Why Do Bees Swarm? How Honey Bee Colonies Reproduce

Swarming is colony reproduction

In the western honey bee, the individual bees cannot start a new colony on their own: a queen cannot build comb or forage, and workers cannot produce new females. The unit that reproduces is the whole colony, so it does so by splitting in two.

A swarm usually contains the old queen and, typically, around 10,000 or more workers of all ages. The workers left behind keep the comb, the stored food, the developing brood and several queen cells, from which a new queen will emerge. If all goes well, there are now two colonies where there was one. A large colony may also release one or more smaller afterswarms, each led by a young unmated queen.

Why and when do bees swarm?

Swarming happens mostly in late spring and early summer, when colonies are growing fast and flowers are plentiful. In much of Europe and the northern United States the peak is roughly May and June. Leaving early gives the new colony time to build comb and store enough honey before winter.

Several conditions push a colony toward swarming:

• Crowding: the brood nest is full of bees, brood and stored food, with little room for the queen to lay.
• Weaker queen signal: in a crowded nest, the queen’s pheromone spreads less well, which allows workers to start rearing new queens.
• Plenty of food and young bees: a strong flow of nectar and many young workers.
• Genetics: some subspecies, such as African and Africanized bees, swarm much more often than others.

How a colony prepares to swarm

Before swarming, workers build queen cells and the queen lays eggs in them. Workers feed the queen less, and she slims down so that she can fly, as a laying queen is too heavy for a long flight. Workers about to leave fill their honey stomachs with enough food to build new comb and survive for a few days.

The swarm usually leaves on a warm, sunny day, often around midday, shortly before the first new queens are due to emerge. Thousands of bees pour out of the entrance in a roaring cloud, then settle nearby, often on a branch, fence or post, in a cluster around the queen.

How scout bees choose a new home

While the cluster hangs quietly, a few hundred of the oldest, most experienced bees act as scouts. They search the area for cavities and inspect each one, walking around inside to judge it. A good home usually has:

Back on the cluster, each scout performs a waggle dance that points to the site she found, the same dance foragers use for flowers; see how bees communicate. The better the site, the more times she repeats the dance, so good sites attract more scouts. Scouts also gradually stop dancing over time, which allows weaker options to fade.

The swarm’s vote

The American biologist Thomas Seeley, who studied swarms for decades and described them in his book Honeybee Democracy, showed that swarms decide by a kind of vote. The decision is made at the candidate site itself: when the number of scouts present at one cavity reaches a quorum, about 20 to 30 bees at the same time, those scouts switch from dancing to signaling. They make piping sounds that tell the cluster to warm up its flight muscles, and after about an hour of warming, the swarm takes off.

Scouts lead the flying swarm by streaking through it toward the new site, which may be several kilometers away. The process usually takes from a few hours to two or three days, and swarms almost always choose the best site on offer.

Why are swarming bees so calm?

Honey bees defend their nest, honey and young, but a swarm has none of these. The bees have also just filled up on honey. As a result, a resting swarm is usually remarkably gentle, and beekeepers can often collect one with bare hands. They can still sting if they are poked, sprayed or knocked about, so it is best to leave a swarm alone.

If a swarm settles in a garden, a local beekeeping association can often collect it and give it a home in a hive. Otherwise it will usually move on within a day or two.

Do other bees swarm?

Swarming is typical of honey bees and also occurs in some tropical stingless bees. The buff-tailed bumblebee, wasps and hornets do not swarm: their colonies are founded each spring by a single queen, and only new queens survive the winter. The mass flights of locusts are sometimes called swarms, but that is a completely different phenomenon; see why locusts swarm.