Another thought:

The use of heat to kill or knock down varroa is not new.

How about poking in a very accurate temperature probe, sealing the hive,
giving it a couple of thumps, and waiting for the temperature to hit the
critical level?

Then light large smoker before opening the entrance!

Low cost and environmentally friendly.

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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