At least two experiments about bottom boards letting the varroas drop through have been made in France.
The first one uses a 3 mm (or 1/8 inch) hardware cloth and was reported in July-August issue of “L’Abeille de France et l’apiculteur”. In the very beginning, there was a lot of bees with withered
wings but later on everything became okay again. Besides, this hive without any treatment is told to give more honey than the others.
The other experiment is the one I personally carry out with the bottom board which have been designed by Mr. Legris. Instead of hardware cloth, you have 34 mm diameter tubes under each frame. The
space between the tubes is 3,5 mm which is large enough to eliminate wastes as well as varroa mites. Bees cannot however go through. (for more details and pictures :
www.apiservices.com/plateau-anti-varroas/).
The immediate advantages are to save money (no more chemicals to buy) and to save time (no more treatment to apply, bottom boards always clean).
I prefer the bottom board with tubes because isolation against cold is good and I personally think that varroas elimination is better through larger holes. Moreover,  an industrial production would
lead to prices not much higher than ordinary bottom boards. Plastic tubes are cheap but stainless steel hardware cloth is not.
I ordinary count between 50 and 200 varroas mites dropped each week depending on the hive and they are in very good health.
Of course, two experiments are not sufficient to tell the problem is away but these first result should lead other people to try this method and report their own observations.
Jean-Pierre LE PABIC
RUEIL-MALMAISON (10 km from Paris)
FRANCE
 
Ernest Gregoire a écrit:
 
> I found time to search the Archives for Wire Mesh Bottom Boards,
> WMBB,and thought this would be a good time to challenge the group
> to do some research into just how they work in various parts of
> the world.
>
> Letting the Varroa drop through the wire mesh sounds like a good
> idea to me. Having been told that this will eliminate up to 15%
> of the mites, I am willing to give it a try.
>