Oxalic acid preparation

This is the second of three articles on midwinter treatment of colonies with oxalic acid to minimise Varroa levels. In a recent post I explained why a midwinter treatment was necessary, even if you’d treated three months earlier. Essentially this is because:

  • there will still be some residual Varroa, particularly if you treated in late summer rather than early autumn (and this post explains why early treatment is preferable)
  • midwinter is the time when brood levels are at a minimum, so most mites will be phoretic and readily accessible to the miticide treatment

Midwinter is the time to use oxalic acid-containing treatments. It can be delivered in a variety of ways; by sublimation (vaporisation), spraying or trickling (dribbling).

Trickling or dribbling

This post is about the preparation and storage of oxalic acid-containing solutions for trickling. Sublimation is covered elsewhere and spraying is not approved or widely used in the UK.

The process for trickling is very straightforward. You simply trickle a specific strength oxalic acid solution in thin syrup over the bees in the hive. The oxalic acid kills the mites. How isn’t entirely clear – it’s thought to corrode the mouthparts and soft tissue. It’s more than 90% effective in killing phoretic mites when used like this.

Beekeepers have used oxalic acid for years as a ‘hive cleaner’, as recommended by the BBKA and a range of other official and semi-official organisations. All that changed when Api-Bioxal was licensed for use by the Veterinary Medicines Directorate (VMD).

Oxalic acid and Api-Bioxal, the same but different

Spot the difference ...

Spot the difference …

Api-Bioxal is the VMD-approved oxalic acid-containing miticide. It is widely available, relatively inexpensive (when compared to other VMD-approved miticides) and very easy to use.

It’s very expensive when compared to oxalic acid purchased in bulk.

Both work equally well as both contain exactly the same active ingredient. Oxalic acid.

Api-Bioxal has other stuff in it (meaning the oxalic acid content is a fraction below 90% by weight) which actually makes it much less suitable for sublimation.

How much and how strong?

To trickle or dribble oxalic acid-containing solutions you’ll need to prepare it at home, store it appropriately and administer it correctly.

I’ll deal with how it is administered next time. This is all about preparation.

The how much is easy. You’ll need 5ml of oxalic acid-containing solution per seam of bees. In midwinter the colony will be reasonably well clustered and its likely there will be a maximum of only 8 or 9 seams of bees, even in a very strong colony.

Hold on … what’s a seam of bees?

Two seams of bees

Two seams of bees …

Looking down on the colony from above, a seam of bees is the row visible between the top bars of the frames.

Remember to prepare ~10% more than you think you need. You’ll inevitably spill some when using the Trickle 2 bottle to administer it to the colony. It’s not that expensive, so don’t risk running out.

And the how strong? The recommended concentration to use oxalic acid at in the UK has – for many years – been 3.2% w/v (weight per volume) in 1:1 syrup. This is less concentrated than is recommended in continental Europe (see comments below on Api-Bioxal).

My advice – as it’s the only concentration I’ve used – is to stick to 3.2%.

Listen very carefully, I shall say zis only once

A bit of basic chemistry coming up. Skip to the warning in red below and then the recipes if you want, but this explains some important things about working out how much to use.

The molecular formula of oxalic acid is C2H2O4. The molecular weight of oxalic acid is 90.03 g/mol. However, the oxalic acid you purchase – including Api-Bioxal – is the dihydrated form of oxalic acid.

Di as in two, hydrated as in water.

The molecular formula of oxalic acid dihydrate is C2H2O4.2H2O and oxalic acid dihydrate has a molecular weight of 126.07 g/mol.

Therefore the weight of oxalic acid in 1 g of oxalic acid dihydrate is 90.03/126.07 = 0.714 g.


Oxalic acid is toxic

  • The lethal dose for humans is reported to be between 15 and 30 g. It causes kidney failure due to precipitation of solid calcium oxalate.
  • Clean up spills of powder or solution immediately.
  • Take care not to inhale the powder.
  • Store in a clearly labelled container out of reach of children.
  • Wear gloves.
  • Do not use containers or utensils you use for food preparation. A carefully rinsed plastic milk bottle, very clearly labelled, is a good way to store the solution prior to use.

Recipes : oxalic acid

The standard recipe is 100 g water plus 100 g white granulated sugar. Mix well and then add 7.5 g of oxalic acid. The final volume will be 167ml i.e. sufficient to treat over 30 seams of bees, or between 3 and 4 strong colonies (including the 10% ‘just in case’).

