I lost a nuc last winter to excess moisture. My fault, the hive wasn't slanted forward, it was actually slanted back, and it had a solid bottom. I found a small pool of water and frogs eggs in it.... It made me realize that I don't have to worry about that as much with a screened board.. I have a hive with a solid board, and it had a ton of crap on it come spring to clean out.. The screened boards weren't nearly as bad. Don't know about the mites... I just want to keep breeding from survivors and not worry about it too much.. who knows..