>>A question I have is if I find even 1or 2 
>> relatively high mite counts in a yard 
>> of 24 hives is it best to treat the whole yard?

> That is a logistics/economic decision.

That reply pre-supposes that the "drift" issue is well understood for
varroa, and that there is significant drift within a yard to promptly and
equally infest every colony in the yard.   There's been a stack of evidence
growing for the last 20 years showing that robbing is far more a force in
varroa reinfestation than drift within a yard.

It follows that one might want to save the across-the-board formic
treatments for the fall dearth and robbing.

The charts I show my class are from these papers:

"Invasion of Varroa destructor mites into mite-free honey bee colonies under
the controlled conditions of a military training area" by Frey, Schnell, and
Rosenkranz, in JAR, 2011.  They had colonies at 1 meter, 30m, 400m, 1300m,
and 1500m (about 1 mile) from a weak and varroa-infested colony in the
middle of a known bee-free area.  There was quite a bit of robbing at 1
meter when a very strong colony was placed next to a weak one, but not
nearly as much from a "normal strength" colony at one meter.  There numbers
went down drastically for colonies further away than 1 meter, but shot back
up again at 1500 meters.  The conclusion here was that bees engage in
robbing at distances of about a mile or more, except for the special case of
a very very strong colony overpowering a weak one in the same yard, a
condition that a competent beekeeper can easily avoid.

There's also the paper "Reinfestation  of an acaricide-treated  apiary  by
Varroa"  by Greatti in Experimental  & Applied Acarology, 1992, where mite
count data went up only during dearth periods.

There's also "Seasonality of  honey bee colony invasion by  Varroa
Jacobsoni" by Sakofski, Koeniger, and Fuchs in Apidologie, 1990, which did
not show much varroa "re-infestation" at all during blooms, but quite a bit
during dearths, pointing to robbing as the primary mechanism by which a
colony reinfests itself via its own foragers.

There's also "Population growth of  Varroa jacobsoni Oud in Mediterranean
climates of  California" by Kraus and Page in Apidologie, 1995, which has a
nice chart showing when mites coming in the door on constantly-treated hives
went up or down, and these events synched up with dearth periods.

And Eric Mussen's Nov/Dec 2011 newsletter " We experimentally removed older
Apistan strips and placed two new strips in a couple hives every month for a
year and monitored the mite drop on sticky boards. There were some mites in
the spring, but basically none through the summer. Then, in mid-September,
when robbing started and heavily infested colonies began to die off, we
found sticky board counts that were impossibly high for mite reproduction
within the colonies considering the previous mite levels. We went from
practically none to thousands of dropped mites within a week."  And you can
read on in that same newsletter to see his cogent analysis of
"laissez-faire" and "treatment-free" practices, entitled "Nuisance
Beekeeping".

None of them found varroa reinfestation at a steady rate.  None found
reinfestation increased during blooms and periods of heavy foraging, which
would be what one would expect if "drift" was a significant issue.  They all
found pretty clear evidence that robbing was the sole proximate cause of
varroa reinfestation.

What to do?  Take some of the money you'd save by buying fewer Formic pads,
and use some of it to buy entrance reducers for all the novices in the
county.  Tell them that they want to protect their colonies against robbing
in the fall, and they do.  The colonies that are not dwindling due to varroa
will not be harmed at all by this, and those that are dwindling will be able
to defend a smaller entrance until the bitter end, when both the colony and
the varroa they carry, die.  Your colonies will not be reinfested as much.

Yes, one could carpet-bomb the whole yard with formic "to be sure", and this
may make sense given the cost of labor vs the cost of the treatment, but I
think it is important to realize that the varroa treatments work much better
than beekeepers think, and what appears to be a failure of a treatment to
control may well be the result of the success of the treatment creating a
colony strong enough to go out and rob a dying colony well away from the
yard.

I can't see doing ether-rolls on every colony in a yard, but sticky-board
drop counts are a breeze in these days of high-rez digital cameras - pull
the board, shoot it, scrape it off, re-spray with PAM, and reinsert, and do
the counting in the evening. Gotta mark the sticky boards with the colony
designation, but it allows someone who is "detail oriented" to do the
counting on a computer display, and allows those able to handle the physical
labor and the heat to do the apiary work.

Not sure that the concept of "sentinel colonies" stands up to the findings
of these papers, sadly.  The use of sentinel colonies may be one of the big
reasons that we still struggle to control varroa after nearly 30 years.

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