A new thought.

This weekend at a bee meeting someone suggested caging the queen to
interrupt the brood cycle and thus lower the varroa numbers.

You really don't need to eliminate varroa, you need to keep the level down.
Pretty labor intensive for commercial guys but as part of an Integrated pest
management plan .. maybe your fall treatment when the hive is boiling over
and you could use a few less bees?

Comments .. timing?

Dave