Just a brief input on this subject. We have been trialling these for a couple of years now. Whilst we cannot comment on their effect on varroa, there appear to be no adverse effects on colony development and possibly even a benefit in wintering. The floors we are using are made in Germany and Sweden and have been produced there for many years, so it is not a new thing. Particularly in the case of Sweden, where they are apparently very popular, I cannot see how they would be much used if the cold caused problems. The floors in question are permanent year round migratable floors made from tough, hard, expanded polystyrene, and the centre portion (approx 50%) of the floor is replaced by a mesh screen. They are not completely compatible with wooden hive bodies as they are manufactured as one component of a fully polystyrene hive. Murray -- Murray McGregor