Given - that 2 OAV treatments seven days apart gives 90% control.

We know there is a significant mite drop after the OA treatment, usually
the second day, so it is reasonably fast acting. After the initial drop,the
numbers drop. So it appears to kill most all phoretic mites with the
treatment fairly quickly.

We still are not sure how OAV works but it is assumed that it is through
the contact of the mite's foot pads with OA crystals. In any case it seems
to be a contact pesticide.

Why does it work over such a long period with only two treatments?

Hypothesis:

You want to kill as many mites as possible over the capped brood period
that starts with the first treatment, or 11-12 days after the first
treatment.

Persistence is the key here. If you look at the literature on OA, it stays
around for a long time so it will be in the hive for the seven days, but
not on the emerged bees with their mites. So a seven day interval will
allow mites on the emerged bee to leave that bee in about 5 days to start
the cycle again,  but it is probably longer since you have 5-11 days.

So the issue is -what happens to the mites in that 2-3 max day period as
well as after the next treatment.

My hypothesis (guess) is that many are killed by residual OA on the frames
when they look for a cell ready for them. Since it seems to take about a
day to see mite drop, they can easily get a lethal dose of OA and never
show up on a mite drop count since they are dead in the cell.

So, after seven days, when the next treatment occurs, you have knocked down
about 7/12 of the phoretic mites but none that stayed on the emerged bees
which have come out over that time. But they get killed with the next
treatment. So you have effectively knocked down all the mites over that 7
day period, except those which are still in the last of the capped cells.

When you treat on the morning of the eighth day (seven days later) the
treatment only need be effective over the next four or five days.

But Medhat noted that this is the same amount of time that the OA would be
effective against the mites. plus my guess still is in effect, so the
highest number of mites that could still be around are those on bees which
emerged in the next 4-5 days. But not all of those will survive since mites
do not necessarily stay with the same bee, and can transfer to an OA bee.
Or detach themselves from their bee and walk on the OA.

It is easy to see how you get to 90% kills (some put it at 95%) because you
are leaving a persistent pesticide in the hive that does the mites in as
soon as they leave their original emerged bee.

Then, even after all this, you still have residual OA all over the place.
My guess is that moisture in the hive eventually does it in. But then
serendipity steps in. OA breaks down into Formic Acid and Carbon Monoxide.
Probably not enough to do much, but it is nice to see that OA still can
thumb its nose at the mites even in its departure.

Bill Truesdell
Bath, Maine

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