Why colonies swarm

Reproduction at the level of the colony

A honey bee colony is the reproductive unit, not the individual bee. Making more colonies means dividing, and the mechanism is that the old queen leaves with roughly half the workers while the colony raises a new queen from a cell it prepared in advance.

The swarm that leaves has no comb, no stores and no brood. It is a package of bees with a queen, which is exactly what a package is, and it behaves the same way.

What triggers it

• Congestion. A brood nest with nowhere to lay and a hive with nowhere to store.
• Population and season. Peak is spring build-up, which Penn State puts at mid-April to mid-June in the mid-Atlantic.
• Queen pheromone dilution. In a large or congested colony her signal no longer reaches every bee, and the colony behaves as though it is queenless enough to act.
• Age of queen. Older queens produce less pheromone and swarm more readily.

What happens on the day

The colony gorges on honey, the old queen is slimmed down so she can fly, and a large part of the workforce leaves in a few minutes. They cluster nearby, often on a branch, while scouts look for a permanent home.

That cluster is temporary, usually hours. A swarm on a branch is calm, full of honey, and about as unthreatening as bees ever get, which is not what it looks like to a neighbor.

Where they go

Scouts assess cavities on volume, entrance size, height and dryness, come back, and the swarm eventually agrees. Then they are gone, and with them roughly half your bees and your season's crop.

Catching one

A clustered swarm can be collected into a box, and MSU publishes guidance on what to do when you find one. It is free bees with unknown genetics and unknown disease history, which is the trade.

Preventing your own is swarm prevention, and doing the division yourself is making splits. Both are the same biology with you choosing the timing.

References