The two websites/blogs I follow most closely to help with my beekeeping are written by the wonderful Rusty Burlew (https://honeybeesuite.com) and the informative and entertaining David Evans (http://theapiarist.org). Both are very much of the view that Varroa destructor is a serious and continuing problem for our bees, which makes a refreshing change from the views of these who don't count, don't treat, and say "Oh, I just keep strong colonies". If you don't already read them, I encourage you to follow Rusty's and David's blogs.
Following their advice I decided for the first time to apply oxalic acid to my hives this winter to help control the varroa. You can read what Rusty has to say about it here (she uses a slightly weaker solution than we do in the UK). David has recently posted three articles on the rationale and technique of use of oxalic acid which you can find here, here, and here. I bought Api-Bioxal, the form of oxalic acid now approved for treatment of varroa in the UK (generic oxalic acid was previously used as a "hive cleanser"). After discussion I decided to use the manufacturer's recommended dose of 4.2% w/v although David Evans argues in favour of the more traditional 3.2%. The 35mg sachet is dissolved in 500ml of 1:1 sucrose syrup (made from 308g sugar in 308ml water). The dose is 5ml per seam of bees so this is enough to treat 100 seams, perhaps 12-20 hives.
I have four hives in the garden, all on double brood. I waited until this week as the treatment is best applied in cold weather when the bees are not flying. It is done in December when there should be little or no brood as oxalic acid can only kill phoretic mites, ie those living on bees rather than in brood cells. I was a little bit apprehensive as I have never looked in a hive in the winter before but in the end it was all pretty straightforward. Having made up the solution I loaded five 5ml syringes before opening the first hive. In two hives the cluster of bees was in the top box and the other two clusters were in the bottom so I had to split the boxes to get at them. There were 6, 7, 7 and 8 seams of bees in the four hives so I think they are all doing OK. They seemed pretty active and were not as tightly clustered as I had expected, even though the outside temperature was only 1℃. It probably took less than minute to trickle the oxalic acid solution along all the seams and close up so the bees didn't seem disturbed by the intrusion. I was also able to judge that they all have plenty of stores.
Having treated all my hives I had used only 140ml or so of the solution so I had arranged to meet Sue, Ian and Jonathan at the association apiary where we treated another seven colonies. They were a little smaller, perhaps 4 or 5 seams, but all seemed OK. The advantage of the others being there was that I could take some photos, not really possible on my own hives as I was working quickly. Here are the bees emerging from the hole in the crown board once the insulation has been removed
Here are two relatively small clusters of bees, each occupying around 4 seams.
Here are the bees in the seams between the frames.
We applied the oxalic acid with a 5ml syringe.
Here the bees are all discussing what has just happened to them.
All my own hives were treated last summer with formic acid (MAQS) as well so it will be interesting to see what the varroa counts are like next spring. I am hoping they will be very low.