>
> >The phenomenon defies definition by a simple formula.  That is why I still
> hear educated people speculating that there are safe thresholds higher
> than 3 to 5%.


Dr. Eischen compared winter survival of colonies in Calif under various
regimens of mite treatment, fumagillin feeding, and protein supplementation.
  He compared survival of colonies above and below an arbitrary daily
natural mite drop of 50 in October.  Unless treated for mites, those
colonies did not survive.  Most of those below that number did.

Over a wider range of mite drops, there was a strong linear correlation for
survival.  But as you say, there is no sudden cut off at a safe
threshold--the more mites, the poorer the survival.

He did not attempt to determine an economic threshold, other than that 50
mites falling per day was clearly too many.

My own take-home interpretation of the presented data was that the 3% (or
20-30 mite natural drop) infestation level that you mention is in the right
ballpark for those depending upon making a living off of overwintered
hives--meaning almond pollinators.

Good nutrition was #2
Nosema management, since it was the only treatment left in his trials, came
in #3.  He didn't test pesticide exposure (these were hives from a large
commercial migratory operation that is exposed to plenty of pesticides) or
any other variables.  However, well-nourished colonies with low mite levels
had high winter survival and graded strong going to almonds, despite having
been run in a multi-crop pollination rotation the previous season.  This
result suggested to me that the pesticide insult to the colonies was of far
less consideration than that of controlling mite levels (Bob, I'm not saying
in any way that pesticides are harmless--just that it appears that one can
take strong bees to almonds despite pesticide exposure).

In his data on the Coordinated Action Project, in which eight research
facilities around the country set up apiaries of colonies started from
packages, on fresh foundation, without appreciable exposure to agricultural
pesticides, and without mite treatment, that only in the two apiaries from
the warmest areas (Florida and south Texas) did mite levels grow high enough
to cause serious mortality in the first year (dead serious).

His research was clearly targeted to answer questions about supplying bees
for almond pollination.  But the results are useful for all beekeepers.

Randy Oliver

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