I am a hobbyist beekeeper and this year I lost my bees. The hive was healthy going into the fall. The hive still has plenty of honey (the uppper super is almost completely full). It has been a long winter here, during one 3 week period, the high temperature stayed close to 10 F.
My entire bottom board was covered in dead bees. I am wondeirng if they could have frozen to death, or if it was something else, i.e. mites, disease etc.
There are, of course, a lot of possibilities, but in recent years I've found it's usually Varroa mites that are the cause of winter losses of otherwise strong colonies. Since it was strong and since it had plenty of stores that would be my first guess. But to investegate more, did you look through the debris for mites? If it was Varroa there will be LOTS of them in the debris. Varroa are visible but small and easilly overlooked. They are about the size of this period. "." and are purplish brown and slightly oval shaped.
Sometimes a cluster get's stuck in a long really cold snap (like we had in February here). But usually a large cluster manages to move under those circumstances and a small one gets stuck. From the dead bees stuck in cells, does it look like the cluster was large or small when it died? If it was small, then the question is, how did it get that small? T-mites? V-mites? Nosema? If it was larger, is there a space of no honey all around the cluster? If so, they may have died because they couldn't move to the stores. As I said generally a large cluster doesn't have this problem.
Also look through the dead bees for shriveled and deformed wings. Of course some will be missing (from drying up and breaking) but shriveled ones will look different from that.
All in all, winter kills some of the bees no matter what you do, but you want to make sure they are strong and well stocked and you did that. And you need to make sure the bees going into winter are young strong and not infested with mites.
You can make sure there are young bees going into winter by making sure there is a fall brood rearing. If there is a dearth or drought you may have to feed them to get this.
You can make sure they don't have a bad infestation of Tracheal mites by using either the FGMO regularly or grease patties regularly or treating with menthol in the fall or treating with formic or oxalic acid.
You can make sure they don't have a bad infestation of Varroa mites by monitoring and, if needed, using some kind of treatment and then monitoring more to see that it worked.
Treatments for V-mites are:
"Chemical" treatments are Apistan and Check-mite.
soft-"chemical" treatments are FGMO (used regulalrly) oxalic acid (spring or fall or both), formic acid (spring or fall or both), Thymol (sometimes used in various forms from straight crystals to mixed with FGMO fog)
non-chemical control is really only acheived with small cell and genetics. These are long range solutions, not short range ones.
mechanical methods: Screened Bottom Board (SBB), and inert dust (powdered sugar dusting).
Details of these methods are discussed at great length on this board and you can search for those using the search feature.