Why and How Do Bees Swarm?

Swarming is the way that honeybees reproduce their colonies. It is, as Robert often says, a "blessed event" for them--like a birth. When they sense that their colony is strong enough to divide, they stop feeding the queen so that she will be light enough to fly. The old queen takes off with about one-third to one-half of the worker bees, leaving behind several developing queens in swarm cells.

A number of factors can trigger honeybees to begin making swarm preparations. One of the most common is congestion in the hive. If they sense that they're running out of room for the queen to lay eggs, they'll prepare to swarm.

Though a blessed event for honeybees, for beekeepers, a swarm means losing up to half the bees in a colony and so beekeepers try their best to manage swarms using a number of techniques. One way they try to prevent swarming is to be sure the colony has plenty of space. They will intersperse empty frames and add more supers (boxes) to the colony to add more room for the colony to grow. They will also make "artificial swarms" or "splits" to give the bees the impression that they have already swarmed. Here's a great YouTube video of how to make a split:

For more information about swarms, check out this link from the University of Nebraska's Department of Entomology.