Tom,

I don't think you can rely too much on departing swarms taking varroa mites
with them. Remember that the mites spend most of their time in cells.

Losing a swarm has some advantages.  You have achieved natural requeening.
If you catch the swarm you can use it in many profitable ways.  You don't
have to worry about swarm control for the rest of the season, thus saving the
time and stress of opening the colony which has been shown to reduce honey
production.

Regards,

Chris Slade

Chris Slade