In message <001101c2e75a$8dbde320$18ac58d8@BusyBeeAcres>, Bob Harrison
<[log in to unmask]> writes
>It is very hard to get (in my opinion) an accurate varroa count in a strong
>hive when large scale brood rearing is going on *because* of the amount of
>varroa in sealed cells.
Advice from CSL/DEFRA (Bee Inspection, testing and research, UK) is to
multiply the daily mite fall by 30 to get a reasonable estimate of the
total mite population during brood rearing; 400 without brood - once
everything has settled. It seems reasonable enough, when high
effectiveness treatments are used, the figures are reasonably close
(accurate is a relative term - 20% or even 30% would be good enough?).
The factors should be reasonably stable so long as we are in a steady-
ish state, not just at the start of any rapid rise in brood, nor at the
finish of all brood rearing. The in-between times are much more
difficult as the multiplying factor is between 30 and 400, too wide a
variation to trust during transition times - hence the very approximate
100x suggested. The 30x allows for mites in the cells.

How you get a threshold is another matter. The size of the colony and
its susceptibility to viruses are variables. Our Regional Bee Inspector
suggested (for a typical UK colony) that we revise down the previously
recommended threshold of 2500 to 1000. I will follow up the reasoning.
The original threshold was based on research which showed an increasing
probability of colony demise above 2500. Presumably they are now finding
the demise occurring at 1000 mites. The earlier estimate was in earlier
days - perhaps the viruses are now endemic and kick in earlier.

Our RBO has given us a table for urgent treatment, delayed or less
potent treatment and no treatment. One figure given was that if there
were more than 15% drone brood with mites, we should use an appropriate
treatment immediately. This seems to be getting towards your approach,
where, if my memory serves me, Bob, you referred a year or two ago to %
mites being a better guide to the need for treatment. Absolute numbers
are not reliable, since they must be interpreted. % cells infested is
perhaps rather more reliable.

I hope this helps the discussion. Thank you all for the ideas.
--
James Kilty

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