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Fast Facts
Unlike their cousins Western bumble bees have white patches of hair on their abdomen</li>
They live in a variety of habitats, including flowering grasslands, savannas and alpine meadows
Western bumble bees eat nectar and pollen; carrying pollen from plant to plant, Western bumble bees are excel
lent pollinators making them an important part of the ecosystem
They are currently under assessment in Canada (COSEWIC); main threats include habitat loss and fragmentation,
disease, pesticides, invasive species and climate change
Having round, fuzzy bodies and bright colours, Western bumble bees (Bombus occidentalis) are an endearing and essential part of the grassland ecosystems along the west coast of North America. One of 30 different species of bumble bees
found in Canada, the Western bumble bee is easily distinguished from its cousins by unique white patches of hair on its
abdomen. Like other insects, bees have three main body regions (“segments”): head; thorax and abdomen. These white
hair patches are on the abdomen which is at the opposite end of the bee from its head. The black and yellow colour
pattern on the bee’s body alerts potential predators to the bee’s ability to sting to protect itself.
The name “bumble bee” was given to this family of insects because of the humming sound they make when they fly.
Contrary to popular belief, the humming sound is not produced by the beating of their wings, but by rapid vibrations of
their flight muscles. Bumble bee wings act more like helicopter blades than airplane wings, they don’t just flap their
wings, they twist them! Their tiny wings push the air downwards, lifting the insect up, allowing them to fly. Still, keeping
their furry, robust bodies in the air is hard work. Bumble bees need to beat their wings an astonishing 200 times per
second to stay airborne! It’s A LOT of work being a bee!
The Western bumble bee lives in a variety of habitats including flowering grasslands, savannas and alpine meadows.
These areas provide ideal feeding (“foraging”) ground for the adults, but Western bumble bees may also be seen in
agricultural fields and searching for wildflowers on forest floors. Bumble bees are considered generalist flower visitors in
that they visit and pollinate flowers from a wide variety of plant species, including food crops such as tomatoes, blueberries, cranberries and cherries, among many others.
To house the colony, Western bumble bees also require nesting sites. Typically, Western bumble bees nest underground
or in dense tufts of grass. Some of the best nesting sites for Western bumble bees are abandoned rodent burrows. Near
the end of the summer, the next generation of queens will begin searching for overwintering spots, sometimes choosing
hollowed out trees or other underground animal burrows to wait out the winter months.
Bumble bees are not typically the target of many predators, perhaps due to the colour of their bodies which warn other
animals to stay away. This warning colouration (“aposematic colouration”) is a good thing too because bumble bees have
a very important role to play in our ecosystem.
Collection of nectar and pollen by the Western bumble bee makes them excellent pollinators. For flowering plants to
produce seeds, they must exchange pollen with the same or different flowers of the same species. When bumble bees
land on flowers in search of food, they inadvertently gather pollen from the anthers of the flower onto the hairs covering
their bodies. Then, when they move to a different flower, some of this pollen is left behind on the stigma, fertilizing the
egg cells of that plant. Once plant egg cells have been fertilized by pollen, the fertilized eggs develop into fruits containing seeds. When the seeds are released and germinate they develop into new young plants. Without pollinators like bees
it would be difficult for many plants to produce seeds; in fact, some plants rely so much on bee pollination they cannot
transfer their pollen in any other way. Although some plants are pollinated by other means such as by butterflies, flies,
mammals or the wind, the humble bumble bee plays one of the most important roles of all in grassland ecosystems.
Different castes (groups or classes) play different roles in a bee nest or colony. Three different castes make up the social
structure of a bee colony and include the queen, workers and drones. It’s the job of the worker bees to collect pollen and
nectar for the family. To aid them in pollen collection, they have a special “pollen basket”, called the corbicula, on their
hind legs. It is a small depression surrounded by long hairs which help them carry pollen back to the nest. Upon their
return, the worker bees share what they’ve collected. The nectar is a source of carbohydrates and the pollen is an excellent source of protein for the bee larvae in the nest.
