Bill Truesdell wrote:

"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)."
Bill:

This french-canadian study was done by Jean-Pierre Chapleau from "Les Reines
Chapleau" here in Québec,who is a very active and reknown queen producer.
Perhaps Jean-Pierre can bring more precision to what I am writing here but,
as we can read in his study, the explanation of the increased Varroa
population with an OPEN SBB would be that the lowering of temperature affect
the larva maturation time, perhaps delaying 2 to 3 days the normal adult
emerging time.The cell being closed 2 or 3 days more would allow MORE young
Varroa female to become mature before the adult emerge.
Hence, the increase of Varroa population during colder temperature with an
OPEN SBB.
Hope I have helped.

Docteur André Simoneau, b.sc.,d.m.v.
CQIASA
Laboratoire de pathologie animale
867, boulevard L'Ange-Gardien
L'Assomption, Québec
CANADA    J5W 4M9
Tél. (450)-589-5745 poste 275
Télécopieur:   (450)-589-0648