I treated the colony in the city against Varroa the Saturday before last by what here is called the dripping method. In this way you can start the season with a colony relatively free of Varroa. This treatment is meant to be performed in the middle of winter December till January. Even though here the winter has hardly shown its face, with temperatures as high as 10°C (50°F). The day I performed the procedure it was around -1°C (30°F) but felt like -5°C (23°F).
The bees were relatively calm, a few flew up, no more than about 15, one landed on my sock on the edge with my shoe and stung me when I started walking because it got stuck between my ankle and my shoe. The procedure took no time at all. I filled a 60 ml syringe with the oxalic acid mixture (9 grams oxalic acid, 150 grams sugar, and 150 ml water) and dripped about 5 ml per “lane” (room between the frames) that was occupied by the bees.
This method needs to be done in the winter time, which feels all wrong because at the beekeeping course I followed I was taught that you do not bother your colony when the temperature drops bellow 14°C (57°F). You perform this method in winter because in this season there should be no brood, any mites present will be on the bees. According to the organization that does research in the Netherlands on Varroa treatments (and on bees in general) there are no detrimental consequences for the colony by opening the hive in winter. I guess I’ll trust their judgment/research. What I think happens is that when the bees clean each other of the sticky sugary liquid they also ingest the oxalic acid. The oxalic acid enters their hemolymph on which the Varroa feed and this will then kill the mites.
After the oxalic acid treatment I placed a styrofoam sheet on top of the top cover (supposedly against the cold somehow) and replaced the lid. I can now only hope that the colony will be able to survive the rest of the winter. The colony was no longer as large as I would have liked. In a few months I will reduce the size of the hive and place the honey super back on top of the hive (I trust that they will start to use it in stead of using the second brood box as honey super).
I found a video of the kind of treatment I performed, it’s a video of Dutch origin so don’t expect English commentary.