Bombus occidentalis

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Bombus occidentalis is commonly known as the Western Bumble Bee. Bombus occidentalis is just one of the approximately thirty bumble bee species currently present in the western United States and western Canada [1] . Bumble bees are considered to be true social insects as they have communal nest sites and share division of labor.


General Information

In the taxonomic hierarchy, all bumble bees belong to the Genus Bombus within the Family of Apidae. Apidae also includes honey, carpenter, cuckoo, digger, stingless, and orchid bees [2]. Apidae are specialists for feeding on flowers and gathering nectar and pollen. Bumble bees belong to the Order Hymenoptera (greek origin hymen = membrane + ptera = wings). Hymenoptera are the only other Order, besides Isoptera (termites), to have evolved complex social systems with division of labor. Aculeate Hymeoptera (certain wasps, bees, and ants) are the only insects that can sting. Unlike honey bees, bumble bees can sting repeatedly.

Taxonomic Hierarchy[3]


Bumble bees are homometablous (complete metamorphis).

  • Immature Stage
                - Grub-like
                - Well developed head
                - Chewing mouthparts
                - Legless and eyeless
  • Adult Stage
                - Maxillae and labium form proboscis for collecting nectar
                - Well developed compound eyes
                - Tarsi usually 5 segments
                - Hind wings smaller than front wings and linked together by small hooks (hamuli) 

The color markings of bumble bees act as a warning to predators that they can sting if threatened. Bumble bees are covered with branched or feathered hair that pollen sticks to.

There are three types of bumble bees; queens (female), workers (female), drones (male). Although the queens and workers tend to have similar colorations and the same morphological (physical) features, the queens tend to be much larger in size [4] .

Bombus occidentalis workers have three main color variations [4].

  • 1st Color Variation - Found from northern CA, north to BC, east to southwest Saskatchewan and to Montana [4]
                     - Yellow hair on front part of thorax
                     - 1st through basal section of 4th abdominal segments have black hair
                     - Lower edge of the 4th and 5th abdominal segment are whitish
                     - 6th abdominal segment often has sparse, whitish hairs, but may still appear black
                     - Hair is entirely black on head
  • 2nd Color Variation - Found along the central coast in CA [4]
                     - Yellow hair on the sides of the 2nd abdominal segment and all of 3rd abdominal segment
                     - Reddish brown hair on 5th abdominal segment
  • 3rd Color Variation - Found form Rocky Mountains to Alaska [4]
                     - Yellow hair on the thorax behind the wings 
                     - Yellow hair on the rear of the 2nd and all of the 3rd abdominal segments


  • Queen (female)
       - Lay eggs
       - Fertilize eggs to produce more queens and workers (unfertilized eggs will become drones)
       - Initiate new colonies
       - Initial food collector and care of larvae until colony is established
  • Workers (sterile female)
       - Collect nectar and pollen 
       - Protect nest
       - Feed queen, larvae, and drones (only until Fall)
  • Drones (male)
       - Mate with queen
       - Cannot feed themselves

Life Cycle

Bumble bees have an annual eusocial colony [5]. In Hymenoptera the females develop from fertilized eggs while the males develop from unfertilized eggs. Females control egg fertilization therefore, regulating the sex ratio of offspring (haplodiploid method of sex determination) [2] .

  • Autumn - Queens and males mate, males die, queens find wintering site and enter diapause ( a form of hibernation)
  • Spring - Queens emerge from diapause and initiate new nest sites (often built in abandoned rodent burrows)
  • Summer - Increased colony size where workers become the majority
  • Late Summer - Reproductives produced and colony size decreases

Bumble Bee Species Identification

All insects have three main body parts; the head, thorax, and abdomen [4]. Bumble bee 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 down. Bumble bee abdominal segments can be colored with black hair, orange/red hair, yellow hair, white hair, brown hair, mixed hair, or bald (no hair). Bumble bees facial hair color (black, yellow, or mixed) can also aid in the identification between species.

