'Swarm control', in the world of beekeeping, actually refers to swarm prevention, keeping the bees from ever getting to the point of swarming in the first place. If you are being chased by angry bees, this writeup will do you no good at all.
When a honey bee hive gets too crowded, the bees start getting ready to split the hive. The worker bees allow more virgin queens to develop (normally the virgin queens kill each other, so the workers have to keep them separated if they want a good supply). Next, the queen bee will take most of the older workers and drones and set off to start a new hive. After the prime swarm with the old queen has left, sometimes there will be one or more afterswarms, with virgin queens heading out to start their own nests. This is how bees spread, and new hives are formed in the wild. A bee swarm is a good and healthy thing.
Why not to swarm.
A swarm may be 1,500 to 30,000 bees in size, and after a swarm the old hive will expend most of its energy in rebuilding its population. The worker bees will also be working overtime to construct and fill the new hive. Honey production drops dramatically, and there is no guarantee that the beekeeper will have possession of the new hives, or that any given one of the hives will survive. Honey bees seem to be genetically programmed to swarm every couple of years, and not surprisingly, they usually do this in the spring, during prime honey season. If the prime swarm isn't big enough to drain the honey producing capabilities of the hive, well, that just means the hive has the resources to support an afterswarm (secondary swarm) -- or two, or five. Needless to say, beekeepers want to prevent this.
Healthy hives with lots of room to expand into will be less likely to swarm -- but it is natural for hives to swarm every couple of years, so good hive-keeping is not enough to keep your bees in place. Here are some of the more common methods of swarm control.
Requeening: Queen bees release a pheromone that suppresses the queen rearing behavior in the worker bees. Younger queens release more of the pheromone than do older ones, so changing your queens yearly is a great way to suppress swarming. If you are buying replacement queens, you can also order ones with positive genetic qualities, for example, low swarming tendencies. Of course, this means you have to find and kill all of your queens, and care must be taken in introducing foreign queens, or the workers may try to kill them.
Removal of queen cells:
Hives can't swarm unless they have at least two queens. If you are a very attentive (perhaps even obsessive), you can monitor the hives for any developing queen cells, and kill them before they hatch (or kill the queen and all but one queen cell). You will have to be inspecting your hives every 7-10 days during swarming season to make certain that no new queen cells have been started. Expanding the hive will help keep the production of new queens down, so if you don't have too many hives, destroying the queen cells and expanding your hive as soon as you see them appearing may be an effective method of preventing swarming.
A swarm will always leave the brood (larvae and pupae) behind. If you have a hive that looks ready to swarm, you may be able to trick it into staying put by removing the brood.(The nurse bees should migrate to the brood on their own). You can the make the brood into a hive of its own, add the brood to weaker hives that you wish to strengthen, or add it back to the nest later in the season, when the risk of swarming is over. There are many different variations on the artificial swarm method, the two most common being Pagden's method and Heddon's method.
The Demaree Method:
This method also involves moving the brood, but this time you're not moving it into another hive. Instead, you move all but a couple frames (frames are those boards full of honey comb that you see beekeepers lifting out of their hives) of uncapped brood away from the queen, to the top of the hive. You place at least one queen excluder, a grid with holes large enough for a worker bee to pass through, but too small for a queen, between the brood and the rest of the hive. The queen still has a small brood with her, but she cannot reach the majority of the hive's brood. This upper section of the brood is checked for developing queens, and any found are killed. (The bee keeper will have to check for developing queens again in 5-10 days, but after that there will be no larva in the upper brood of the appropriate age to be raised as queens). In between these two sections is left an area into which the queen can continue to expand her brood.
The Snelgrove method is closely related to the Demaree method, but it leaves one developing queen bee in the upper brood, allowing the hive to be eventually split into two. This is often an improvement over the Demaree method, in which the hive grows very large and strong, but hard to manage.