I don't speak from a ton of experience but I don't think it is the mold that kills the bees. It is probably the constant moisture that is needed for the mold to form that is chilling the bees. The girls can stand some pretty cold temps in their cluster but if they are wet (damp) they will chill and die quickly.
I had some mold on one side of one outside frame in one box of my hive. I am thinking that there must have been an opening that didn't get propolized that allowed excess moisture in that area for much of winter. I did not have any significant die-off as a result and before I could get back and swap out the frame, they had it very well cleaned up.
Don't over insulate and be sure to ventilate adequately even in winter.
Yes, moisture can come from condensation caused when the warm air generated by the cluster meets the cold surfaces of the hive. In my case, I suspect some kind of outside water because the location of the mold was so isolated...
Hives need ventilation even in winter. How you do it and how much depends on your equipment and a few other factors like your location, hive location, etc.
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