This final concentration is 3.2% w/v oxalic acid … (7.5 * 0.714)/167 * 100 = 3.2. Check my maths.

0.01 g to 500 g

0.01 g to 500 g

If you have more colonies to treat, or have trouble weighing 7.5g, scale everything up ten-fold. Or buy a small, accurate set of digital scales – like these for £9 which work very well. 1 kg of sugar plus 1 kg (1 litre) of water requires 75 g of oxalic acid and makes 1.67 litres … enough to treat all the colonies in the association apiary.

Which is not such a bad idea. Make it up carefully once and share it with your fellow beekeepers. Storage details are provided below.

Recipes : Api-Bioxal

Warning – the recipe on the side of a packet of Api-Bioxal makes up a much stronger solution (4.4% w/v) of oxalic acid than has historically been used in the UK. Stronger isn’t necessarily better. The recipe provided is 35 g Api-Bioxal to 500 ml of 1:1 syrup. By my calculations this recipe makes sufficient solution at a concentration of 4.4% w/v to treat 11 hives. 

To make a 3.2% Api-Bioxal-based oxalic acid-containing solution using the 35 g pack of Api-Bioxal you need to mix the entire contents of the pack with 691 ml of 1:1 syrup.

Here’s the maths:

  • 35 g of Api-Bioxal contains only 22.14 g of oxalic acid. 88.6% of the 35 g is oxalic acid dihydrate (the remainder is cutting agents like glucose and powdered silica) and so the oxalic acid content is ((35 * 0.886) * 0.714) = 22.14 g.
  • To calculate the volume of syrup you need to divide it by the final percentage required i.e. (22.14 / (3.2/100)) = 691 ml. I don’t know the exact amount of sugar and water needed to make this amount … it’ll be about 430 g of each (I think).

A 35 g packet of Api-Bioxal is therefore sufficient to treat about 15 colonies (assuming 5 ml per seam, 8 seams per hive and 10% ‘just in case’) at the recommended concentration of 3.2% w/v.

Api-Bioxal is sold in three pack sizes (35 g, 175 g and 350 g). If you are wealthy enough to be able to purchase the larger pack sizes you’ve probably got your own beekeeper (or mathematician). Relax on your yacht while they do the calculations for you 😉

On the other hand … if you have a smaller number of colonies either make a full 35 g packet up and share it, or use accurate scales and the following table:

Api-Bioxal recipes for 3.2% OA trickling

Api-Bioxal recipes for 3.2% OA trickling


Storage of oxalic acid syrup at ambient temperatures rapidly results in the acid-mediated breakdown of sugars (particularly fructose) to generate hydroxymethylfurfural (HMF). As this happens the colour of the oxalic acid-containing solution darkens significantly.

This breakdown happens whether you use oxalic acid or Api-Bioxal.

Stored OA solution and colour change

Stored OA solution and colour change …

HMF is toxic to honey bees at high concentrations. Studies from ~40 years ago showed that HMF concentrations below 30 mg/l were safe, but above 150 mg/l were toxic1. HMF buildup is one way overheated honey is detected.

At 15°C HMF levels in OA solution can reach 150 mg/l in a little over a week. At room temperature this happens much faster, with HMF levels exceeding 150 mg/l in only 2-3 days. In the dark HMF levels build up slightly less quickly … but only slightly 2,3.

Only make up OA solutions when you need them.

If you must store your oxalic acid-containing syrup for any length of time it should be in the fridge (4°C). Under these conditions HMF levels remain well below toxic levels for at least one year. However, don’t store it for this long … use it and discard the excess. Don’t use discoloured oxalic acid solutions as they’ve been stored incorrectly and may well harm your bees.

Please re-read the comments above about the toxicity of oxalic acid. If you are going to store it in the fridge it must be very clearly labelled and there must be no chance that children can reach or open the container.


Api-Bioxal is the least expensive VMD-approved miticide and powdered oxalic acid is much, much cheaper. Both contain the same active ingredient, oxalic acid, which is highly effective against phoretic mites.

In midwinter, with very low levels (or no) of brood, a single oxalic acid-containing treatment minimises mite levels for the coming season.

Oxalic acid-containing solutions are easy to prepare. I recommend you make sufficient for your own colonies and those of your beekeeping friends and association members. My previous BKA used to distribute litres of the stuff for use in midwinter. Use this solution in midwinter and then discard any that is unused.