Honey is also part of a bumble bee’s diet. Honey is formed when the bees ingest and regurgitate nectar a number of
times. It is then stored in the hive for a day when the weather doesn’t allow the bees to collect other food sources.
Bumble bees don’t make as much honey as honey bees because only the queen bumble bee overwinters. Therefore,
there is little need for bumble bees to stockpile food like honey bees do.
Western bumble bees are considered generalist foragers, meaning they don’t depend on one flower for food. They
collect pollen from a wide variety of plants such as Melilotus, Cirsium, Trifolium, Centaurea, Chrysothamnus and Eriogonum genera, which include thistles, clovers and wild buckwheats.
The Western bumble bee is a social insect that lives in very structured colonies. There are three classes, or castes, of
Western bumble bees in any colony: the queen bee, worker bees and drones. In the early spring, queen bees emerge
from hibernation in overwintering sites and look for places to build nests for their new colony. Once they find a good
place – in an abandoned rodent burrow or dense grass, for example – the queen lays her first eggs and collects food for
the larvae when they hatch. The larvae eventually grow into female worker bees, which assume the responsibility for
caring for the nest and finding nectar and pollen for the colony as the queen lays more eggs. Near the end of the
summer, new queens and male drones hatch—these male drones mate with the new queens, who will then seek out
their own overwintering sites before the end of the fall. As winter settles in, the old queen, worker bees and drones die,
leaving the next generation of queens to start new colonies the following spring.
Conservation Status
Under Assessment (COSEWIC) – No recovered strategy yet but IUCN has a special sub-committee examining this
species in Canada
A combination of habitat loss and fragmentation, disease, pesticides, invasive species and climate change are
threatening the Western bumble bee
As recently as 1998, the Western bumble bee could be seen over a wide range in the Western and central parts of North
America, found throughout Alaska and continuing as far south as Arizona and New Mexico. Once a common sight, the
Western bumble bee has begun to disappear from much of its historical range in recent years. This species is thought to
be close to disappearing completely in many areas including California and Oregon. This rapid and alarming decline in
the species’ population size is thought to have come about from a combination of threats that have been building up
over several years.
As with most animals facing threats to their survival in the wild, the Western bumble bee has faced many successive
years of habitat loss and fragmentation. Loss of habitat makes it more difficult for the bumble bees to find foraging and
nesting sites, and the fragmentation of their remaining habitat into smaller patches means that what food remains is
farther away and more dangerous to access. Habitat fragmentation can also make it more difficult for drones and queens
of different colonies to mate with each other, leading to inbreeding and a loss of diversity.
Although loss and fragmentation of Western bumble bee habitat has made life more difficult for the species, they are
also facing several other threats. Diseases introduced to wild bumble bees from ones raised in captivity in other parts of
the world are infecting large numbers of colonies, as are non-native parasites like the tracheal mite and other diseasecausing pathogens. Further threats come from the widespread use of pesticides—bumble bees are exposed to these
chemicals when they attempt to feed on plants treated with pesticides or when the pesticides wash off farm fields and
into nearby wild spaces during rainy weather. Invasive plant species that are pushing out native wildflowers are also
continually decreasing food sources for bumble bees and many other pollinating insects.
If you want to learn more about insect pollination check out:
The Xerces Society for Invertebrate Conservation. The Western Bumble Bee.
Colla SR, MC Otterstatter, RJ Gegear, and JD Thompson (2006). Plight of the bumble bee: pathogen spillover from commercial to wild populations. Biological Conservation 129:461-467.
Koch J, J Strange, P Williams. Bumble Bees of the Western United States. US Forest Service and the Pollinator Partnership. (page 5)
IUCN Red List. Bombus franklini.
Encyclopedia of Life. Bombus occidentalis.
BBC One. Richard Hammond’s Invisible Worlds: Speed Limits. “Impossible Flight”.