Western bumblebee
Bombus occidentalis, the western bumblebee, 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.
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TerrestrialTerrestrial animals are animals that live predominantly or entirely on land (e.g., cats, ants, snails), as compared with aquatic animals, which liv...
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OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
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starts withWestern 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 (T7 if male) starting from the abdominal segment closest to the thorax and then working ventrally.
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. occidentalis 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.
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 successful foragers returning to the nest can stimulate their nestmates to forage, although presumably like other bumblebees, they cannot communicate the actual location of resources. This phenomenon is often referred to as 'foraging activation'. 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. The mechanism by which foraging activation occurs is not well understood, but it is possible that the returning forager, which before unloading its cargo will spend some time running around the nest and interacting with its nestmates, releases a pheromone that induces foraging behaviour. Furthermore, the sudden influx of high-quality nectar may itself stimulate foraging behaviour.
Although bumblebees 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.
Threats to this species include:
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 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.
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. The California Department of Fish and Wildlife evaluated this petition in a report for The California Fish and Game Commission completed in April 2019. On June 12, 2019 the California Fish and Game Commission voted to add the four bumblebees, including Bombus occidentalis, to the list of protected species under the California Endangered Species Act. 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" was eventually overruled, because the explicit intent was for all invertebrates (therefore including insects) to be qualified for protection under this legal definition.