Oxalic acid-containing solutions are inexpensive and easy to administer by trickling. As I shall demonstrate next time.

Please re-read the safety instructions highlighted in red above.

Michelle Dubois

Michelle Dubois

† Listen very carefully, I shall say zis only once was a catchphrase used by “Michelle of the Resistance” in the 1980’s comedy ‘Allo ‘Allo! Michelle (Dubois) was rarely seen without a trench coat and beret, had a corny French accent and was played by Kirsten Cooke.

‘Allo ‘Allo! ran for 85 episodes in the decade from 1982 on BBC one. It was about a café in Nazi-occupied France and the French Resistance, just about. It mixed bawdy humour with gross stereotypes (posh British twits, sex-obsessed French) and was a parody of ITV’s series Secret Army (’77-’79).

Early episodes had obvious and rather dull titles. In the later series the individual episodes had some quite good puns like Awful Wedded Wife.

Michelle – Listen very carefully, I shall say zis only once

René – Well, in that case, could you please speak slowly?

You had to be there … 😉

‡ Oh alright then, since you insist. The 175 g pack of Api-Bioxal (~£39) needs to be made up in 3.459 litres of 1:1 syrup and the 350 g pack (~£65) 6.919 litres of 1:1 syrup. Determining how much water and sugar to mix to make these amount is, as they say, an exercise for the reader. Assuming a 3.2% solution and 8 seams of bees per colony Api-Bioxal costs between 63p and 41p per hive (see note below), depending upon the pack size you purchase. I know that beekeepers moan on and on about the outrageous cost of Api-Bioxal (as do I), but is 63p per colony really an unreasonable amount to spend on VMD-approved medicines to keep your colony as clear of Varroa as possible? I don’t think so.

Note – the costs in the paragraph were calculated using the lowest prices I could currently find for Api-Bioxal. C Wynne Jones has the 35g packets for £9.50 and Maisemores have the 350g packets for £64.79. Prices correct on 9/10/17.

1 Jachimowich T., El Sherbiny G., Zur Problematik der verwendung von Invertzucker für die Bienenfüttering, Apidologie 6 (1975) 121-143.

2 Bogdanov S., Kilchenman V., Chamere J.D.. Imdorf A. (2001) available online.

3 Prandin, L., Dainese, N. , Girardi, B., Damolin, O., Piro, R., Mutinelli, F. A scientific note on long- term stability of a home-made oxalic acid water sugar solution for controlling varroosis Apidologie, 32:) 451-452


27 thoughts on “Oxalic acid preparation

  1. Emily

    It was so much easier when you could buy oxalic acid pre-mixed – grumble grumble. Api-Bioxal is making life harder for us. I’m not worried about the price, just the inconvenience. Why can’t ApiBioxal be sold premixed too?

    1. David Post author

      I suspect this has a lot to do with what was licensed. This covers both the product, the way it is packaged and the routes by which it can be administered.

      As indicated above, you can’t prepare oxalic acid (or Api-Bioxal) solution in a form that is stable for long-term storage without refrigeration (or freezing). Since the manufacturer cannot guarantee the storage conditions I suspect they opted not to offer (or licence) a solution form suited for trickling. Instead, they chose to licence something that – at the point of sale – they could be sure of.

      How was it possible before? Presumably, someone just made up a big bucket of it, put it into bottles with simple labels and distributed it. Since it was not approved there were no standards to meet. It was still an oxalic acid solution whether it had been stored for a week in a fridge, or a year at the back of a warm shelf. I’ve never used the stuff sold by Thorne’s … we made it up from powder and so could be certain it was at the right concentration and had been stored properly.

      I hate to think why price Api-Bioxal would have been if they’d made a solution and guaranteed the ‘cold chain’ from manufacture to sale.

      However, as shown here, it is really easy to prepare … a small consolation perhaps, but still far from perfect.

  2. Mike

    I’ve heard people say you should avoid hard water as it makes the oxalic solution cloudy and that’s a scary thing. It makes some sense that the mineral ions could bind with the acid to form salts and this may reduce the strength of the acid (but it’s decades since I did chemistry).
    Should I be using the water collected in the tumble dryer or save that for wax processing?

