Kill the drone laying queen.
Uncap a few drones and check for varroa.
Unite the colony with a sound colony over a sheet of newsprint.
Make a split at a later date to replace the "lost" colony.
Of all the methods I have tried this works the best so far .
When I set the queenless portion on the newsprint in addition to a slit or
two I offset the box so they have a way out.