Katie Stepp said:
<My own answer comes from a Swiss beekeeper I know who uses open varroa
floors himself.  As this is sort of a new thing in the States I was curious
about it.  He said the open bottoms have not had any ill effect for
overwintering. He lives near Geneva, so winters are pretty harsh.  His hives
are also in a bee house year round, so they are a little more protected than
if they were standing outside.>

Consider this: If the bottom of the hive is basically an open screen mesh,
wouldn't that leave hives especially vulnerable to another "new thing" in
America: small hive beetles, which live part of their life cycle in the dirt
around the hives? Seems like such a open mesh would create especially easy
access.
John