Why bees swarm, and what to do if you see one

A swarm is how a colony reproduces

An individual honey bee is not the reproductive unit — the colony is. A colony that has grown large enough divides: the old queen leaves with roughly half to two-thirds of the workers to found a new home, and the original nest raises a new queen from the brood left behind. One colony becomes two. Everything about swarming makes sense once you hold that frame, and almost nothing about it makes sense if you think of a swarm as an angry mob.

The trigger is congestion combined with abundance. A colony that has filled its cavity, has no room to expand the brood nest, and is bringing in more forage than it needs will begin building queen cells. This is normal, healthy behaviour and a sign of a colony doing well.

What actually happens

1. 1Queen cells are builtWorkers construct several large downward-hanging cells and the queen lays in them. The colony now has replacement queens developing, and about a week to eight days before the first emerges.
2. 2The old queen is slimmed downWorkers reduce her feeding so she loses weight and can fly — a laying queen is normally too heavy. This is a deliberate preparation, not a side effect.
3. 3The swarm leavesOn a warm day, usually late morning to early afternoon, the old queen and a large part of the workforce pour out of the entrance and take to the air. The bees gorge on honey before leaving, which is central to why they are so docile.
4. 4The cluster formsThe swarm settles nearby — a branch, a fence post, a hedge — in a dense hanging cluster around the queen, usually within a hundred metres of the old nest. This is a staging post, not a destination.
5. 5Scouts search and the colony decidesHundreds of scout bees search for cavities and return to dance on the surface of the cluster, advertising what they found. Better sites are danced for more vigorously and recruit more scouts; the process converges on one site over hours or days, and the swarm flies to it. It is a genuine collective decision, and one of the best-studied examples of distributed decision-making in any animal.

Why a swarm is not dangerous

Defensive behaviour in honey bees is about defending a nest — brood, stores and a cavity. A swarm has none of those. It has no comb, no larvae, no honey stores and nothing to lose except the queen at its centre, and the bees have filled their crops with honey before leaving, which makes them physically less able and less inclined to sting. A settled swarm is the most placid a large mass of honey bees ever is, and beekeepers routinely collect them wearing very little protection.

What to do if you find one

1. Keep people and pets a reasonable distance away, and close nearby windows.
2. Do not spray it with anything — not water, not insecticide. A wetted swarm is a distressed swarm, and killing them is both unnecessary and a waste of a healthy colony.
3. Call a local beekeeping association. Most countries have a swarm collection service and most will come free, because a swarm is a free colony.
4. Note whether it is hanging in the open or going into a structure. A cluster on a branch is straightforward to collect; bees entering a wall cavity or chimney is a removal job rather than a collection, and is more urgent.
5. If nobody comes, a cluster in the open will usually move on by itself within a few hours to a couple of days.