Oxalic acid is a very effective treatment for Varroa but only during broodless periods when the kill rate can be above 90%. When brood is present the kill rate is closer to 30% as most of the Varroa are in the brood where this acid cannot reach them. Click here for more on oxalic acid.

For this reason oxalic acid tends to be used during the mid-winter broodless period – if there is one!

However,  winter is not the only time bees are broodless and oxalic acid can also be used during summer broodless periods when the Varroa are phoretic (out and about on the bees).

Summer Treatment

Imagine a swarm…

Technically, that swarm is Varroa-free. Note I say ‘technically’ – because there are of course a few Varroa present attached to the flying bees but most are left behind sealed in the brood or creeping about on the comb and on the young bees.

This point accepted then you can deal with the other half when all the brood has hatched three weeks later. If you treat with oxalic acid at this time, before the new queen starts to lay, you can expect a spectacular knock-down of mites.

These conditions occur each time you allow a swarm to get away or when you perform an artificial swarm. An oxalic treatment can thus be incorporated into your artificial swarm routine at, or shortly after day 21. Click here for detailed artificial swarm instructions.

However, regard must be given to the possibility of honey contamination.

Taking Precautions

Oxalic acid is a natural constituent of honey. The concentration varies depending on the botanical origin of the honey but it is at levels that cause no harm to humans and experimental work has shown there is no significant increase in honey due to spring or summer oxalic acid Varroa treatment (Rademacher &Harz). And they must have had their supers on.

Now imagine your way through an artificial swarm…

After you have moved the original box to the opposite side and it loses that second cohort of flying bees it simply doesn’t have the ‘man-power’ to be putting anything into supers. It doesn’t need supers so move them to the parent stand at least for a couple of weeks until the new queen starts to lay. If it is short of stores in the meantime – feed it.

Because oxalic acid has hydrophylic or water-loving properties, it is thought unlikely to accumulate in wax either (Rademacher &Harz).

Trickle or Vaporise/ Sublimate?

I prefer the trickling method, especially for summer, as this allows you to tailor the dose according to the strength of the colony ie only trickle active seams of bees and if they are particularly weak give only 2mls of a 3.2% solution per seam instead of 5mls if they are strong. Click here for oxalic acid recipe.

It is less easy to tailor the dose if you are vaporising.

Snelgrove Boards

If, like me, you use Snelgrove boards then oxalic acid can be incorporated into methods 1 or 2 at day 21 as above but it might be an idea to temporarily cover the metal grille throughout the treatment.

Click here for Oxalic Acid for Beekeepers

Click here for Winter Oxalic Acid Treatment

Click here for more on Snelgrove

Click here for pictures and how to make your own Snelgrove board

Click here for Varroa Floor Flaw

Click here for recipes for both concentrations and instructions.

Click here for pictures and how to make your own Snelgrove board

Sources

Aliano, N. An Investigation of Techniques for using Oxalic Acid to reduce Varroa mite populations in Honey Bee Colonies and Package Bees. (2008) University of Nebraska

Nanetti, A., R. Büchler, J.D. Charrière, I. Fries, S. Helland, A. Imdorf, S. Korpela, and P. Kristiansen.  Oxalic acid treatments forVarroa control (Review). (2003) Apiacta 38: 81-87

Rademacher, E.R & Harz, M. Oxalic acid for the control of Varroosis in honey bee colonies – a review. (2006) Apidologie 37: 98–12

Rashid, M, Wagchoure, E.S., Mohsin, A.U., Raja, S., Sarwar, G. Control of Ectoparasitic Mite Varroa destructor in Honey Bee (Apis mellifera. L.) colonies by using different concentrations of Oxalic acid. (2012) Journal of Animal and Plant Sciences, 22(1): 72-76

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Trickle treated 35 hives of bees with oxalic acid for Varroa today at 4 apiary sites. The temperature was 6 degrees, windy and beginning to rain. It took from 1.15pm to 4.15pm including travelling time so it has to be said it is the quicker of the two methods. If we had only the one Varrox Vaporiser it would have taken 10 minute per hive – which is nearly 6 hours and without the travelling time.

It has to be said the bees were not pleased to see us. It also has to be said that they are still very well stocked with stores. Most of them were on double boxes or one-and-a-halfs and because they had so much stores they were well down in the boxes so that was a bit of a dilemma: either you disturbed them more by separating the two boxes or you took a gamble and trickled from on high. We did some of each. I’d say the hive is open for no more than 1 minute each if you’re organised and have everything ready.

The only other thing to say is that there was very little problem with brace comb but then we have most of the bees on double boxes or 1.5’s – perhaps with that extra space they don’t feel the need to pack the space between the top bars and the crown board.

We must have found at least 8 hibernating queen wasps and often – stangely enough or perhaps not – 2 per roof – common wasps I’d say, going from the marks on their faces.

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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Be aware of the possibility.

We could leave it a bit later but the bees are very much attuned to the changing of the seasons and lengthening days. Before we know what’s hit us the garden heathers, snowdrops, hazel and willow will be breaking bud and the queen will really get into her stride.

We’re too late to go earlier now! But there would have been brood then too – they’ve been bringing ivy in for weeks.

Wouldn’t it be great if we had formic acid to fall back on?

Another thing to bear in mind is that all this activity will be burning a hole in their stores so go forth and heft your hives. If they feel a bit light get yourself a bit of fondant for later.

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