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26 February 2011
In February each year, weather permitting, we begin to see the first bumble bees of the calendar year. If you take note, these bumble bees seem to be really big compared to the ones you will see later in the spring and throughout the summer. That is because these first bumble bees are the queens who have emerged from their winter hibernation.
In San Francisco, we see two types of bumble bees, Bombus vosnesenskii and Bombus californicus, although the Bombus vosnesenskii seems substantially more prevalent (pictured above).
They are similar to one another. They both are black with a single stripe at the end of the abdomen, but the head of the vosnesenkii is yellow while the head of the californicus is black (pictured to the right).
Like honey bees, bumble bees are social. They live in families, “colonies”, headed by a queen. Unlike honey bees, however, bumble bee colonies are considerably smaller and have an annual lifecycle.
During late summer, bumble bee colonies produce a number of new virgin queens who leave the colony, mate and find a suitable winter hibernation site. Typically, this is a small sheltered cavity in the ground, under debris or in an opening of a building/structure. The new queen hibernates and emerges in late winter/early spring, forages for nectar and finds a suitable nest site to raise her colony.
Once the queen finds her nest site, she constructs a wax cup or “honey pot” in which she stores nectar. The honey pot remains open so she can feed on the nectar as needed. She also collects pollen, clumping it into a ball mixed with nectar. She lays fertilized eggs on the pollen ball and covers it with wax. Then, like a bird, she incubates the egg ball by sitting on it. In a few days, the eggs hatch into larvae. To keep the larvae from chilling, the queen goes out only on short foraging trips for nectar and pollen, and returns to the nest to feed the larvae and keep them warm.
The larvae pupate and hatch as adult worker bees (all female). The workers get to work provisioning the colony with nectar and pollen, they feed and care for the queen and larvae, and otherwise help the colony population to grow. The bumble bee queen remains in the hive and devotes herself to laying more eggs and incubating them.
In late summer, the colony raises a number of new virgin queens as well as male bees. The virgin queens go on mating flights, finds other males and mates with them. Like honey bees, the male bumble bees die after fulfilling their reproductive role. The newly-mated queen looks for a winter hibernation site. In the meantime, the mother queen, workers and unsuccessful males reach the end of their life span and die. The newly-mated bumble bee queen hibernates over the fall and winter, emerging in late winter to raise her own colony. The annual cycle begins once again.
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written by Madeline Marrow, May 23, 2011
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