Bombus vosnesenskii in Bacopa

Bombus vosnesenskii          Family: Apidae
(BOM-bus  voz-nez-SEN-ski-eye)
Subgenus: Pyrobombus
Common name: yellow-faced bumble bee

Adult size: Queens: 0.7 – 0.8 in (18-21 mm)
                       Female workers: 0.3 – 0.7 in (8-17 mm)
                       Male drones: 0.4 – 0.6 in (10-15 mm)

Type: Beneficial

Life cycle phases:
              Generations per year: one
              Egg: 4 days
              Larva: 14 days
              Pupa: 14 days
              Adult: Queen: 9 – 12 months; Female workers: 5 – 28 days; Males: a few weeks

Description: The yellow-faced bumble bee has short, even hair and a short, square face. The fringed hairs on their hind legs form the pollen basket, called the corbicula. Antennae are medium length on the queen and females (workers), longer on the males (drones). The eyes on the queen, females, and males are all of similar size and shape. All of them are patterned with black and yellow hairs. The females and queen are relatively identical in colouring with black thoraxes marked with yellow strips at the head and the T4 segment of their thorax. Colouring on the males is almost the same but varies with more yellow on their sides which extends further up the back on their thorax. Males also have an additional yellow stripe on their abdomen and no pollen sacs on their hind legs. They also do not have a stinger. Only the female workers and queens have a stinger…which is barbless, meaning it can be retracted and extended again and again…allowing them to sting multiple times. However, in general, bumble bees are not aggressive, therefore unlikely to sting unless their nest is disturbed.


Special Notes: One of about 40 species which are native to the west coast of North America, Bombus vosnesenskii is found in British Columbia, Canada…south to Baja, California and Mexico…and touches into western Nevada.

Due to their large size and hairy bodies, they are able to fly in cold, damp weather which makes them a very important early pollinator in our gardens. This species has been a popular pollinator for the commercial greenhouse trade, especially for tomato growers. Even so, the extent of urban sprawl has impacted on nesting density of this bumble bee species.

Where once Bombus occidentalis (western bumble bee) was the most common species, diseases brought to this part of the country by commercially-raised bees from eastern United States, has infected the population of B. occidentalis almost to the point of extinction. Now Bombus vosnesenskii is the most commonly seen bumble bee on the west coast.


Bumble Bee Life Cycle

Queens are the only bumble bees who survive at the end of the season. After mating, they search for a suitable spot where they will hibernate through the winter.

Emerging in late winter or early spring, depending on the bumble bee species, the queen begins foraging for nectar and pollen. Nectar is vital for energy during this chilly period. Pollen is for the replacement of her body fats, plus it provides protein for the maturing of her ovaries. Once she has secured enough pollen for those purposes, she will collect it to horde for feeding her offspring. Supplying plants in your garden such as early spring crocuses, winter heathers, rosemary, pussy willow, berberis (barberry), mahonia (Oregon grape), and daffodils will ensure a food supply for these early pollinators.

When the queen has fed and she is ready to lay her eggs, she searches for a suitable nesting site. This is almost always underground, and typically, they will utilize an abandoned mouse hole. However, they have been known to nest in tussock-type grasses and wooden bird nest boxes.

Once a nest site has been selected, the queen builds a small wax cup which, when complete, she fills with nectar. This will be her food supply for the duration of her egg incubation period. A second wax cup is constructed and a mound of pollen, called a pollen ball, about one inch (2.5 cm) in diameter is built inside the cup. The queen lays her eggs…usually 200 to 300, but sometimes as many as 500…on the pollen ball, which will be the food source for the larvae when they hatch.

Bumble bee eggs are poikilothermic, meaning they are not able to regulate their own temperature independently of the ambient air. The queen must, therefore, incubate her eggs. To do this, she lays her abdomen on the clump of eggs and begins vibrating the muscles in her thorax. Once heat has built up…which can take up to 15 minutes when the ambient air is only 6 °C (43 °F)…she transfers it from her thorax to her abdomen where it will warm the bald patch located on her underside.

Naturally, the queen must leave her eggs for short periods of time in order to feed from her nectar cup. During the time it takes the queen to sate her appetite, the temperature of her eggs will have dropped to the ambient temperature level. These periods of temperature drop do not have any adverse effect on the development of the young larvae, however.

Four days after the eggs are laid, the larvae hatch and begin feeding on the pollen ball. The queen continues to incubate the larvae, leaving them only when she must feed herself. This is when the queen is likely to leave the nest again, if she has run out of nectar in her wax cup.

The larvae will moult three or four times as they continue to eat and grow over the next two weeks. At the end of that time, the larvae transform into pupae. During this stage, the larvae go through metamorphosis to change from a grub-like larva into an adult female bumble bee.

This first generation of female workers then take over incubation and feeding duties from the queen, who in turn, focuses on laying more eggs through into mid-summer.

With subsequent generations of workers becoming adults, the work duties are split. Some workers head out to forage, traveling as far as 3.2 km (2 mi) from the nest as they forage for flower nectar and pollen. The rest of the workers remain in the nest to rear the young, clean the nest, and maintain the temperature of the nest…either by producing heat through their thorax and sending it out from their abdomen or cooling the temperature by fanning their wings near the entrance to the nest.

Come mid-summer, the queen lays fertilized eggs, which will grow into new queens, and unfertilized eggs which will grow into males (drones). These specialized adults emerge from the nest in late summer and do not return to the nest. The males lay their scent on plant material relatively near the ground, in the hopes of attracting a new queen with which to mate. Once a queen is fertilized, the male will die soon afterwards and the queen begins her search for a good hibernation spot to wait out the coming winter.


Hibernation emergence by species:

Bombus vosnesenskii – yellow-faced bumble bee

  • as early as the end of January in some areas; as late as towards the end of February in others


Posted on March 24, 2017