Oxalic acid (Api Bioxal) preparation

This post updates and replaces one published three years ago (which has now been archived). The registered readership of this site has increased >200% since then and so it will be new to the majority of visitors.

It’s also particularly timely.

I will be treating my own colonies with oxalic acid in the next week or so.

Mites and viruses

Varroa levels in the hive must be controlled for successful overwintering of colonies. If you do not control the mites – and by ‘control’ I mean slaughter 😉 – the viruses they transmit to the overwintering bees will limit the chances of the colony surviving.

The most important virus transmitted by Varroa is deformed wing virus (DWV). At high levels, DWV reduces the lifespan of worker bees.

This is irrelevant in late May – there are huge numbers of workers and they’re only going to live for about 6 weeks anyway.

In contrast, reduced longevity is very significant in the winter where more limited numbers of overwintering bees must survive for months to maintain the colony through to the Spring. If these bees die early (e.g. in weeks, not months), the colony will dwindle to a pathetic little cluster and likely freeze to death on a cold winter night.

Game over. You are now an ex-beekeeper 🙁

To protect the overwintering bees you must reduce mite levels in late summer by applying an appropriate miticide. I’ve discussed this at length previously in When to treat? – the most-read post on this site.

I’d argue that the timing of this late summer treatment is the most important decision about Varroa control that a beekeeper has to make.

However, although the time for that decision is now long-gone, there are still important opportunities for mite control in the coming weeks.

In the bleak midwinter

Miticides are not 100% effective. A proportion of the mites will survive this late summer treatment 1. It’s a percentages game, and the maximum percentage you can hope to kill is 90-95%.

If left unchecked, the surviving mites will replicate in the reducing brood reared between October and the beginning of the following year. That means that your colony will potentially contain more mites in January than it did at the end of the late summer treatment.

Mid September

Late summer mite treatment and no midwinter treatment.

Over several years this is a recipe for disaster. The graph above shows modelled data that indicates the consequences of only treating in late summer. Look at the mite levels (in red, right hand vertical axis) that increase year upon year.

The National Bee Unit states that if mite levels exceed 1000 then immediate treatment is needed to protect the colony. In the modelled data above that’s in the second year 2.

In contrast, here is what happens when you also treat in “midwinter” (I’ll discuss what “midwinter” means shortly).

Two optimal treatments

Two optimal treatments

Mite numbers remain below 1000. This is what you are aiming for.

For the moment ignore the specific timing of the treatment – midwinter, late December etc.

Instead concentrate on the principle that determines when the second treatment should be applied.

During the winter the colony is likely to go through a broodless period 3.

When broodless all the mites in the colony must, by definition, be phoretic.

There’s no brood, so any mites in the colony must be riding around on the backs of workers.

A phoretic mite is an easy mite to kill 4.

A “midwinter” double whammy

A single oxalic acid based treatment applied during the winter broodless period is an ideal way to minimise the mite levels before the start of the following season.

Oxalic acid is easy to administer, relatively inexpensive and well-tolerated by the bees.

The combination – a double whammy – of a late summer treatment with an appropriate miticide and a “midwinter” treatment with oxalic acid should be all that is needed to control mites for the entire season.

However, “midwinter” does not mean midwinter, or shouldn’t.

Historically, winter mite treatments were applied between Christmas and New Year. It’s a convenient time of the year, most beekeepers are on holiday and it’s a good excuse to avoid spending the afternoon scoffing mince pies in front of the TV.

Or with the outlaws inlaws 😉

But by that time of year many colonies will have started brooding again.

With sealed brood, mites have somewhere to hide, so the treatment will be less effective than it might otherwise have been 5.

Why go to all the trouble of treating if it’s going to be less effective than it could be?

The key point is not the timing … it’s the broodlessness of the colony.

If the colony is broodless then it’s an appropriate time to treat. My Fife colonies were broodless this year by mid-October. This is earlier than previous seasons where I usually have waited until the first protracted cold period in the winter – typically the last week in November until the first week in December.

If they remain broodless this week I’ll be treating them. There’s nothing to be gained by waiting.

Oxalic acid (OA) treatment options

In the UK there are several approved oxalic acid-containing treatments. The only one I have experience of is Api-Bioxal, so that’s the only one I’ll discuss.

I also give an overview of the historical method of preparing oxalic acid as it has a bearing on the amount of Api-Bioxal used and will help you (and me) understand the maths.

OA can be delivered by vaporisation (sublimation), or by tricking (dribbling) or spraying a solution of the chemical.

I’ve discussed vaporisation before so won’t rehash things again here.

