I looked at Hive F's removable floor again on Tuesday 9th May. I counted 24 Varroa mites which had fallen out of the colony. That's 8 per day. Will they survive?
UPDATE: I visited the hive on 12th May after dark. There were another 32 mites. I've counted 88 mites in 7.5 days, an average of about 12 per day. That's very high. This colony has been untreated for a couple of years but it was from a bought queen before that. I don't think much of its' ability to manage Varroa.