Swarming is how honey bee colonies reproduce at the colony level. When a hive grows large enough, roughly half the worker bees leave with the old queen to establish a new colony elsewhere, while the remaining bees stay behind and raise a new queen. It looks dramatic, thousands of bees pouring out of a hive and forming a buzzing cloud, but it’s a natural and essential part of honey bee biology.
Why Colonies Swarm
The root trigger is overcrowding, but the mechanism is chemical. A queen produces a blend of pheromones that spread through the hive via worker-to-worker contact. These pheromones essentially signal “the queen is here” and suppress the colony’s urge to raise new queens. In a small or moderate colony, every worker gets enough of the signal to stay content.
As the colony grows, workers at the edges of the hive receive less and less of this pheromone. Research on pheromone dispersal shows that in populous colonies, bees at the periphery pick up a significantly lower dose than bees near the queen. When enough workers perceive that the queen’s signal has faded, they interpret it as a cue that the colony is too large for one queen, and they begin preparing to split. This typically coincides with spring and early summer, when nectar and pollen are abundant and the colony’s population is booming.
How the Colony Prepares
Weeks before the swarm departs, workers begin building special queen cells along the bottom and side edges of the comb frames. These swarm cells are distinctive: they hang vertically, point downward, and a colony preparing to swarm often builds 5 to 20 of them at once. The queen lays an egg in each cell, and workers feed the developing larvae a diet that turns them into queens rather than workers.
Meanwhile, the colony gradually reduces how much it feeds the old queen. She slims down enough to fly, something she hasn’t done since her original mating flights. Workers also begin gorging on honey, filling their stomachs for the journey ahead. This stored fuel gives the swarm enough energy to survive while searching for a new home.
The Day the Swarm Leaves
When conditions are right, usually a warm, calm day, 50% to 70% of the workers rush out of the hive, herding the old queen to the entrance. They launch into the air in a massive cloud that can contain tens of thousands of bees. Despite how intimidating this looks, swarming bees are typically docile. They have no brood or honey stores to defend, and their stomachs are full, which makes them less inclined to sting.
The swarm doesn’t fly far at first. Within minutes, the bees cluster on a nearby surface: a tree branch, a fence post, the side of a building. This temporary resting spot, called a bivouac, can look like a football-sized clump of bees hanging in a mass. The cluster usually stays put for a few hours to a couple of days while scout bees fan out to find a permanent home.
How Scouts Choose a New Home
Scout bees are experienced foragers that switch roles to become real estate agents. They fly out from the bivouac searching for suitable cavities, and they’re remarkably picky. Honey bees prefer a nesting site that is dry, elevated off the ground, clean, unoccupied, and well insulated, with a small, defendable entrance and a volume of about 40 liters (roughly the size of a large moving box).
Cavities that meet all of these criteria are relatively rare in nature, which makes the selection process competitive. Multiple scouts may return to the cluster and “advertise” different sites by performing waggle dances on the surface of the bivouac. Each dance communicates the direction and distance of a site, and more enthusiastic dances recruit more scouts to inspect it. Over hours or days, scouts converge on a consensus. Once enough scouts agree on the same location, the entire swarm lifts off and flies to their new home, sometimes traveling several kilometers.
Primary Swarms vs. Afterswarms
The first swarm to leave a hive is called the primary swarm. It carries the original, mated queen and the largest share of workers. This is the biggest swarm the colony will produce, and it has the best chance of establishing a successful new colony because the queen can begin laying eggs immediately.
Back in the original hive, the remaining bees are left with all the brood, the honey stores, and multiple developing queen cells. The first new queen to emerge often kills her rivals while they’re still in their cells. But sometimes multiple queens emerge in succession, and each one may leave with a group of workers. These smaller, secondary departures are called afterswarms or cast swarms. Afterswarms are led by virgin queens that still need to mate before they can lay fertilized eggs, so they carry more risk. A colony that throws off several afterswarms can weaken itself badly, leaving fewer bees in the parent hive than can sustain it.
What Swarming Means for the Bees
For the species, swarming is the only way honey bee colonies spread genetically. A single hive becomes two (or more), each with its own queen, its own workers, and a chance to thrive independently. It’s colony-level reproduction in the same way that cell division is reproduction for single-celled organisms.
The process carries real costs for both groups. The departing swarm must find and furnish an entirely new cavity, building comb from scratch and stockpiling enough honey to survive. The parent colony has to successfully raise and mate a new queen, a process that takes several weeks during which no new eggs are laid. If the new queen is lost on her mating flight, or if bad weather delays mating too long, the colony can fail. Despite these risks, swarming has been the honey bee’s primary reproductive strategy for millions of years, and healthy colonies in favorable conditions pull it off reliably.
Swarm Cells vs. Emergency Queen Cells
If you keep bees or are learning about them, it helps to know the difference between the queen cells built for swarming and those built in an emergency. Swarm cells are planned. Workers build them on the bottom edges and sides of frames, they’re well formed and large, and there are usually many of them at once.
Emergency queen cells look quite different. When a queen dies unexpectedly, workers scramble to convert an existing worker larva into a queen. They pick a larva already sitting in a regular cell and extend that cell outward to accommodate a queen’s larger body. These cells appear wherever eggs and young larvae happen to be, scattered across the face of the comb rather than clustered along the edges. They tend to be smaller and rougher looking because the bees are working in a hurry, often starting with a larva that’s already two or three days old rather than freshly hatched. Spotting swarm cells early is one of the main ways beekeepers anticipate and manage swarming before half their colony flies away.