Something to consider, not yet mentioned other than jiminy. Varroa damage to in the fall, from mid August through end of September. In addition to the array of virus that the mite can propagate through the hive (invisible killers), the damage done to pupae during the critical period of raising the `winter bees` has a significant impact on winter survivability by shortening the lifespan of the bee.
I see many dead-outs with the characteristics you have described and shown in the pictures. The hives start the winter with good populations. In January they look ok. By mid February there are no bees left in the hive. Those deadouts occur end of January through end of February. The lifespan of the winter bee is significantly shortened by mite damage in the fall. Instead of living 6 months in winter, they live 3 months. As a result the hive dies out suddenly 2/3 of the way through winter with no obvious signs of distress.
To check or validate this possibility on this hive, just check back into your inspection notes through 2020 on this hive. This is when and why records are important. Did it have varroa counts? When did you check for mites? When did you treat for mites? How do those dates align with the critical time of when winter bees were being raised? The hive may have been treated for Varro, however too late; meaning the damage was already done to the winter bees population before the varroa load was addressed.
Great job on the nosema check.
... for your considerations.
Hope that helps!
Hope that helps!
