>In my experience, placing any product that depends upon evaporation
(formic, thymol, other essential oils) under the lid is unreliable.  Better
to place in the center of the brood nest, where the temperature is
constant, and where it is more difficult for the bees to simply fan the
fumes away from where they might do some good.

I have used only thymol crystals since we found varroa here in 1997.  Based
on a paper by Higes, I place 8g of crystals on an old honey jar lid (Petri
dish substitute!) placed on the excluder directly above the cluster; this is
repeated weekly for 3 weeks (rather than Higes original 4 weeks).  An eke
raises the crown board to provide an air space.  This has served me well for
16 years.  With a single brood box on most of my colonies, it is difficult
to see how I could get closer to the centre of the brood nest.

Best wishes
 
Peter
52°14'44.44"N, 1°50'35"W

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