Why not put a queen separator under the box to prevent swarms leaving?
Q. One of my swarms left a day after I brought them to their new home. What do you think about putting a queen separator under the box to make sure they don’t take off?
A. Some beekeepers recommend that, but there’s risk. A swarm is often composed of multiple swarms—a large primary one with the old queen, and smaller ones with virgin queens. If you know you have the old queen (maybe she’s marked with a previous year’s color?), a queen separator would keep her at the new location.
But, if you retain a virgin queen, you’ve captured the swarm for nothing. If she can’t get out and mate, the colony is doomed.