[threesixandnine]

If you have some trees around the hives the swarm will usually hang there for a few hours so it is entirely possible to catch it. These swarms are the best since they are ready to build new home and they build really really fast if you catch them and put them into new hive. What is left in the old hive are half of the bees and new queen.

Now about hive health. There is this thing called Varroa and if you let the hive swarm half of it goes with the swarm and the other half stays. Varroa I mean. What is really good with swarming is that the hive now has a virgin queen and she doesn't lay eggs right away and a lot of Varroa dies off because they can't reproduce. They need eggs and bee larvae to do so. The swarm with old queen has an interruption as well because they need to build comb and cells where the queen lays eggs.

In my beekeeping years ago I kept hives healthy ( fight against Varroa ) entirely with letting them swarm.

Just to write it once again since by your writing I think you are confused...Old queen leaves with the swarm.

[luhn]

To clarify the parent comment, Varroa is a parasite that lives on honey bees and can spread diseases harmful to the bees. It's one of the suggested causes of colony collapse disorder.

[tptacek]

They also eradicated feral honeybees several decades ago (all honeybees in the US are alien to the continent; almost all today are livestock, not wildlife).

[bjelkeman-again]

We manually spilt hives and get the same effect, without the swarming and worried neighbours and hunting down the swarm.

[threesixandnine]

One of the ways is to manually split indeed.

Although I must say that natural swarms show greater vigor when building new comb than splits. Even packaged bees built faster than splits. That is of course my experience and you have to keep in consideration that I didn't add any foundation to new colonies so that was maybe one of the reasons. I let them build their own comb on a narrow strip in the frame/top bar and never tried with full foundation.

[shostack]

How do you catch a swarm?

[pygy_]

You make it fall into a box. Shake the branch they are resting on, or brush them off until they fall. You cant tell if the queen made it into the box depending on the behavior of the workers already in there. If you missed it, goto step 1.

[pvaldes]

You can just pick up gently and put it in a box, they do not sting in this phase unless you treat directly the queen.