Did mites kill my bees?

You went into autumn with an exuberant and populous hive. You weren’t worried about them because they had a first-year queen and loads of honey and pollen. You counted your mite drop and it seemed okay—you saw a few mites but not many—so you dusted the bees with powdered sugar just to be on the safe side.

Somewhere toward early spring you decided to check the colony’s food supply, just in case. You even prepared some candy boards as an insurance policy. So on the first warmish day when you thought it was safe to open the hive, you did. But to your amazement, no one was home.

What you found was an empty hive with very few dead bees and frame after frame of untouched honey. Or maybe you found no bees and no honey either. Instead, all of the frames that had been full were now empty with ragged and torn edges. What went wrong?

The very first thing I would suspect in a situation like this is Varroa mites. Mites can take down a colony quickly, and the mites that you count on a sticky board are just the tip of the iceberg. But to be sure—or at least more sure—here is a list for your post mortem:

  • If there are very few dead bees in your hive, it may mean the colony worked hard at removing them until the last minute. Try to find some dead ones on the bottom board, alighting board, or even on the ground nearby. Sift through them and look for bees with deformed wings.The presence of many deformed wings is a good indicator of Varroa.
  • If you have a bottom board or Varroa tray in place, look for mites. If the colony died from mites, you will find mites in the debris.
  • Look for frames of honey. A hive with plenty of honey and no bees can be a sign of Varroa. A hive with no bees and honeycombs with jagged edges indicates a weak or dead hive that was invaded by robbers, which can also be a sign of Varroa.
  • Examine the brood frames. Adult bees that died while they were emerging, or just before, may have been weakened by Varroa. These bees will have their heads facing up. (Bees that starved while searching for food in the cells will have their tail ends up.)
  • Hold up the empty brood frames with the sun at your back so you can see inside the cells. If you find bright white deposits adhering to the inside of brood cells, you can be sure of a Varroa infestation. These white spots are patches of mite excrement that contain about 95% pure guanine, an amino acid.

Mite feces
The white mass in lower right is mite feces. Photo by USDA ARS

If you conclude your bees died of a Varroa infestation, review your management strategy and try something different next year. Mites can be handled in many different ways, but you must be diligent. If you do nothing—or next to nothing—the mites you reared will soon spread to other colonies, both feral and managed.