In a conversation with Diana Sammataro last year she mentioned that Varroa
(and I believe mites in general) tend to be a bit "leaky". Good air flow
when a mite is "questing" for a host could lead to a higher mortality
rate.

We are in the middle of our high heat season in Florida right now. I just
split a large hive and in the burr comb between two supers I broke open
well over 100 drone cells. A good inspection found no sign of mites in
this hive. This hive has seen no treatment this year.

I am about to inspect a couple of hives for mite load in the next few
weeks. I will have to see how the other hives are doing. I had to treat
some hives this year so I will have to compare the ones treated with
Apistan to the untreated hives.

I am considering an experiment where I put in a sticky board and lift the
brood frames one at a time and spray them with sugar water to induce
grooming. Very labor intesive but if I find a high mite drop it may be
something to add to SOP for a hive inspection.