Trickling has a lot to commend it. It is easy to do, very quick 6 and requires almost no specialised equipment, either for delivery or personal protection (safety).

Trickling is what I always recommend for beginners. It’s what I did for years and is a method I still regularly use.

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.

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

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

Spot the difference ...

Spot the difference …

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) and these additives make it much less suitable for sublimation. I’ll return to these additives in a minute or two. These additives make the maths a bit more tricky when preparing small volumes at the correct concentration – this is the purpose of this post.

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’ve dealt with how to administer OA by trickling previously. This is all about preparation and storage.

The how much is easy.

You’ll need 5ml of oxalic acid-containing solution per seam of bees. In cold weather the colony will be reasonably well clustered and its likely there will be a maximum of no more than 8 or 9 seams of bees, even in a very strong colony.

Hold on … what’s a seam of bees?

Three 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.

So, for every hive you need 5ml per seam, perhaps 45ml in total … with an extra 10% to cover inevitable spillages. It’s not that expensive, so don’t risk running out.

And the strength?

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

Calculators at the ready!

The oxalic acid in Api Bioxal is actually oxalic acid dihydrate. Almost all the powdered oxalic acid you can buy is oxalic acid dihydrate.

The molecular formula of oxalic acid is C2H2O4. This has a molecular weight of 90.03. The dihydrated form of oxalic acid has the formula C2H2O4.2H2O 8 which has a molecular weight of 126.07.

Therefore, in one gram of oxalic acid dihydrate powder (NOT Api Bioxal … I’ll get to Api Bioxal in a minute! Have patience Grasshopper) there is:

90.03/126.07 = 0.714 g of oxalic acid.

Therefore, to make up a 3.2% oxalic acid solution in 1:1 syrup you need to use the following recipe, or scale it up as needed.

  • 100 g tap water
  • 100 g white granulated sugar
  • Mix well
  • 7.5 g of oxalic acid dihydrate

The final volume will be 167 ml i.e. sufficient to treat over 30 seams of bees, or between 3 and 4 strong colonies (including the 10% ‘just in case’).

The final concentration is 3.2% w/v oxalic acid

(7.5 * 0.714)/167 * 100 = 3.2% 9.

Check my maths 😉

Recipe to prepare Api-Bioxal solution for trickling

Warning – the recipe on the side of a packet of Api-Bioxal makes up a much stronger solution 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. 

There’s an additional complication when preparing an Api-Bioxal solution for trickling. This is because Api-Bioxal contains two additional ingredients – glucose and powdered silica. These cutting agents account for 11.4% of the weight of the Api-Bioxal. The remaining 88.6% is oxalic acid dihydrate.

Using the same logic as above, 1g of Api-Bioxal therefore contains:

(90.03/126.07) * 0.886 = 0.633 g of oxalic acid.

Therefore, to make up 167 ml of a 3.2% Api-Bioxal solution you need to use the following recipe, or scale up/down appropriately:

  • 100 g tap water
  • 100 g white granulated sugar
  • Mix well
  • 8.46 g of Api-Bioxal

Again, check my maths … you need to add (7.5 / 0.886 = 8.46) grams of Api-Bioxal as only 88.6% of the Api-Bioxal is oxalic acid dihydrate.

Scaling up and down

8.46 g is not straightforward to weigh – though see below – and 167 ml may be too much for the number of hives you have. Here’s a handy table showing the amounts of Api-Bioxal to add to 1:1 syrup to make up the amount required.

Api-Bioxal recipes for 3.2% trickling in 1:1 syrup

The Api-Bioxal powder weights shown in bold represent the three packet sizes that can be purchased.

I don’t indicate the amounts of sugar and water to mix to make the syrup up. I’ll leave that as an exercise for the reader … remember that 100 g of sugar and 100 ml of water make 167 ml of 1:1 (w/v) syrup.

Weighing small amounts of Api-Bioxal

The amount of Api-Bioxal used is important. A few grams here or there matter.

If you are making the mix up for a limited number of hives you will have to weigh just a few grams of Api-Bioxal. You cannot do this on standard digital kitchen scales which work in 5 g increments.

Buy a set of these instead.

Digital scales … perfect for Api-Bioxal (and yeast)

These cost about a tenner and are perfect to weigh out small amounts 10 of Api-Bioxal … or yeast for making pizza dough.

A few words of caution

I don’t want to spoil your fun but please remember to take care when handling or using oxalic acid, either as a powder or when made up as a solution.

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 well rinsed plastic milk bottle, very clearly labelled, is a good way to store the solution prior to use.


