?Thanks, Randy.  Good info.

> He compared survival of colonies above and below an arbitrary daily 
> natural mite drop of 50 in October.

That is quite high. we used to freak at a 5 or 10 drop.

Doing a very rough conversion, assuming steady-state and using a 100-day 
mite time to live, that gives us a 5000 mite number, mostly phoretic at that 
date, I assume.  I also assume Frank is still working out of Bakersfield, so 
maybe there is some brood?

Anyhow, this is very rough, so take the 5,000 and a late October population 
of 30,000 (8 lbs. of bees) and we get 5/30=17%.

> Unless treated for mites, those colonies did not survive.  Most of those 
> below that number did.

In a 300-bee sample that would be 50 mites.  OK.  No argument there.  The 
odds are *very* poor for such hives.  I'm surprised it is that high, but 
then my calcs are very rough.  Could be out by 50-100% either way.

> 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.

The problem in such studies is "n".  "n" is necessarily small and the hives 
in the study necessarily share a common history and location.  History is 
likely a wild card, as is location.

Genetics is another and chances are that the study did not use all the 
strains available.  I'm thinking of the Russians here. properly managed, 
they are hard to get much above 6% according to what I hear.

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

I think that any commercial beekeeper could have saved him the effort, and 
most would halve that number or quarter it.

My own take-home interpretation of the presented data was that the 3% 
(or20-30 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.

Seems we calculate differently.  I'd be interested in how 20-30 converts to 
3%. in your situation.  I gather we are using different assumptions.  I 
checked the drop against washes in my outfit last year and got a good fit 
with my method and timing, albeit on a small sample.

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).

Impressive.  I guess that answers my questions about history and location.

One huge caveat in this is the use of packages.  They are a special case, 
and seem to have less vulnerability to varroa collapse than overwintered 
colonies, possibly due to different virus profiles (which is confirmed 
anecdotally).  I suppose that may explain why most of us would think hat 50 
is twice too high and maybe 4X. 

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