Hi Jay and everyone,
You need to use a mesh size that will allow the mites to drop through but not allow the bees to go through.  I use 8 mesh hardware cloth with 1/8 inch wire spacing.  This replaces the solid wood of the bottom board.  I wintered two colonies last winter here in St Paul, MN outdoors with the mesh bottoms and currently have 4 on the mesh bottoms.  I only use a regular solid bottomboard under the screen bottom for treating with formic acid ( gel packs here ) otherwise the bottom is open all year.  My colonies are on stands that are about 8 inches high with no wrapping but good wind protection.  We sometimes but not always get enough snow to reach to the bottom of the hives but didn't last year and don't yet this year.  The colonies reared brood this summer to the bottoms of the combs on the bottom and even raised queen cells (swarm cells) from the bottom bar of the bottom box.

They really do seem to help reduce the increase of varroa mites but I have not done any measurements.

FWIW

blane

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Blane White
MN Dept of Agriculture
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