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 toxic 11.

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 12.

Therefore 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 should remain well below toxic levels for at least one year. However, don’t store it for this long … use it and discard the excess.

Or prepare excess and share it with colleagues in your beekeeping association.

Don’t use discoloured oxalic acid solutions as they’ve been stored incorrectly and may well harm your bees.

Another final few words of caution

I assume you don’t have a fridge dedicated to beekeeping? That being the case please ensure that the bottle containing stored oxalic acid is labelled clearly and kept well out of the reach of children.


A quick trawl through the Veterinary Medicines Directorate database turns up several oxalic acid-containing solutions for managing Varroa. These include:

  • Oxuvar – approved for trickling or spraying, an aqueous solution of oxalic acid to which you add glucose if you intend to use it for trickling.
  • Oxybee – approved for trickling (and possibly other routes, but the paperwork was a minefield!), contains oxalic acid, glycerol and essential oils and is promoted as having a long shelf life.
  • VarroMed – approved for trickling, contains oxalic acid and formic acid and can be used throughout the year in one way or another.

I’ve not read the documentation provided with these and so don’t know the precise concentration of oxalic acid they contain. It will be listed as an active ingredient. I have not used these products. As with everything else on this site, I only write about methods or products I am familiar with. I therefore cannot comment on their relative efficacy compared to Api-Bioxal, to Apivar or to careful siting of your hives in relation to ley lines … or 5G phone masts.



  1. This is not necessarily due to resistance, but reflects the fact that not all mites come into contact with the miticide, and of those that do, not all are killed.
  2. Note that the colony was primed with just 20 mites at the beginning of the first season. How many of your colonies have only 20 mites at the beginning of the year?
  3. Not always, and it depends very much upon latitude … and altitude.
  4. Which should perhaps be engraved on the reverse of every hive tool sold to beginners, the front side being of course engraved with the immortal words “Knocking back queen cells is not swarm control”.
  5. Did you not read your hive tool?
  6. Yes, faster than vaporisation, before you ask.
  7. Also see the additional note at the bottom of the page.
  8. Most readers will be able to recognise the H2O at the end, meaning ‘water’. C is carbon, H is hydrogen and O is oxygen. The subscripted numbers indicate the numbers of each atom in the oxalic acid. The 2H2O at the end indicates the di (i.e. two) ‘hydrate’, molecules of water.
  9. Actually 3.206586826347305% but we’ll round it down for convenience
  10. 500 g x 0.01 g
  11. Jachimowich T., El Sherbiny G., Zur Problematik der verwendung von Invertzucker für die Bienenfüttering, Apidologie 6 (1975) 121-143 … unsurprisingly, a paper I’ve not read.
  12. See Bogdanov S., Kilchenman V., Chamere J.D.. Imdorf A. (2001) available online and 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.

60 thoughts on “Oxalic acid (Api Bioxal) preparation

  1. Barbara Keating

    This is really clear and good advice, thank you ! I’m now sold on the GasVap as it it very quick and easy enough to use, once you are used to putting on the mask… Have you tested this method yet, and if so, what are your thoughts ?

    1. David Post author

      Hi Barbara

      Not the GasVap, but I’ve got a Sublimox. The price (and efficiency) of the Sublimox means I have no intention of ever buying or trying another vaporiser again 😉

      I’ll probably return to this topic again in the winter sometime. The Sublimox is an excellent piece of equipment.


      1. Geoff

        I have tested the gas vap.

        I think it is basically a great idea….it is quick to use and cheap but there are problems ….I would make a few comments that might be helpful……

        My main issue is the use of a butane torch. Not only does butane fail to vaporise and produce a flame at lower temperatures, (i.e. as applies in the field with a winter treatment) but the chemical reaction that produces the flame rapidly cools the torch to the point where it fails or is unreliable.

        I recently had a very frustrating experience when using the gas vap …..the temperature was 9 degrees centigrade and the torch spluttered after just two unreliable treatments….. it took me a while to realise that butane was the problem. If you are treating just a few hives, u might get away with it but it will help if u just keep the torch warm by carrying it next to your body or storing indoors before going to hives etc….I’m sure u can think of a safe method.

        If u are treating a number of hives then it is best to use propane ….this vaporises reliably at much lower temperatures and provides enough uummmph to spread the vapour reliably round the hive. You will need to adapt propane torch to fit the gasvap but this it isn’t difficult to do…..

