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The two subspecies are occidentalis and mckayi which are threatened and of special concern respectively. Subspecies occidentalis ranges from California to British Columbia and east to Saskatchewan and South Dakota. Subspecies mckayi ranges from British Columbia to the Yukon and into Alaska and the Northwest Territories. They are holometabolous insects with three adult forms. The forms include the queen, the non-reproductive female workers, and the male workers.

Photo by Matt Bowser

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Nosema bombi is a obligate microsporidian parasite found in bumble bees that affects the gut and tissues. It causes reproduction performance as the gynes readiness to mate is reduced which lowers the fitness while lowering the survival rate of the individual. N. bombi reproduces in the Malpighian tubules and affects the thorax muscles, the muscle tissue surrounding the around the gut epithelium, the body fat, nerve tissue, and the midgut. Their abdomens become bloated and, in some cases, may become paralyzed. The tissue structure is destroyed due to the heavy infection and the N. bombi spores are then expelled through
feces. Infected workers are observed to appear slow, clumsy, dirty and may die early. This is assumed to be from the inefficiency of their cleaning behavior and the diarrhea induced by the parasite. The dead individuals are also not carried out of the nest. The inability of the workers to fulfill their duties may have led to the decline of the colony. Even if the colony is heavily infected, they may also produce healthy sexual individuals thus maintaining the population. This was observed to occur to approximately 35.1% of the colony.

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The parasite can spread rapidly through the colony if the queen is infected as the queen is the main source of transmission for the next generation as N. bombi appears to have little effect on her ability to reproduce. In some cases, the transmission of N. bombi is done by through trophallaxis, or by ingesting contaminated comb, water sources, and honey stores. The rate of infection increases dramatically within high-density breeding facilities for commercial colonies which often have few genetic variances.

Photo by gailhampshire

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As Bombus occidentalis pollinates a variety of species and their colonies are easy to rear and manage, they were a common chosen pollinator in greenhouses in North America. The native bees were captured and brought to Holland to turn into rearing stocks. An accepted hypothesis is that N. bombi was introduced during the domestication of commercial colonies through contact with the European species Bombus terrestris but there is no hard evidence of this. Between 1992-1994 due to an expansion in the market queens within the colonies were exported to Europe for rearing within the same facilities as B. terrestris. The colonies were then moved back to North
America for greenhouse and open field pollination use. The colonies were abandoned in North America in 1997 after the infestation of N. bombi. Soon after, the wild populations of B. occidentalis began to decline. N. bombi in the wild populations is from pathogen spillover from the escapees from the commercial
colonies. It is known that the commercially reared colonies frequently foraged outside of the greenhouses overlapping in range with the wild population. N. bombi is believed to be the cause of the managed commercial colony collapse in North America in the 1990s. Since the decline of B. occidentalis was not consistent with all populations of bumble bee
species, it is assumed that it was more of a targeted decline of Nosema rather than from environmental causes.

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High levels of are prevalent within 35% of individuals of declining species but may also be apart of the normal host-pathogen dynamics. N. bombi was noted as low even before the population declines in North America. The subspecies mckayi in Alaska has a prevalence of 44% and the population is currently deemed as healthy. This may be also due to the fact that commercial colonies are less used in the north or that the mckayi subspecies
responds differently than occidentalis subspecies in the south, or that there could be multiple strains of N. bombi. More research would be needed
to reach a conclusion.

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N. bombi may not be the causing factor of decline in the commercial
greenhouse colonies but may have greater effect on the wild populations. This species is ecologically significant as they pollinate a variety of native plants, and the commercial colonies are useful for the pollination of greenhouse crops as well are culturally significant to Aboriginal groups. N. bombi requires more study and as the sustainability of B. occidentalis should be a point of concern as they are an important species in the ecosystem and economy.

Photo by Stephen Ausmus