Re: Catch them today, and they leave the next day.
I did my first swarm retrieval last week where someone called me that they had a swarm in a bush in their yard. I have had to re-collect cut outs that absconded and swarmed to a nearby tree which is very similar.
I really thought this swarm would not stay. I put a piece of queen excluder over the entrance, and a few days later I put in a frame of open nectar and capped honey. I got them last Thursday and they are still in the cardboard nuk and bringing in pollen. No brood added yet. I'm going to do that when I move them to their wooden hive.
I think that a QE works to a degree for a swarm, but as the queen is slimmed down for swarming I don't see it as a guarantee. I think it works well for cut outs. I have a piece cut that covers all the entrance and tape/push pin it to the hive.
I also stayed out of the box except to add the feed frame a few days later.
"Rule Three of beekeeping...Never cease to feel wonder" Laurie R. King--
March 2010; +/- 30 hives, TF