        Finally, the gas vap needs to be held at the hive entrance …..this means u are standing very near and pretty much directly above the vaporiser…..be careful! I always hold my breath for the duration of the treatment and have also developed a simple device to minimise seepage from the entrance i.e. to trap the vapour in the hive. If u have a spare hand, then use this to press mask firmly onto face …u need a mask of the correct grade etc etc etc

        1. David Post author

          Hello Geoff

          Thank you for those comments. I looked at the videos on the sellers website and was concerned about the amount of OA vapour he exposed himself to. While the principle of the machine appears OK, the practice certainly looks flawed (as reflected in your comments about holding your breath while treating 😉 ). The beauty of the Sublimox is that, with a properly sealed hive, almost no vapour escapes. The vaporiser is only inverted once the delivery nozzle is inserted through the sidewall of the floor.

          I own and use a Sublimox and I recommend it.

          My comments on the gas vap above, in response to yours, are only posted here for information … they are not an endorsement or recommendation by me for the equipment. Vaporised oxalic acid is very unpleasant – to mites and beekeepers – and anyone using either homemade or commercially purchased vaporisers must be aware of the potential risks and take all of the necessary safety precautions.


    2. Alan Lucas

      I have a Gas-Vap and it’s quick and easy to use though the basic supplied gas torch is very poor quality, they sell a better one now but I prefer my chefs torch from the kitchen. With a little looking around I found a 3M full face mask with acid filters for about £100 and I wouldn’t use the gas vap without one, it just isn’t worth the risk of getting a lungful of OA. On the other hand the trickle method does mean you gat a quick look at your bees. Curious division on opening a hive in cold weather but then that’s bee keeping. If you ever get the chance go and listen to an older bee keeper like Roger Patterson as much of what he does (successfully) is a bit different from what I was taught by a younger generation of well qualified and just as successful bee keepers.

      1. David Post author

        Thanks for the info Alan … I’ll still be sticking to the Sublimox. I’m never sure with some of the more ‘homemade’ pieces of kit quite where liability lies should something go wrong, or how much testing has been done to demonstrate safety, efficacy and longevity.

        And yes, there’s a wide variety of views on opening hives in cold weather. You have to do a sort of cost/benefit analysis … the cost is that the colony is disturbed, the cluster broken up a bit, the propolis seal cracked etc. However, if we’re considering OA trickling, the benefit – assuming it’s done at the right time of the winter – is dramatically reduced mite levels to start the season ahead.

        If I had to choose between trickle treating a broodless colony on a cold, drizzly day with a gusty wind or a colony rearing early brood on a calm day with with some lukewarm sunshine I’d always choose the former. If you’re going to treat, why bother treating at a suboptimal time of the winter?


  2. Karl and Ella Kusta

    Fantastic article as always and a joy to read. We are in our 2nd season of Beekeeping and have a question: Given we don’t inspect when the temperatures are low in November and December, how do you know if any one of your hives are broodless? We live in Wiltshire and use poly hives and so just wonder if we need it to be later on in December before our hives would be broodless.
    Thanks in advance.

    1. David Post author

      Hello Karl and Ella

      Tricky, but not impossible. I’ve written about this previously. The most certain way is to look. If you don’t want to do this then slide in a clean Varroa tray and look for biscuit coloured cappings – in which case brood is being uncapped. You can also take the temperature of the cluster with suitable equipment (something I’ll write about at some point).

      In Wiltshire there’s a possibility the colonies are never broodless. My guess is that colonies are more likely to be broodless earlier in the winter than after the solstice. That’s certainly the case in Scotland.

      For starters I’d try a Varroa tray for a few days … see hat that looks like. I’m going to return to this topic in the next week or two as I’m expecting to treat some of my colonies in the next 7-10 days.

      If you’re in your second year you’ve already overwintered colonies, so know it’s not a real problem if the colonies are prepared properly.


  3. Jean Stewart

    I use oxalic acid in the winter. I use a vaporiser; as a somewhat “lone” beekeeper, I have never used a dribble. My question is about the temperature. We are cold here in Canada; isn’t it a) super hard on the colony even opening the hive in winter letting their heat out and the cold in, and b) drenching them with liquid, even warmed liquid, will turn cold quickly and the time the lid is open to do the drizzle is longer.

    My beehives are not inside a shed like yours are or were at the old site.

    Isn’t this method more suitable to summer temperatures?

    1. David Post author

      Hello Jean

      This post was specifically about preparing the solution. Please see this post, referred to above, about how to administer it.

      I don’t know how cold your part of Canada is but OA trickling is very quick. It’s faster than vaporisation in my side-by-side tests, but I acknowledge you have to open the hive. I’m not suggesting do this and do not vaporise. I use either, as appropriate depending upon the conditions.


