For at least the last 2 years I have observed a definite pattern in Hive A. In spring there are lots of crawling bees outside the hive. Some are clearly affected by Deformed Wing Virus (DWV); others maybe by another paralysis virus or by exhaustion.
They crawl around near the hive. If you launch them into the air they fall back to earth. They never make it back to the hive and have probably been ejected by the other bees. The Sparrows eat some of them. The others presumably die out of site.
By mid-May they have all disappeared. Mite drop on the hive floor has diminished so much that I've started wondering whether Ants are removing them.
I don't have an explanation. Maybe there is greater transmission of viruses when the bees are confined to the hive during the early spring build up. Maybe these bees have been in the hive for a while and are only ejected in spring.
I'm going to write to a DWV researcher to ask whether seasonal expression of DWV and other diseases is usual, and if she has an explanation.