The Screened Bottom Board, leave open or closed in winter?

Both our hives have a plastic screened bottom board. I think screened bottom boards are a must for controlling varroa mites.

Varroa mites are a big problem for bees, and with a screened bottom board, mites that fall off bees fall thru the screen and out of the hive. You can have even more mites fall thru the screened bottom board if you dust your bees with powdered sugar. - Here's a video we did on that.

But I've been wondering about the screened bottom board being wide open in the winter. There are different opinions on this, some bottom boards have a wood slide that goes in to close off the screen.

Our bottom boards do not have a slide to close the screen. So I decided to make one out  of thin plywood and a foam gasket.

To get this closer board up against the screen, i used cedar wedges to lift the board up to touch the screen.

Whether this is a good thing to keep the hive warmer in the winter, or not as good a thing because some mites that fall off the cluster of bees may be able to climb  up into the hive again.

Beekeepers, what are your thoughts here? Do you have a type of bottom board you like? The plastic bottom board will not rot, but I don't like the entrance reducer it uses, very clunky.

Update:  I now believe in areas with winter, one should close up the screened bottom board in winter. See our complete winter beehive prep video here.

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