Bumble bees form new colonies every year. Only young fertilized queens overwinter. They emerge early in the spring and look for a nesting site – generally underground, in an existing cavity.
A queen builds two thimble-size wax cells. One is for the first eggs. She fills the other one, a pot, with nectar and uses it to store food while she cares for her eggs.
There are from 5 to 20 eggs, laid in a cell on top of a supply of pollen and nectar. The queen then covers them with wax and settles on the cell to incubate the eggs. They hatch from three to five days later. The newly hatched larvae feed in the cell. After seven to eight days, each larva forms a cocoon, inside of which it turns into a pupa. Adult workers – sterile females – emerge after 12 to 14 days and will care for the next generation of eggs produced by the queen.
The queen continues to lay eggs and to build cells for her brood. She lays three or four eggs in each cell, which will all hatch into workers.
In late summer, the queen begins laying unfertilized eggs that will hatch into males and fertile females. Once they reach adulthood, these insects will leave the nest and mate. The fertilized future queens then seek shelter to overwinter. The rest of the colony dies off when temperatures plummet.