Ten thousand bees suddenly buzzing outside your house or flying around your playground may seem like a scary thing on an otherwise beautiful spring day. So might coming across a branch in the woods teeming with 20,000 bees. Your first instinct might be to run and hide, to swat at them, or to try and kill the bees. But there's no need to do any of those things. Those bees are swarming and the only thing they're hunting for is a new home.
Most bees you see swarming are honeybees (Apis mellifera). Honeybees live in groups called colonies in dwellings called hives and generally are pretty docile creatures. There are three types of bees: female worker bees, who go out of the hive to collect nectar and pollen from plants for food and who care for and protect the hive and its residents; male drones, who live in the hive to mate with the queen; and the single queen bee herself, who lays eggs and emits pheromones, chemical scents that offer instructions to the rest of the bee hive about social behavior, mating, and maintenance of the hive. As long as all the other bees in the hive can sense these pheromones, which are carried through the hive's food, the colony can lead a harmonious life.
During the summer months, a healthy beehive contains 40,000–80,000 bees. However, as cool weather sets in, bees conserve food rations by evicting the drones from the hive. The remaining members of the colony huddle together in what's known as their winter cluster, a big mass of bees with the queen bee protected at its center. They consume the honey stored in their hive as food, and to keep warm, they rapidly contract their wing muscles, causing them to seem to shiver together. The worker bees shift places to keep the core temperature of the hive warm enough to support them, ranging from 80°F near the queen to 45°F at the outside of the group. In late winter and through spring as the weather warms, the queen will begin repopulating the hive by laying more eggs.
As a queen ages and as the population of the hive swells, it becomes harder for the bees to sense the queen's pheromones. This triggers a response in the worker bees in charge of feeding the queen, who cut back on her diet. She lays a few more eggs, including one that will become a new queen for the hive. Then, she and the majority of her worker bees, who have stocked up on honey from their reserves, leave the hive to find a new place to nest. This behavior is known as swarming.
Because the queen does not often leave the hive, her wing muscles are weak and she likely won't be able to fly far from the hive. She and her worker bees will look for a place to rest nearby, often a tree branch. Much like during the winter, they will huddle together with the queen at their center to protect her. At times, this group can be as big as a basketball! Scout bees will fly off in all directions in search of a new home and will report back regularly. When the scout bees find a new home, the bees will fly to it and quickly move in to resume their normal hive activities.
The original hive still holds the recently hatched new queen bee, a small group of worker bees left behind to guard and care for the queen and the other young bees, and those young, who will replenish the supply of workers and drones. In this way, one over-populated hive becomes two sustainable colonies, something we are in desperate need of, since colonies of bees have died off in recent years in alarming numbers.
If you encounter swarming bees, they are nothing to worry about. Their buzzing can be loud and frightening, but, particularly after they've emerged from their hive, they are quite gentle, so long as nothing threatens their queen. In fact, they're so laden down with honey that it's difficult for them to use their stinger. (The longer they remain without a hive, however, the more of their stored honey they're forced to use and more agitated they may become, so it's best to leave swarming bees alone.) There is no need to kill or move the swarm, since they'll find a new home soon on their own. Do not throw anything or spray water at swarming bees, because they will do anything to protect their queen if they think she is endangered.
It is worth noting two other things: One, bees may find a new home that you don't consider suitable, such as inside the walls of your house. In this case, you should contact a local beekeeping group, who will be able to help you relocate the colony safely. Two, there are some areas of the country, particularly in the Southwest, that have colonies of Africanized honeybees. These bees are more aggressive than other honeybees and are more likely to become defensive when swarming. If you live in an area where Africanized honeybees are common, it is best to quickly and quietly leave the scene of a bee swarm without interacting with the insects in any way.
If you're interested in learning more about bees, check out the ways worker honeybees communicate in Dances with Bees. You can also listen to Science Update podcasts about bee maps, beehive monitors, and how honeybees prefer caffeinated nectar.
Photo Credits (from top): "Swarming bees," by Charles Roper under the license CC-by 2.0, via Flickr; Bee swarm on a tree trunk, by Kevin Platte, All rights reserved; "Swarming bees," by Lotus Johnson under CC-by-NC 2.0 license, via Flickr; "Bees Swarming," by Lars Plougmann under the CC BY-SA 2.0 license, via Flickr; "Swarm" by Justin Leonard under the CC by 2.0 license, via Flickr; Bee swarm being contained for introduction to a new hive, by Kevin Platte, All rights reserved.