Hi All,

I've been monitoring a couple hundred test hives all summer, allowing them
to maintain varroa counts (mites per 1/2 cup of bees, 100% recovery) in the
20-50 range and up. Colonies with mite counts up to 10 appear to be
relatively healthy and productive.   I start to see dying brood when counts
get into the 20-30 range.  At 60, a serious virus epidemic generally
begins, and adults with deformed wings are easily found.  At counts above
75, colonies typically go downhill quickly.

The above are for mid- to late-summer counts.  Data from Germany indicate
that success at winter survival drops strongly at over 1000 mites/colony
going into winter.  As the colony approaches fall, at some point it begins
to restrict or cease broodrearing (in cold-winter areas).  At that point,
all mites are forced into phoresy, so it's then easy to figure the total
varroa population in the hive.

For a broodless colony with enough bees to cover 10 frames on a warm day
(the tight cluster will be smaller), at a conservative 2000 bees/frame,
that would be 20,000 bees.  1000 mites/20,000 bees = 5% infestation rate,
or an alcohol wash of 15 mites per half cup of bees.

Here in California, I wouldn't dream of taking hives intended for almonds
into winter at such a high infestation rate.

As Brion noted, we see high hive-to-hive variation in mite counts within a
yard at this time of year.  About 10% of our hives have excessively high
counts.  We work to bring counts down to below 5 per wash, and may move
high-count hives home to blast them with formic acid.

We begin summer treatments for mites in late June, and continue (with
different treatments) as necessary through late summer and fall.  We treat
the entire yard, based upon the highest counts found in the yard.

With regard to Pete's comment about treating by the calendar--we knock
mites down to near nothing in April, and then my calendar is blacked out in
advance to dedicate mid August to mite treatment each year.  So why
monitor?

We monitor all season for three reasons:
1.  To see whether we need to begin treatments prior to mid August, and
2.  To confirm that the treatments were effective, and
3.  To see whether there is an influx of mite immigration in late summer
and fall from hives outside of our operation.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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