Some background-

I do a single OAV with a tray for mite drop in mid spring and mid-summer
and get near zero mites on the tray. I do another after extracting in
July/August (still near zero) and extracting in late September. I do
another in Mid-October and the final one around Thanksgiving when the
colony is near broodless. So six one time mite check treatments and all
also act as mite killers, especially the last.

Usually there are few is any mites, but Fall is the time of year when
untreated hives in my area crash and invade my colonies. Hence the check in
mid-October. Last year all was fine but this year the October count was
over 800 on one hive and 400 on the other (I treat all three hives but only
monitor two).

I treat every third day and the counts by the fourth treatment were 25 and
4. I have one more treatment to go because I go on the assumption that I
want near zero mites because of virus and not just the mites. Plus I like
to treat over the capped brood period plus a little extra (15 days). Had I
not got the near zero results, I would treat until I got there, but I have
never needed to go further.

I use to go every 7 days (when there was no recommended dose) then 5 (the
recommended)  and now three days between treatments if I need more than
one, as I did this year.

To boil it all down, here in Maine, and I would extrapolate it to most cold
climates with decent winters, if you do not check for varroa after the
temps go down and clusters start, you will fool yourself if you go by early
fall mite counts. You can go from zero to sixty in a matter of days and
those healthy colonies will collapse either now or later because of
invading bees carrying both varroa and virus.

The nice thing about OAV with a tray for mite counts is it gives you a much
more accurate representation of trends than formic and alcohol washes. You
see sudden increases with the OAV but not necessarily with alcohol washes.

I have not lost an over wintered hive to Varroa  since I started using OAV.

Bill Truesdell
Bath, Maine

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