Dan, I would not be concerned about the bees on the outside of the screen
bottom boards.  They seem to congragate there, but will move into the hive
once they figure out the situation.  This has been my meger experience with
screened bottoms.  Mites, it seems will not crawl back into the hive once
they hit the skids, but lay in wait for another ride, so I would remove the
plastic and let the air flow.