Oregon, Washington, Alaska, Idaho, Montana, western Nebraska, western North Dakota, western South Dakota, Wyoming, Utah, Colorado, northern Arizona, and New Mexico. Since 1998, it has been declining in population. The areas of greatest decline have been reported in western and central California, western Oregon, western Washington, and British Columbia. From southern British Columbia to central California, the species has nearly disappeared. However, the historic range was never systematically sampled.
Agricultural and urban development has resulted in bumblebee habitat becoming increasingly fragmented. All bumblebee species have small effective population sizes due to their breeding system, and are particularly vulnerable to inbreeding which reduces the genetic diversity within a population, and theoretically can increase the risk of population decline.
Between 1992 and 1994, B. occidentalis and B. impatiens were commercially reared for crop pollination, shipped to European rearing facilities and then shipped back. Bumblebee expert Dr. Robbin Thorp has hypothesized that their decline is in part due to a disease acquired from a European bee while being reared in the same facility. North American bumblebees would have had no prior resistance to this pathogen. Upon returning to North America, affected bumblebees interacted and spread the disease to wild populations. B. occidentalis and B. franklini were affected in the western United States. B. affinis and B. terricola were affected in the eastern United States. All four species' populations have been declining since the 1990's. Additionally, these four bumblebee species are closely related and belong to the same subgenus; Bombus sensu stricto. Dr. Thorp has also hypothesized that the B. impatiens species may have been the carrier and that different bumblebee species may differ in their pathogen sensitivity. In 2007, the National Research Council determined that the major cause of decline in native bumblebees appeared to be recently introduced non-native fungal and protozoan parasites, including Nosema bombi and Crithidia bombi.
- Pocket Guide to Identifying The Western Bumble Bee Bombus occidentalis 
- P. H. Williams et al. (2012). "Unveiling cryptic species of the bumblebee subgenus Bombus s. str. world-wide with COI barcodes". Systematics and Biodiversity 10: 21–56. Retrieved May 30, 2012.
- The Xerces Society for Invertebrate Conservation
- Status Review of Three Formerly Common Species of Bumble Bee in the Subgenus Bombus 
- Invertebrate Conservation Fact Sheet - Bumble Bees in Decline