The mites in that high testing colony may be resistent to the effects of apivar. If it were me, I would immediately retreat with Apiguard. Put it between the brood boxes right in the middle and seal off any upper entrances or mid level entrances so the bees need to haul it thru their brood nest to dispose of it outside. Putting it under the cover often allows the bees to just wall it off and then it is not effective. I have a dose on the top bars of a colony with a 2 1/2" rim but it is cool enough that this strong colony is just leaving it in place. Today it gets moved and supplimented to between hive bodies. If you can borrow an Oxalic vaporizer and operator for the first application of three, that would be good too. I think hopguard and formic products to be dangerous and too hard on queens.
I'd go with OAV, if you can. If the queens has reduced her laying, which is typical this time of year, it would be an easy way to knock mites down fast.
If not, you can immediately apply another type of treatment with a different chemical makeup. You may also want to refrain from using Apivar for a few years if you're having that type of infestation after treatment.
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