I've only owned a handful of colonies that appeared to "groom" off mites in high volume (thousands of mites) to a screen bottom board...and that was in the late August/early September time frame when the bees were reducing brood area and mites are at a high in population here.  I've ran mostly screen bottoms for over 10 years and have always saw a mite drop of at least several thousand per colony when treating after brood rearing ceases here.  No hard data, but my solid bottom colonies appear to be struggling with varroa a lot more this year than my screened colonies.  

Location:  Atlanta, GA


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