    1. David Post author

      Decades since I did chemistry as well … 😉
      If there is a precipitate or a cloudy appearance then clearly something’s wrong (though I’ve no idea what the colloidal silica in Api-Bioxal does in syrup). I’m sure it can’t do any harm to use some soft water (which may be rainwater).
      Maybe someone a little better qualified can comment?

      1. Dorothie

        I’ve just made some up for the first time, as per David’s instructions and the solution is definitely cloudy. We have very hard water here so I can only conclude this is why. If you look at the Dave Cushman site you will see a reference to the formation of insoluble calcium oxalate when using hard water. Don’t know how this might affect the bees but off to try some rainwater!!!!
        Will keep you posted

          1. Dorothie

            So, I have used rainwater today and apart from the yellow colour of the water itself, the solution is clear. So I would suggest that precipitates are formed in hard water.
            I don’t see that this will bother the bees (but happy to stand corrected!) and most of my fellow beekeepers locally seem not to worry about it.
            But if you are concerned and live in a hard water area then go for rainwater or deionised.
            I did try boiled and cooled which I use in my iron, but that made no difference.
            Merry Christmas everyone!

          2. David Post author

            Many thanks for reporting back Dorothie
            When I update the pages – or next time I cover dribbling – I’ll include reference to using rainwater.
            Merry Christmas

  3. calum

    here in good old Germany, they supply the oxalic treatment in a handy pack:
    1 syringe
    1 baggie of sugar
    1 wide necked shallow bottle already containing the oxalic acid solution.
    The recommendation is to warm the solution to just warm to the touch and add the sugar onsite. And apply immediately. I’d recommend following that, think about the container you want to use before hand, and make the acid solution first, then add the sugar when you are going to use it, so it cannot go off.

    1. David Post author

      Hi Calum
      That’s a good way to do it – at least separating the sugar from the OA. Less certain about the ‘wide necked shallow container’. There are easier ways to administer it than that which I’ll discuss in an upcoming post (in which I’ll link this this suggestion about OA in water + sugar).

  4. Mike Street

    Hi David this is my first winter as a bee keeper and I have followed you posts on oxalic acid with great interest. However I am confused is treatment with oxalic acid (or Api-Bioxal) solution the same or is one safer than the other? by safer I mean for the Bees. The SDS for both are quite clear and I will take great care in handling this material. I haven’t as yet had any high infestation problems but the chance to start the new season almost mite free has got to be good for the colony. regards Mike S

    1. David Post author

      Hi Mike
      Oxalic acid and Api-Bioxal are equally good (or bad) for your bees. They’re effectively exactly the same as far as the bees (or Varroa) are concerned. Oxalic acid, bought as a 99.something percent powder is the active ingredient in Api-Bioxal. The latter contains additional glucose and some sort of anti-caking agent (powdered silica?), neither of which are harmful for your bees.

      Actually the only thing that will distinguish between them is your wallet 😉

      But only Api-Bioxal is approved for use. Remember that all use of medicines should be noted in your hive records.

      Used properly, taking appropriate care and following the recommended instructions, neither is dangerous for the beekeeper.


  5. Mike Street

    Thank you David thats put my mind at rest, as I said before the the vertical learning curve can be a bit daunting for new Bee keepers sometimes and sites like yours are an absolute god send thank you Again. On a different subject I notice from the pictures on the site there is a lot of OSR near you, have you got any articles on OSR in your archives? Like best time to get the honey off before it goes rock hard!! I am in Suffolk and our 1st hive is about to be at the centre of 100 acres come next spring. It was wheat last and having spoken to two local farmers I have a map of where OSR is growing and one of the sites we were going to use as an expansion apiary is also slap bang on the edge of another 100 acres so any advice would be most welcome. All the best Mike S

    1. David Post author

      Hi Mike
      Don’t be mislead by the pictures … some of those pics are a few years old, before I moved North. OSR is also quite photogenic so perhaps features rather more than is truly representative. I think with the neonic ban there’s probably less being grown as well. You need to make sure your colonies are strong to fully exploit OSR. This might mean feeding them thin syrup in February/March. Harvesting can be tricky … probably don’t wait until every cell is capped over, just do a shake test and if the stored nectar stays in the open cells it’s ready. Keep the supers warm to help spin the honey out. If you leave it too late the honey sets in the frame and you’re pretty much stuck with melting it out or possibly feeding it back.
      In a good year, with strong colonies, yields can be fabulous … make sure you have enough buckets to store it all in. It’s a bit smelly when fresh, but improves with age, and makes reasonable soft set honey.
      Have fun