  4. Simon Kellam

    Oxalic acid is toxic!! I really do wonder how this can be good for the bees. Secondly the more we treat for varroa we must be hindering the bees to be able to adapt to the mite. Just a thought.

    1. David Post author

      Of course it’s toxic. If it wasn’t the mites would be unaffected. That’s why it’s “good for the bees”. The OA may be bad, but the mite and the viruses it transmits is much, much worse.

      To be a bit more specific … it’s toxic for mites and humans (for different reasons). If it is toxic for bees, it’s at a much lower level than it is for mites. It’s known to be toxic to open brood … that’s probably the acidity. However, I stress the importance of treating when broodless.

      Why is it important to get bees to adapt to the mite? We don’t (seriously) get humans to adapt to measles and the current pandemic shows that adapting to SARS-CoV-2 is going to result in many hundreds of thousands of deaths. There are ‘mite adapted’ bees – I’ve written about them extensively elsewhere on the site – but they’re probably not much use for beekeeping. Or pollination. Small, swarmy colonies. Mites are a human inflicted scourge on our bees. We have the ability to do something about it. I’m going to have a little rant about this later in the winter …


  5. Jackie Elliott

    Good read. I’ve never made up a solution of oxalic acid, I’m a MAQS fan and find that generally this annual treatment, where required, is adequate to control Varroa.

    Despite this I remember being taught that if you are making the solution up you add the oxalic acids crystal (legally licensed product!) to the liquid rather than the other way round, to avoid mini explosions. Was this just another old wives’ tale?

    Always an interesting read. 😊

    1. David Post author

      Hello Jackie

      I’m pretty sure it doesn’t explode whichever way you add them together. Certainly the Api-Bioxal label (that’s the only one I’ve looked at recently) doesn’t have “caution – risk of explosion” written on it.

      Sodium and potassium are the two chemicals well known to react ‘excitingly’ when you add water. Here are some others if you’re feeling reckless 😉


  6. vince poulin

    David – another extremely valuable post. Personally you can’t write enough about mites, miticides, and the results of not-treating for mites. Clearly it does take a few years to get ones head around the issue and approaches to address the risks. Your posts benefit a lot of people. Currently we are going through a serious mite “pandemic” with respect to our local hives. Very significant losses regardless of bee keeper skill level – and treatment option used. This post – as timely as all your other best practices advice. Readers should not waste a second taking advantage of strategy outlined.

    1. David Post author

      Thanks Vince

      Unfortunately, where mites are concerned we can’t afford to take our eye off the ball …


  7. Peter Thompson

    I’ve heard the message many times, which you have clearly conveyed in graphic form; this being that, left untreated, mite numbers will increase to the point that if not gone earlier, colonies will die after three or four years. But is this strictly true, since the turnover of bees during that time is vast, and after that length of time it will have had many changes of bees, may have seen bees and brood redistributed to and from other hives, might have been split and will probably have a different queen?

    1. David Post author

      Hello Peter

      There’s ample evidence in the literature that, left untreated, the majority of colonies die in 2-3 years. There are some large scale experiments that support this rather gruesome statistic. I’ll maybe try and get a post finished (it’s half written) on the “Bond experiment” (as in Live and Let Die) before the end of the year.

      Yes, the turnover of bees is vast, and there’s a significant number that move in and out of our colonies. But, at some point the colony has to get through the winter. Before the winter there’s lots of brood. Most (~90%) of the mites are infesting the brood. If they are infesting the winter bees, those bees will die. It’s as simple as that.

      Very high mite levels leads to very high infestation levels which results in very high viral loads in the winter bees. There’s no turnover of bees over winter (at least, no significant turnover). High viral loads either kill the pupae outright, lead to developmental defects, or reduce the longevity of the winter bees. If the bees in October have all died by February then the colony is dead.

      Thousands of beekeepers every winter demonstrate this. Colony losses in the US are running at 22% (it was 37% last year beeinformed.com). Losses in the UK average about 20%, often significantly higher.

      Where do you think the mites go during all that splitting, uniting, drifting, requeening etc?

      If they stay in the hive they carry on replicating.

      At some point they’ll get you.


      Sorry to be the bearer of bad news … all I’m trying to do is help beekeepers get their bees through the winter so they remain beekeepers.


