Tom's suggestion of adding a frame of open drone brood to queen rearing
colonies should solve the problem of Varroa infestation.  However, the drone
larvae will have to be fed and they will be competing with the queen larvae
for a share of a finite quantity of royal jelly/ brood food.  Without human
interference the normal hive will produce only a few queen cells but a few
hundred drones.  It is likely therefore that the drones may be fed
preferentially although this is an untested hypothesis.
Perhaps the drone brood should be added when it is fully fed and ready for
sealing.
Chris Slade