So...

We know that screened bottom boards are not a varroa control by
themselves, but we knew that last year.

We know that they increase the efficacy of a Varroa treatment (Delaplane
et al), even when the bees exhibit resistance to the treatment!

We know they are a good tool for easily checking mite levels.

We know they have been used in cold climates (Denmark and New Hampshire)
for the past twenty years with no problems.

We know that there might be an increase (French-Canadian study) in
Varroa over winter but not sure since the data was not statistically
significant. But we know there is a decrease in warmer weather
(Delaplane) but not statistically significant. So both studies tell us
little because screened bottom boards are not a Varroa control by
themselves.

We know that screened bottoms lead to increased brood production
(Delaplane) in the beekeeping year as well as, when left open over
winter, during spring buildup with more surviving bees and brood (Danish
study), which yields more bees. (And more Varroa, which may explain the
French-Canadian results, but just speculating.)

We have conflicting data on keeping the bottom open all winter. The
French-Canadian study would say no while the Danish study and experience
in New Hampshire would say yes (as well as my own experience). If the
increased varroa of the F-C study is because of increased brood, which
is a result of keeping the bottom open, then there would be a
correlation (my supposition).

Conclusion: They are a good tool for beekeepers and have few, if any
drawbacks as long as you use them as a part of and not exclusively for
Varroa control. Which is what we knew last year.

Bill Truesdell
Bath, Me.