        1. vince poulin

          Peter – David is 100% right. Listen to every word. Two years ago in our area hive losses were up to 70% amoung urban and commercial bee keepers. Last year was a low mite-year and most hives over-wintering well. I had 100% survival. This year a far different story. I have 86% hive losses (6 out of 7) all due to high mite infestation levels. My experience has been shared with many other keepers. We will soon better understand the losses but it looks grim. Listen to David! Probably the most important lesson a bee keeper can learn is mites are one challenging problem and one not easily controlled.

          1. David Post author

            Hi Vince

            There’s a current thread on Bee-L about high mite levels even post-treatment, discussing links to late season robbing/drifting.

            I’ll write something about the Bond experiment sometime this winter, together with a post on treating and not treating as a strategy.


  8. Keith

    Hi David, interesting and informative as usual. Can you advise on the specification that must be adhered to in mask(S) selection and any particular recommendations perhaps. I also seem to recall that “straight” Oxalic acid is not “licensed” for treatment and api bioxal must be used now instead , which makes it a nightmare for use with a sublimator due to the glucose ?

    1. David Post author

      Hello Keith

      That’s an area I cannot, or will not, provide recommendations on. If beekeepers are going to use chemicals that require it then it’s critical they do their own research to determine that the precautions they take are sufficient. I’m content with the PPE I use and I take a lot of care. However, it might be insufficient for a beekeeper who was less careful, or one with a respiratory condition.

      With apologies

  9. Neil

    Hi David,
    Thanks. After returning to beekeeping having had a break since Varroa first reared its beautiful (under a microscope) head I am learning all i can about them. Your article reminded me of some recent reading… Chronic Bee Paralysis Virus and Nosema ceranae Experimental Co-Infection of Winter Honey Bee Workers in Viruses 2013, 5(9), 2282-2297; https://doi.org/10.3390/v5092282
    which introduced IIR viruses. It gets interestinger and interestinger!

    1. David Post author

      Hi Neil

      Not sure what you mean by IIR viruses … I’m involved in some collaborative studies with Giles Budge in Newcastle looking at CBPV. One of the things Giles will be investigating is synergies between CBPV and other pathogens. We published our first paper earlier this year and I wrote about in in May.


  10. John Bolger

    Beautiful calculations.
    Perhaps a silly question but would 6-8 weeks of Apivar applied about this time be successful as it is not temperature dependant? Especially as colonies may fail to become bloodless.
    As a supplementary, why do you think your hives became bloodless in October? Was it so cold?

    1. David Post author

      Hello John

      You’re correct that Apivar isn’t temperature dependent, but it is dependent on the bees being in contact with it. In the winter the cluster moves about the hive and it can miss the strip entirely. There was a picture on Twitter of this recently, but I can’t now find it (typical!). In addition, it’s good to vary/alternate treatments and mine had Apivar in late summer.

      I think hives are more likely to be broodless (they’ll always be bloodless 😉 ) after the nectar flows stop and before the solstice. The queen then starts gearing up again … she has to or there won’t be a strong enough colony in the spring. I’m sure there are loads of places where colonies are not broodless overwinter, but – fortunately – in Scotland it’s expected. My bees on the west coast were rearing brood later than those on the east coast. The nectar on the east shuts down much earlier (no heather where I am).

      Finally, it differs from year to year … the weather will also influence things.


  11. Dafydd Jones

    Hi David,
    What’s the legal situation (if there is one?) Regarding making up and applying an oxalic acid treatment solution based on oxalic acid dihydrate sold for example as a timber boat deck cleaner, as opposed to spending about 30 times more on buying “approved” preparations like API boxal?
    Regards, Dafydd

    1. David Post author

      Hi Dafydd

      That’s very clear … oxalic acid, other than as one of the proprietary solutions/powders listed above (Api-Bioxal, Oxyvar, Oxybee and VarroMed) and on the VMD website, is not approved. I provide the ‘historical’ recipe for oxalic acid to help explain the dihydrate and the calculations to get the correct strength.

      You’re absolutely correct in saying that Api-Bioxal is – when you consider the chemicals alone, ignoring the licensing costs etc. – an outrageous cost when compared with buying 25 kg of oxalic acid. The latter is £50-75 in the UK compared with ~£85 for 350 g of Api-Bioxal (which I make 80 x more 🙁 ).

      But do the calculations the other way round … bought in bulk, Api-Bioxal costs about 72p per hive to treat (or perhaps half that for vaporisation … these figures are off the top of my head). That treatment could prevent mite build up and save the colony. That’s a trivial price to pay. How much honey does that hive produce a year? 25 kg ? Sold at what? £10/lb. That’s less than 0.2% of honey sales on this miticide.

