Winter Oxalic Acid Varroa Treatment

When the shortest day is upon us there is a brief broodless period in the honeybee colony. This is the time conditions are right for oxalic acid treatment. There are two reasons for this:

  1. When a colony is broodless, all the Varroa are at their most vulnerable out there in the open, either on the bees or creeping about on the comb – Varroa are protected from the effects of oxalic acid when they still inside the cells of the brood nest;
  2. Oxalic acid can damage open brood so these effects are minimal when there are no or few larvae.

On December 9th 2013 the bees were flying quite strongly and still working the last of the ivy and the Mahonia but there has been little activity since then. Any eggs laid during that spell of mild weather will have hatched by the end of December so that might be the best time to treat.

The following methods can be used during any broodless period, even in the middle of summer, but supers need to be removed. Click here for more about summer oxalic acid treatment.

There are two oxalic acid treatment methods – vaporising or trickling.

Vaporising – Possible mitefalls of 98% when broodless.

If you are vaporising you will need an Varrox vaporiser which is a little pan with a heating element which can (technically) be inserted through the hive entrance. 1 gram of oxalic acid dihydrate is placed in the pan and the heater is plugged in – either directly into the cigarette lighter in your car – if you can get that close – or into a spare car or lawnmower battery. The vaporiser is turned on for 2.5 minutes after which the hive needs to be left sealed for about 10 minutes. During that period of time the vapour settles in a fine white layer on the bees and the Varroa and all over every interior surface of the hive. It kills the mites but the bees don’t seem to even notice it.  The vaporiser can then be removed and plunged briefly into a bucket of water to cool it otherwise if you prime it immediately with another dose of crystals it will vaporise right there and then – do not breathe the vapour.

Before you can start this procedure you will need to make sure the hive is sealed or the vapour is going to come billowing out of every crack and don’t whatever you do breath that stuff in because it is EXTREMELY corrosive.

I have used a Varrox vaporiser in the past and although in theory it works well, there are always problems either getting the hives sufficiently well sealed or getting the Varrox pan through the doorway.

Even if you achieve both of those things, the heat from the pan can melt plasticised Varroa screens or even start to burn a wooden one.

Because of these problems and because I overwinter on mesh floors, I made a special, solid vaporising floor with the vaporiser set into it so I could simply lift a brood box gently onto the top and vaporise away.  It worked quite well.  However, as always with bees – expect the unexpected – once at the end of a treatment I found a single bee scorched in the bottom of the pan – the queen!

It is also very time consuming if you have more than a couple of hives.

It is possible to vaporise through a mesh floor but you need to know beforehand that the floor isn’t covered in dead bees or blocked by debris. Also, bear in mind the acid will corrode a metal mesh.

I prefer trickling. Trickling is great.

Trickling –  Possible mite falls of around 95% when broodless.

A lot of experiments have been done to determine the best concentration of acid to use in terms of effectiveness against the mites and ill effects on the bees. In Europe and in the UK, the recommended concentration is around 3% depending on who you read.

Here’s a recipe for 3.2% from the UK:

  • Make a sugar syrup with 1kg sugar and 1 litre of soft water;
  • Add 75g oxalic acid dihydrate crystals and stir till dissolved;
  • This will make nearly 1.75 litres of solution.

I’ve been using 3.3% for many years with good effect and imperial measurements. This is my recipe:

  • Make a sugar syrup of 1lb sugar and 1pt soft water;
  • Add 1.25 oz oxalic acid dihydrate and stir till dissolved;
  • This will make about 1 and 3/4 pints of solution.

As for method and dosage at temperatures down to 0 degrees C when the bees are well clustered:

  • Pull 50ml of solution into a syringe;
  • Gently remove the crownboard;
  • If you are overwintering in two boxes you may have to remove the top box also. Note which way it came off and put it back the same way round or there will be draughts and the same with the crownboard;
  • Trickle 5ml of solution per occupied seam of bees directly onto the bees;
  • For nucs or very weak colonies use 2ml per seam. Or not at all if you think they are too feeble.

This method is very quick and the disturbance is not as much as you would think so long as you are gentle. You shouldn’t need smoke but have it there in case things turn nasty!

By the way, don’t go scraping off brace comb – I’ve never needed to – just trickle between the holes.

Also, use the mixture as soon as possible – it does not store well and eventually it will become toxic to the bees.

Click  here for more about oxalic acid – where it comes from, what it is, what it does and what it doesn’t…

Click here for Summer Treatment with Oxalic Acid

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