I pulled four deep frames of honey from each of my hives this past summer to prevent the queens from becoming honey bound. I stored the frames in a cardboard nuc box and kept them in my house. Later in the fall I fed all but one of the frames back to the bees (see Feeding The Bees Honey Instead of Syrup). This morning I took a look at the remaining deep frame of honey stored in the nuc box and noticed it had mould growing on it.
My best guess is that some uncapped cells at the bottom of the frame began to ferment and got mouldy. Then the mould spread from there.
I don’t think I can feed mouldy combs of honey back to the bees.
But I couldn’t just throw away the whole frame. Instead, I cut off the mouldy portions of the comb (it was from a foundationless frame), then the not-so-mouldy portions, and sealed them inside plastic bags.
I’ll keep them in the freezer until next spring when I might be able to feed the honey back to the bees. Next time I’ll make sure the seal the frame in plastic wrap if it has any uncapped cells on it.
Should I throw away the mouldy comb? I don’t know.
October 15th, 2015: I don’t worry about mouldy comb anymore. If I found a frame like this today, I’d scrape up the mouldy bits because most likely the honey underneath is fine. Then I’d let the bees eat the honey and clean up the comb. It would have to be pretty awful looking comb for me to throw it away.
March 2019 Postscript: Comb often becomes a little mouldy over the winter when the bees are clustered on other frames. I don’t worry about it. If I found mould growing on any frames as thick as it is in these photos, though, I would spray it with apple cider vinegar, wipe away as much mould as I could, and then give it back to bees or scrape open the cells and then give it back to the bees. Apple cider vinegar (though I’ve used regular white vinegar too) works for cleaning up any bee poop outside or inside the hive. I spray on the vinegar, give it a good scrape or wipe with a scouring sponge, let it dry and we’re good to go.