      I’m not defending the price of Api-Bioxal. It’s robbery for what it contains. But that battle wasn’t even fought and lost. The story is murky and I don’t think it helps UK beekeeping (partly because we’re all so mean 😉 but mainly because it’s an inferior product to powdered OA dihydrate as it makes a real mess of vaporisers) but I think some perspective is needed on the actual cost/benefits vs. the relative costs of a product we’re no longer allowed to use for managing mites.

      Just to be crystal clear again … if it’s not approved by the VMD for treating mites on bees it should not be used (I’ll check with my lawyers if I need to make that bold or a larger font 😉 ).


  12. Dafydd Jones

    Hi David,
    You’ve made the situation clear regarding using OA dihydrate as a miticide. However, I have heard the argument made locally that it is ok to apply a home made OA/ sugar solution if it is done under the guise of a “supplement” rather than a miticide as supplements don’t need “approval”.

    1. David Post author

      Hi Dafydd

      That’s a new one to me … though I’ve heard many suggest that using OA to clean frames in situ is OK. I don’t promote that either as it’s not my understanding of the rules, which I think are pretty clear.

      Is there a way of determining whether bees were treated with Api-Bioxal or generic OA anyway? I expect there is if you spent a lot of money to detect traces of the powdered silica (or whatever it is) used as an anticaking agent in Api-Bioxal. I’d be astounded if anyone had done this, or would bother to.

      It’s worth noting that the complaints about the pricing of Api-Bioxal is because beekeepers were using cheap-cheapy OA dihydrate for many, many years before Api-Bioxal was licensed. If you similarly compare the price of MAQS and that of a few litres of formic acid you’ll find the cost difference is equally big … it’s just that DIY formic acid treatments were not widely used (though they are pretty straightforward to prepare and use … I’m told; again, not something I’ve done or would promote).


  13. Michael Hill

    Thanks for your interesting posts.
    What are the accurate sales pictured in your article?

  14. Elaine Robinson

    Thanks David, good article & excellent timing, on the lead up to treating, just waiting for our first decent cold snap. I usually sublimate my hives in winter but this year I’m also over wintering 3 poly nucs as well. I was thinking of trickling these with oxalic acid (Api-Bioxal), as I’m struggling to find a way of safely sublimating them with the Thornes pan-type vaporiser I have. However, a comment from a Regional bee inspector, who said he has experience of trickling causing dysentery in smaller colonies such as Nucs, has put me off. Have you had any experience of this / any advice for treating smaller colonies?

    1. David Post author

      Hello Elaine

      I’m sure I’ve trickle-treated nucs in the past, though it would be 5+ years ago. I don’t remember any issues with dysentery. Considering the dose is per seam it cannot be due to overdosing a smaller colony, unless the instructions were ignored.

      If your nucs have removable floors you can create a simply plywood floor with a entrance slot to suit your vaporiser. You then just lift the box temporarily onto the wood floor, treat them and (after letting them settle) move them back to their original floor. If you’ve got one of the Maisemore-type boxes with an entrance in the front wall you can use a Sublimox-type ‘active’ vaporiser, but not the ‘passive’ pan types. Some people try and treat through the open mesh floor but my attempts at doing that resulted in a lot of the OA just clogging up the mesh … 🙁

      Could you try a deep eke above the nuc, with a suitable slot to allow the pan vaporiser to be inserted? On a cold day the bees rarely move much at all.


  15. Kate Murdoch

    Hi David

    After such a thorough trail of questions relating to this post, I feel a bit lightweight asking this – but here goes. I have two hives on double brood. Is is critical to dribble onto the seams of BOTH brood boxes, (which would necessitate more disruption to the hives) or do you think the 5ml per seam will work its way down through to the bees in the lower box?

    And, if you aren’t certain about broodlessness, should you or shouldn’t you treat now? I will take your earlier advice and put in the varroa boards for a few days monitoring and hope that the picture becomes clearer. This is my first winter and I am definitely feeling unsure of my skills as a beekeeper… I’m sad to say that I have yet to develop a ‘feel’ for the art of beekeeping.

    1. David Post author

      Hello Kate

      When I trickle treat double brood colonies I treat every seam of bees … and there are potentially seams in both the upper and lower box. However, it’s worth checking … colonies will be shrinking in size by now and, if you have good insulation over the crownboard, the colony tends to cluster high up in the box. You might find the lower box is largely empty.

      Since we’ve not had a proper cold snap I might be tempted to wait a bit, but it would be dependent on the ‘normal’ winter you experience. Check the boards, use your judgement based upon previous years. If I get time I’m going to write some more about this later in the week.


