>  I have everything on screens and have watched a decrease in varroa. Less
> winter loss by 20%. It is not the screens alone.

I use IPM with Russian X My Hygienic bees. Screens sounded like a good idea ,
good enough for me to build 180 of them.
 The varroa situation is more complex than screen bottoms or one line of bees.
Weather is a factor and stress has part to play in it also.Hive location has
shown me a difference in mite load of a hive. I had one yard out of 5 that had
Varroa last june. It was in  a more shaded area at 4,000 feet. Two yards that
were at the same elevation in a very sunny ( southern and no trees in the way )
did not show any varroa in the larva , just a few through the screen. ( 100 in 2
weeks on the bottom board.)
I'm making progress up here in western Washington.This year is the best in 5 so
far. Screens are only part of the equation.
I will say that they help me.Studies that are done give us information. They do
not work in all areas unless the work has been done in all areas.The more
research that goes on ,the off we will all be in regards to the mites.We need to
look at all tools that are out there and don't get hooked on chemicals. That's a
short trip with a bad ending.
The Best
Roy