Facial Coloration Variations
Facial Coloration Variations
Abdominal Segment Coloration
Abdominal Segment Coloration
Abdominal Segment Coloration

Male or Female Determination

There are a few ways to determine the sex of Bombus occidentalis [1]. The males (drones) have seven abdominal segments while the females (queens & workers) have only six [1]. The drones antenna have thirteen segments while the females have only twelve[1]. Drones have no stingers. Additionally, the hind legs of the females tend to be wider and fatter with a pollen basket often visible[1]. Drones have thinner hind legs that do not have pollen baskets[1]. Another clue to sexual identity among the Bombus occidentalis species is the time of year that they are being observed. Queens are the first to appear in the Spring and then the workers appear. All Bombus occidentalis females can then be seen throughout the Summer and into early Fall. The drones will only appear in the late Summer and early Fall.

Pollen Basket
Pollen Basket


Bombus occidentalis are generalist foragers [2]. Because Bombus occidentalis do not depend on any one flower type, they are considered to be excellent pollinators. Bumble bees are also able to fly in cooler temperatures and lower light levels than many other bees [6]. Additionally, bumble bees perform "buzz pollination". This behavior is displayed when a bumble bee 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 [6]. Tomatoes, peppers, and cranberries are some of the plants that require this type of pollination [2]. For these reasons bumble bees are considered to be more effective pollinators than honey bees. Bombus occidentalis have been commercially reared to pollinate crops such as; alfalfa, avocados, apples, cherries, blackberries, cranberries, and blueberries [6].

Workers collect nectar and store it in their stomach. Nectar is then converted into sugars and then spit back up at the nest. Pollen is collected and put into "pollen baskets" located on the hind legs. Nectar provides carbohydrates while pollen provides protein.


  • Spread of pests and diseases by commercial bumble bee 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 bumble bees are particularly susceptible)
  • Invasive species that may directly compete with native nectar and pollen plants
  • Natural pest or predator population cycles
  • Climate change

Status and Conservation

Due to their role as pollinators, loss of bumble bee populations can have far ranging ecological impacts [2]. Bombus 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 [2]. Since 1998, Bombus occidentalis has been declining in population [4] . 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 [4]. The historic range was never systematically sampled [4].

Agricultural and urban development has resulted in bumble bee habitat becoming increasingly fragmented[2]. Size of population tends to decrease as inbreeding becomes more prevalent [2]. Scientific research has shown that as inbreeding increases genetic diversity decreases along with increasing the risk of population decline [2].

Between 1992 and 1994, Bombus occidentalis and Bombus impatiens were commercially reared for crop pollination, shipped to European rearing facilities and then shipped back [4]. Bumble bee expert, Dr. Robbin Thorp, has hypothesized that the Bombus occidentalis decline is in part due to a disease acquired from a european bee while being reared in the same facility [4]. North American bumble bee's would have had no prior resistance to this pathogen. Upon returning to North America, affected bumble bees interacted and spread the disease to wild bumble bee populations [4]. Bombus occidentalis and Bombus franklini were affected in the western United States [2]. Bombus affinis and Bombus terricol were affected in the eastern United States [2]. All four of these bumble bee species populations have been declining since the 90's. Additionally, these four bumble bee species are closely related and belong to the same subgenus; Bombus sensu stricto [2]. Dr. Thorp has also hypothesized that the Bombus impatiens species may have been the carrier and that different bumble bee species may differ in their pathogen sensitivity [2].

Further support for Dr. Thorp's bumble bee hypothesize was shown in 2007 when 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 [2].


  • ISBN 0-03-096835-6
  • ISBN 0-395-91170-2
  • Pocket Guide to Identifying The Western Bumble Bee Bombus occidentalis [1]
  • USDA bumble bee life cycle chart [2]
  • ITIS [Taxonomic Nomenclature] [3]
  • Status Review of Three Formerly Common Species of Bumble Bee in the Subgenus Bombus [4]
  • Invertebrate Conservation Fact Sheet - Bumble Bees in Decline [5]
  • The Xerces Society for Invertebrate Conservation [6]

External links

  • Discover Life [7]
  • Bombus occidentalis Wanted Poster [8]
  • Key to the Bombus of Evergreen [9]
  • Wikipedia [10]