I've now applied 140ml/65% formic acid to 4 hives, used an insulated eke and appear to have had reasonably consistent evapouration over 8 days. I had expected to see lots of dead juvenile varroa on the sticky boards, but only the odd bee larvae and varroa adult. The queens are laying well.
This is a first attempt, how do you measure success or is this a sign of no affect?