In the spring, the bumblebee queens emerge from their winter rest. They bask for a while in the sun to warm up, find some flowers, load up on nectar, and then start looking for a nest site. They’re looking for pre-existing cavities: old mouse nests are fine, but almost anything will do. Once they find one, they start laying eggs. They gather nectar and pollen for their hatchlings until the first group of worker bees are ready to fly. Once the worker bees get started, the queen never leaves the nest again. It’s easy to tell these early season workers apart from queens: the size of a bumblebee is determined in part by how much she had to eat as a larva, and the first cohort of workers is living off of the effort of a single bee. They’re very small, perhaps half the length of their mother.
Over the summer, the workers gather more nectar and pollen to feed themselves, their mother, and new cohorts of worker bees. The nest grows, both in numbers and in size: each new cohort of workers is better fed than the last, and so grows to a larger size before reaching adulthood. Eventually, there are anywhere from a few dozen to a couple hundred workers supporting the nest.
In the fall, the last and largest group of worker bees is hatched. These are next year’s queens. They meet up with males from neighboring nests, and the fertilized queens look for a safe place to sleep through the winter. The other members of the colony, including the old queen and all of the males, die before the onset of winter.
The climate of Puget Sound is warm enough that you can find the odd bumblebee queen at any time of year, but they don’t emerge in substantial numbers until the onset of (relatively) warm spring weather… like the weather we’ve had the past few days. I’ve seen as many as a dozen queens at a time basking in a single Pieris japonica bush, and I’ve seen quite a few crawling around on the ground, looking for nest sites. They’re particularly attracted to mulch: you might think that mulch is good habitat, but it’s not. It’s actually lousy habitat, but all the little shadows confuse the queens. From the queen’s point of view, any one of those shadows might be a mouse hole! … and then it turns out that they’re not.
If you want to create good bumblebee habitat (and of course you do!) it’s easy: just leave the ground alone. Bare dirt, old rockeries, dead shrubs, dry grass, old mouse nests… it all makes for an excellent neighborhood for native bees.