Swarms obviously do spread Varroa, but on the other side of the coin,
since a swarm has no brood to harbor mites, you can get the most bang
for your buck (or pound, or whatever) when you put Apistan on a new
swarm.  All the mites in a swarm are on adult bees, and therefore, more
exposed and suceptible to fluvalinate.
I have very little problem with swarms absconding, so I put the strips
in the hive right away with a new swarm.  Some people wait a day or two
to let the bees settle in first.
 
Good luck to you guys who haven't seen a mite yet.  Don't underestimate
their destructive powers like I did when they first arrive.  If you do,
they'll kick your butt.
 
Michael Reddell
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http://www.hotcity.com/~mwr