>When I worked at the Dyce Lab at Cornell University we tried for about five years to get bottom screens to work. We wanted them to work, but they never did. They never affected mite buildup, honey production, brood, overwintering, neither negatively nor positively. No significant difference was ever found. 

I agree with you. A few studies seemed to show some effect, but as many did not find any.

One suggestion that seems plausible to me is that screens may help eliminate mites which show some resistance to acaricides and are only stunned by the chemicals by getting them out of the hive.

Years back, I only used one strip of Apistan once a year and had excellent control. We seldom saw more than five mites in a several day drop test in the fall. We never used any other control, except a few times we used one mitewipe 3x for tracheal.

The secret??  We did not use the Apistan in the fall like everyone else.  We reasoned that in the fall, the bees are on the bottom board and any mites weakened by the chemical and falling, but surviving can just get back up on a nearby bee.  In the early spring, when the mites are weak and the cluster is small, the fall from that top box down through the cold hive is bound to be fatal for a mite that is only weakened by the fluvalinate.  The brood area was small at that point, too, so getting the strip right where the mites were located was easy, as well.

The smaller clusters required only one strip and, of course we did not treat any hives that had died, so our Apistan use was anywhere from a third to a fifth of what others were using.

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