Why Did My Bees Die?

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Carl Levien of Washington writes:

These bees seemed fine the first of October, and I was getting ready to move them today to a more sheltered place for the winter and found them all dead and all the cells full of dead bees. Any thoughts?

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Rusty Burlew replies:

Dead bees head-down in cells is a common occurrence, but beekeepers seldom agree about the cause. The most commonly heard theory is that the bees could not find food and died licking the remaining molecules off the bottom of the comb.

However, clustering bees that are not starving routinely go headfirst into cells during cold weather. In a healthy colony, the bottom layer of bees goes in headfirst, and the remaining bees cluster on top of them. This tight cluster conserves heat by trapping it and sharing it among themselves. Starvation has nothing to do with it.

If the bees die in that position, they most likely succumbed to cold. But if they were cold, it may be because they didn’t have enough food. Without food, they wouldn’t have enough energy to vibrate their flight muscles to generate heat. And when they get too cold, they become stiff and lethargic. In a state of torpor, they become unable to collect food, even if it’s nearby.

I call this situation a cold/starvation complex because it’s impossible to tell if the bees died of cold because they couldn’t find food, or they died of starvation because they were too cold. It’s a chicken-or-egg kind of question. The only thing you know for sure is they died while clustered.

A related possibility is that the colony was too small. A small colony may die of cold no matter how much food it has because the number of bees isn’t great enough to make enough heat. The only thing you can do is try to figure out why the colony was small. Was it queenless? Did it suffer from varroa and viruses? Was it small because of pesticides or poor management?

Usually, death by the cold/starvation complex is just the last step for a colony that was having other problems. The challenge is to figure out what those problems were.