Randy Oliver (www.scientificbeekeeping.com) ran a trial with screen bottom boards and published the results in the February 2011 issue of American Bee Journal. There was a 4% to 5% reduction in efficacy compared to colonies treated in hives with solid bottom boards.
However, both open screen and solid bottom boards saw over 90% drop in mite loads. Mesh floor/screen bottom boards should be closed off to prevent formic vapours from dumping out. Bees are not built to move air up through a screen, so open mesh floors should not be considered as additional ventilation. The bottom hive entrance must be fully open during treatment.