> The varroa shaker does not extract 100% of the mites. Two weeks ago we 
> compared it to icing sugar.  icing sugar extracted about 45% of mites and 
> the varroa shaker extracted 90% if the mites.  When mite levels are too 
> high extraction of mites is less. The same applies when number of bees are 
> larger than 3oo.

This last point is important.  I am finding that if a sample is too large, 
even by a little, then there are enough bees to act as a filter when 
inverting the jar.  I suspect the maximum accuracy is with a little less 
than 300 bees in the sample.  Anyone making a home-made version, as I did at 
one time should consider that the area of the screen determines the maximum 
sample size that is practical -- a smaller unit would only be able to do 
smaller samples.

Anyone having problems can simple take two sample of 150 bees each and add 
the results.

I also suspect that the two samples may well be sufficiently similar in 
results that many who are just looking for a rough estimate would be happy 
with using 150 or 200 bees per sample instead of 300.

Of course, the math stays the same.  Divide the number of mites seen by the 
number of bees in the sample and multiply by 100 to get the percent 
phoretic.  Even 100s of bees make for easier math.

 

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