Varroa count after the harvest: sugar roll numbers and the treatment choice
Once the supers were off I did a sugar roll on both hives, which I prefer to the alcohol wash because the bees survive it. Half a cup of bees, about 300, scooped from a brood frame into a jar with a mesh lid, a heaped tablespoon of icing sugar, a gentle roll for a minute, two minutes rest, then shaken over a white tray and the mites counted. Hive one: 9 mites. Hive two: 14. That is 3 and almost 5 percent, and the general advice is that anything over 3 percent in late summer needs treating if the colony is to raise healthy winter bees.
So both got treated, which I had half expected. The timing matters more than the choice of treatment: the bees that will carry the colony through to March are being raised in September, and if the mites are feeding on them as larvae, the colony dwindles in January for reasons that look like something else. Treating in October is too late to protect those bees even if it kills the mites.
I used a thymol-based treatment, which works by evaporation and therefore depends on temperature. The instructions want daily highs between 15 and 30 °C. I logged the temperature in the shade by the hives each afternoon for the four weeks of treatment: the first fortnight ran 19 to 24 °C, ideal; the third week dropped to 13 to 16 °C for five days, and I expect the treatment did very little in that spell; the final week recovered to 18 to 21. The bees disliked the smell, bearded on the front of hive two for two evenings, and settled.
Mite drop on the floor inserts during treatment: hive one, 212 over four weeks; hive two, 390. A follow-up sugar roll in late September gave 1 mite from hive one and 2 from hive two. Under 1 percent on both, and a decent position to go into winter from.
What I will do differently: start the treatment a week earlier, in the third week of August, so that the whole course falls inside the warm window. And keep the monitoring going through the summer rather than waiting for the harvest; a count in June would have told me hive two was heading somewhere I did not want it to go.