  6. Mike Street

    Thanks again for the info ok now I have to get my head around extraction storage warming cabinets etc etc. When I retired nearly 2 years ago my colleagues all said you will be bored once the house and the garden are done. Well the house is done and I have not started the garden too busy with the Bees. Bored!!!! Fat chance Take it easy David Mike S

  7. Dorothie

    Hi David, I hope you are well,
    I trickled OA nearly 4 weeks ago and have been doing weekly drop counts. After the initial mega drop the numbers declined (smiley face) but this week they were up again (sad face!)
    The colonies were out and about and quite active as we amazingly had some sun so I wondered if increased drop was a feature of increase bee movement in the hive? I took a peek in the top and they were busy in there!! I have poly hives.
    Is it right that you can’t trickle OA more than once in the winter?
    Any suggestions?
    Many thanks

    1. David Post author

      Hi Dorothie
      Mite drop is a rather inaccurate predictor of infestation levels as it depends upon the activity of the colony, the amount of brood and – if they’ve been treated, as your have – the lingering effects of the miticide. Unless the numbers are high I wouldn’t worry too much and just assume it’s a combination of the warmth of the sun on the hives causing increased activity by the colony.
      The general advice is to only trickle-treat once per season. It damages unsealed brood. I know some people who have trickle-treated multiple times but I haven’t and so I cannot recommend it. This is one of the reasons I’d prefer to use sublimation (vaporisation), which can be used repeatedly and does not harm open brood.
      Keep an eye on the levels, but don’t worry unduly unless you’re talking about dozens or hundreds … in which case I’d be thinking of an additional early-season treatment of some sort.

      1. Dorothie

        Thanks David,
        I’ll continue with my weekly counts along with noting other activity, weather etc. and go from there in the spring

  8. Rich

    David, there are lots of people vaporising acid through the summer to keep mite levels down. Why don’t more people trickle throughout the summer? Perhaps in the evening when the foragers are home? Three weekly treatments would keep the phoretic mite population down, no?

    1. David Post author

      Hi Rich

      Trickled OA damages (kills) open brood whereas vaporised OA does not. Both only target phoretic mites so – as you suggest – multiple treatments are necessary.

      However, there’s a problem if multiple treatments are needed through the season. If one treatment is used properly (and effectively) then the rate of mite replication shouldn’t be sufficient to make another treatment needed until midwinter. This suggests to me that a) the first treatment didn’t work, or b) there are lots of mites being acquired (through drifting/robbing) from nearby infested hives.

      This is one of the reasons that coordinated treatment makes sense.


      1. Rich

        Thanks David. Didn’t know that trickled OA killed larvae and eggs. So grateful for your website. It’s a mine of useful information. Alongside Dave Cushman, most of the internet’s beekeeping profferings are garbage.

  9. Caitlin

    Hi, this is my first year with bees. I caught 2 swarms this June and treated mid August with Apiguard and I am thinking ahead about a winter treatment. I have oxalic acid dihydrate lying around anyway as I use it as a step in a varnishing process for wood, so figure I should try it since I won’t have to purchase any additional treatments.
    I am a little worried about opening the hive in winter. From what I have learned from my local bee meetings people say not to open the hives until spring when things warm up. Is it just a matter of trickling very quickly or do you do anything to bring the temperature of the hive back up? I am located in the southwest of Ireland so we are cold in winter but not covered in snow or anything.
    Thanks for this very informative and practical article!

    1. David Post author

      Hello Caitlin

      Opening the hive in midwinter is OK if there’s a good reason to do so. Far better treating with OA in midwinter when the colony is broodless than waiting for spring ‘when things warm up’. By then the colony will be have brood and the efficacy will be reduced hugely.

      Don’t believe what the local ‘experts’ say unless they can justify it and have done it themselves and shown it to be detrimental … there’s too much dogma in beekeeping.

      OA treatment takes 30-60 seconds per colony. I show how to trickle treat, including videos, on https://www.theapiarist.org/trickle-and-treat/. I’ve opened hundreds of colonies in midwinter over the last few years and all have benefitted from the treatment received.

      There’s lots of other information on the timing etc of winter treatment on the pages – have a look at the Varroa control heading under ‘practice‘.


Comments are closed.