Why colonies swarm, and how to stop it

It's the second week of June. Anne is weeding her garden when she hears a roar from the bee yard. The air above her strongest hive is full of bees, thousands of them, spiralling up like smoke. Ten minutes later they settle in a great brown cluster on a branch of her apple tree. Her best colony has just swarmed.

So what happened, and could she have stopped it?

Swarming is how a honey bee colony reproduces. The old queen leaves with a large share of the workers, often half to two thirds of them, to start a new home. The bees left behind raise a new queen and carry on. It's natural and healthy for the bees. For a beekeeper, it means losing half the workforce just before the main honey flow.

Here's the key idea, and it surprises people. It isn't strong colonies that swarm. It's crowded ones. When the brood nest fills up, the queen's scent, her pheromone, gets spread too thin among all those bees, and the workers take that as the signal to raise new queens. In Ontario, swarming comes in late spring and early summer, just before and during the main flow.

So how do you see it coming? You look for queen cells. Swarm cells are large, peanut shaped cells, usually hanging from the bottom edges or sides of the brood frames, and there are often many of them. An empty queen cup is normal. A cup with an egg or a larva in it means the colony has started raising queens. Once those cells are capped, a swarm is very hard to stop.

That's why timing matters. Start your swarm checks two to four weeks before the main flow, and inspect every seven to nine days through swarm season. A queen cell is capped about nine days after the egg is laid, so a two week gap can miss the whole thing.

Prevention comes down to room and youth. Add space above the brood before the colony is crowded. Make a split, taking two to four frames of bees and brood from a strong colony, one with five or more frames of bees, to start a new one. Replace old queens with young ones, which produce more pheromone. And use more than one method, because removing queen cells on its own only buys a few days.

And if you do find swarm cells? If the queen is still there, check every frame and remove every swarm cell, then give the colony room or split it. If the queen is already gone, she has left with the swarm. Leave the cells alone, so the colony can raise its new queen.

Now let's change one fact. Suppose Anne had inspected ten days earlier, in late May, and found three uncapped queen cells with larvae in them along the bottom of a frame. The queen is still there. She removes the cells and moves two frames of brood and bees into a new box as a split. The crowding is gone, the pressure is gone, and she has a third colony.

The mistake beginners make is waiting until a colony looks packed before adding space, and checking too rarely in June. By the time a hive looks crowded from the outside, the queen cells are often already capped.

Here is the rule, in one breath. Crowded colonies swarm, so check every seven to nine days before and during the main flow, give room early, split strong colonies, keep young queens, and treat a queen cup with a larva in it as your warning.

Next, what to do when a swarm does get away: catching it.