Bumblebees (Bombus) do not swarm in the same manner as honey bees. Swarming—the mass departure of an existing colony with an old queen to form a new colony—is a behavior unique to perennial bee species like the honey bee. Bumblebees follow a fundamentally different life cycle that makes this kind of colony division and migration unnecessary.
The Annual Life Cycle of a Bumblebee Colony
The fundamental difference between bumblebees and honey bees lies in the longevity of their colonies. Bumblebee colonies are annual, meaning they perish completely each year, while honey bee colonies are perennial and survive the winter. A bumblebee colony begins in the spring when a single, mated queen emerges from her winter hibernation, or diapause. She is solitary at this stage, foraging for pollen and nectar and searching for a suitable nest site, often an abandoned rodent burrow or a cavity.
The queen constructs wax pots to store nectar and lays her first batch of eggs on a clump of gathered pollen. She incubates this initial brood by warming them with heat generated from vibrating her flight muscles. The first generation of bees, which are all small, female worker bees, emerge after about four to five weeks.
Once the first workers emerge, the colony enters its social phase, and the queen focuses solely on egg-laying. The workers take over foraging, nest maintenance, and caring for subsequent broods. The colony grows steadily throughout the summer, reaching a peak size of 50 to 400 individuals, depending on the species.
How Bumblebees Reproduce and Disperse
The annual life cycle dictates that the colony must reproduce before cold weather arrives, which is the alternative to swarming. In late summer, the colony shifts its focus from producing workers to producing reproductive individuals: new queens (gynes) and male bees (drones). The queen achieves this by laying unfertilized eggs, which develop into males, and by ensuring that some fertilized eggs are fed more frequently to develop into new queens.
The new virgin queens and drones leave the nest to find mates from other colonies, ensuring genetic diversity. Drones often patrol specific flight paths, awaiting the emergence of a virgin queen. After mating, the males die, and the newly fertilized queens begin intensive foraging to build up fat reserves for the winter.
This dispersal of individual, mated queens is the mechanism for species propagation, replacing the need for a mass swarming event. As the days shorten and temperatures drop, the founding queen, the workers, and the drones all perish, and the original nest disintegrates. Only the new, fertilized queens survive the winter by locating a suitable spot, such as loose soil or leaf litter, to enter diapause and emerge the following spring.
Behaviors Often Mistaken for Swarming
Because bumblebees do not swarm, sightings of large numbers of bees are often attributed to other common behaviors. One frequently mistaken behavior is nest surveillance, which looks like a small, swirling cloud of bees outside a nest entrance. This group consists entirely of male bumblebees, often 10 to 20 individuals, waiting to mate with new queens as they leave the nest.
This “dancing cloud” is a reproductive gathering, not a colony migration, and the male bees involved cannot sting. Another behavior that causes alarm is sudden, intense buzzing and flying near a nest. If a bumblebee nest, often located in a bird box, under a shed, or underground, is disturbed, workers will exhibit defensive behavior to protect the brood.
These localized defensive responses or reproductive aggregations are temporary and involve a relatively small number of individuals compared to the thousands involved in a honey bee swarm. The largest bumblebee colonies rarely exceed a few hundred bees, lacking the coordinated, long-distance migration characteristic of a true swarm.