All the colonies survived into March but Clarissa did not last into April. Since that happy day of observing traffic from all the hives, we had been uncertain of seeing any from her. Confusingly we would see foragers come to the entrance, flit about a bit, and then suddenly dart off to enter Beatrix or Dorcas. A very few may have entered but we were unsure. The other hives definitely had traffic.

Three Sundays ago we were sitting on our bench, watching bees at work, and finally lost patience. Without suiting up we impulsively opened Clarissa and had not a single bee come to question our intentions. Grabbing a hive tool, we somberly poked through the combs. She resembled a typical winter varroa death as described in this article. [Edit: 2021-Apr-23 Changed to link to a newer version of the article per comment from its author.] Looking at its points in detail:

  1. The colony was big and looked healthy in the fall.

    Yes, it did.

  2. A lot of honey is left in the top supers.

    Equivalently for our horizontal hive, the combs furthest from the winter entrance are full of honey while those nearest have just bit at the corners with slightly moldy cappings.

  3. The cluster is now small, maybe the size of a softball.

    Smaller. And moldy.

  4. There are hardly any bees on the bottom board.

    Definitely dead bees on the floor. Not as thick a carpet as we have seen but more than “hardly any”.

  5. Near or just below the cluster is a patch of spotty brood – some fully capped, and some with bees dying on emergence (heads facing out, tongues sticking out).

    Nearly no brood at all. We did see one cell where the occupant had just cracked open the capping and expired.

  6. If you look closely in the cells around the brood, you will see white crystals stuck to the cell walls, looking like someone sprinkled coarse salt in the brood nest.

    In past years before we took varroa monitoring and treating seriously, we would see such small crystals in many cells. We thought they were sugar until we learned that they were guanine from varroa waste. We see some such cells this time and more with large blobs. Incontinent varroa on diuretics? Could they possibly be larvae that had gone through a few freeze thaw cycles? We have not read of such a thing but the color and volume of these deposits do not quite match varroa poop descriptions or our previous experience.

  7. You don’t have records showing that varroa was under control.

    And there is a bit of a puzzle. We do have records indicating that varroa was under control except in Dorcas, not Clarrisa. We applied the oxalic treatment to all the hives anyway and verified that the mite count in Dorcas had declined to our target level. It is true that we did not check Clarissa’s mite count post-treatment but it had already been fine pre-treatment.

So Clarissa may not tick all the boxes for winter varroa death but it still seems the likeliest explanation at this point. And our fault yet again.