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
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Murray McGregor