I just spent several hours diagnosing the cause of a winter dead out. I went through the enter hive, each frame, top-to-bottom, paying close attention to the contents of cells and the state of the bees themselves. I also reviewed my records for the hive, because the logical first question is, "Did mites kill them?" It's possible the mites did weaken them enough to contribute to their demise. The colony consistently had my heaviest mite load throughout 2017. I got it below 1% by the end of October, but a lot of viral damage could've been done by then. So I'm going to say mites were a factor, but not the sole cause of death. As I explained in my last post, I experimented with this hive by foregoing fall feeding in an attempt to avoid a honey-bound hive in the spring. Here's the unfortunate result as I see it:
- During the fall and early winter, the colony completely exhausted all food resources in the bottom box. It was totally empty.
- Following the food, the bees clustered in the back-left corner of the top box.
- The temperatures stayed well below freezing from December 23 through January 7. I found a lot of dead bees on the bottom board during this time, suggesting they weren't getting the calories to keep fueled. It was too cold to shift the cluster over new honey during this time.
- When the cold spell ended and it got warm enough to fly, the colony was sufficiently weakened to be vulnerable to robbing. I tried to stave off robbers by closing and reducing entrances, but they were overpowered nonetheless.
- The arctic weather returned from January 12 through 17, and by this time the colony was so decimated by winter and robbing that they succumbed to the second round of extreme cold.
The postmortem showed bees with their heads buried in cells (shown in the picture), with empty cells directly under the nest. This is the classic sign of starvation. Sadly, there was still 30-40 pounds of capped honey in the hive--mostly near the center of the top box; frustratingly close to the cluster itself. This would be even more tragic if I didn't learn from it, and what I learned is that fall feeding is not optional. You've got to try to backfill every last cell in the hive to give the bees the best chance of surviving long periods of freezing temperatures. Any downside to overfeeding in the fall is better than a box of dead bees in the spring.