Western Bumble Bee

Appearance

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. "B. occidentalis" in these areas have yellow hair on front part of thorax. They are also marked by black hair segments on the basal section of the fourth abdominal segments have black hair and whitish lower edge of the fourth and fifth abdominal segments. In addition, they also have sparse whitish hairs that may appear black on the sixth abdominal segment, and an entirely black head.

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 and a 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 starting from the abdominal segment closest to the thorax and then working ventrally.

Distribution

"Bombus occidentalis" was once one of the most common bee species in the North West America. They have been found from the Mediterranean California all the way up to the Tundra regions of Alaska, making them one of the bees with the widest range geographic range. However, recently there has been a noticeable decline in population. In the past decade, the population of "B. occidentalis" has dropped by around 40.32%. The disappearance of these bees have been especially significant in California, western Oregon, and western Washington. The range and persistence of "B. occdientalis" has also gone down by around 20%. Some scientists point to the rise of "Nosema", a parasite, as the reason for the decline in population. Others say that the population decline could have come because of the invasion of European honey bees. A recent study in 2016 suggests that the Western bumblebee population is rebounding, possibly due to evolutionary development of resistance to "Nosema".

Status

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 1990s. Additionally, these four bumblebee species are closely related and belong to the same subgenus; "Bombus sensu stricto". Dr. Thorp has also hypothesized that "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".

Behavior

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 flight 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 regurgitate it in the nest. Pollen is collected and put into "pollen baskets" located on the hind legs. Nectar provides carbohydrates while pollen provides protein."B. occidentalis" are social bees and can communicate to other worker bees where to find pollen or nectar. Foraging activation is the increase in colony foraging upon the arrival of a successful forager, and "B. occidentalis" does this through scent. The amount of recruitment a returning bee is able to garner depends on the amount of nectar that the bee is able to bring back. When the bee is able to bring back a big amount, more bees will forage in the same vicinity of the first bee.

Bees that return with higher quality of nectar have been observed to have longer paths upon returning to their nest and thus spend a longer time inside. Longer traveling period inside the nest allows more bees to get in contact with the bees with higher quality foods.The "nectar robbing" behavior is exhibited when the organism obtains the nectar of the flowers without getting in contact with sexual parts of the flowers. "B. occidentalis" can be seen displaying this behavior due to the shortness of their tongues. Instead of going through the normal route, "B. occidentalis" use their mandibles to make holes to circumvent the process. The mandibles of "B. occidentalis" are thus understandably more toothed than that of other bumble species to help them cut into the flowers.

Importance of Nectar

It is crucial for "B. occidentalis" to maintain high levels of nectar for their colony. Not only does the level of stored nectar affect the temperature of the colony, but deficiencies in nectar cause a significant change in behavior due to low energy of the bees. When energy abundant colonies are threatened by predators, they assume the natural defense behavior, moving about loudly to deter the predator. However, low energy colonies will remain still in their colonies. Although temporary low energy periods do not affect the survivability of the larvae, it increases the colonies' susceptibility to predators and increases the time of development for the larvae.

Predators

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
⤷  Invasive plant species that may directly compete with native nectar and pollen plants
⤷  Natural pest or predator population cycles