  16. David

    Great write up and very useful. Thanks.
    Is there any chance of resistance to Oxalic acid developing at some point or any evidence that might be likely in the long haul. Seems like it hasn’t happened as yet.
    Just found your blog and great read, especially nice for an ex PhD student (Drosohila) who spent many hours looking out at St Mary’s quad from the Bute medical building many years ago.
    Seattle , USA

    1. David Post author

      Hi David

      All sorts of changes around the Bute building these days with the new Music Centre being built. Other bits you’d still recognise, and the inside of the Bute is still impossible to navigate around … at least to us arrivistes based out on the North Haugh campus.

      OA appears to damage the footpads and mouthparts of mites, though the precise mechanism by which it causes damage is still unclear (at least to me). Certainly there’s no evidence for resistance yet, and reports of lack of efficacy are probably issues with the way it was administered (or the presence of brood). The footpad and mouthparts are complex anatomical features, the function of which depend upon dozens or hundreds of proteins. It therefore seems unlikely that a single mutation (or a small number) could confer resistance. In contrast, the pyrethroid in Apistan has a single target site, and a point mutation can reduce binding, so rendering the miticide ineffective. This mutation is widespread, and I’d bet that the trace levels of pyrethroids in commercial foundation contribute to continued selection for the mutation in the mite population.

      In many ways oxalic acid is an excellent miticide. It’s cheap. It can be administered in a variety of different ways. It’s a natural product (so compatible with organic beekeeping I believe) and it’s very effective if used properly. Of course, it’s often used incorrectly 🙁 and local or national regulations have restricted some of the benefits, by limiting the authorised routes or contaminated the product with unnecessary additives that make is both more expensive and much less useable.

      Progress … but not really 🙁


      1. David J Peel

        Thanks just seems a possibility what with the lack of rotation of treatments that seems to be common place but as you say its a more complicated pathway but I wouldn’t count out some behavioural change at some point. Hope not anyway.
        I was at the Bute just when they amalgamated all the separate depts into one biological preclinical medicine dept, so lots of hastily built and knocked through entrances to join up pieces that seemed to before have lived separate existences. Was amazed at the separate student/staff/lecturer toilets for each dept and the myriad of dark rooms in obscure places. I hope they’ve done something with the natural history museum, was a bit sorry and unopened to the public but where else in scotland could you see a stuffed thylacine. Anyway I missed the morning and afternoon teas, they laugh when you mention that in a US science lab.

        1. David Post author

          Hi David

          I think most beekeepers do rotate treatments – OA in the winter and something else in late summer (formic acid, Apivar, thymol etc.). If they don’t they certainly should consider doing so.

          The Bell Pettigrew museum is still going strong. There’s some great stuff in it. We use it for a variety of ‘social’ type events, such as post-graduation drinks, and offer tours on Open Days. I’m not sure how heavily used it is between then, or whether it’s routinely open to the public, but it’s certainly looked after well. It’s also available to hire.


  17. Alistair Black


    Hi David,

    Publications always say 3.2% concentration in a syrup for a trickle method. Why this concentration?
    Api-bioxal 35g pack says it is a 4.2% concentration for 10 hives.

    Is it simply a case of diluting the chemical down to a 3.2% or does Api-bioxal need to be applied at a 4.2% concentration?

    The Moray Dinosaurs calculator for Oxalic Acid suggests that 35g is sufficient to treat 25 hives. What is correct? Will a 4.2% concentration cause the bees a problem?

    1. David Post author

      Hello again Alistair

      I can’t comment on the link you provided as I didn’t write it and haven’t read it … and I don’t have time to check their maths or justifications.

      Mine are clearly explained above.

      Historically (as it says above) the UK treated using a 3.2% solution. This predates my involvement in beekeeping and I’m not sure how the figure was arrived at – I suspect someone did some empirical studies and that gave the best results.

      The 3.2% relates to the concentration of the active ingredient, which is oxalic acid. Api-Bioxal has the same active ingredient, so if you want to use it at a final concentration of 3.2% you need to dilute it more than is recommended on the packet.

      Yes, this is “not following the instructions” but it’s not the first time that a miticide (labelled for use across Europe) has carried instructions that were either unsuited to UK users, UK historical precedence, or UK hives. MAQS was the same.

      I don’t know whether 4.2% is more toxic to bees than 3.2%. It is toxic, so it might be. I always dilute Api-Bioxal to 3.2% as that’s the concentration that hundreds of thousands of hives have been treated with over the last 20+ years in the UK. That’s good enough evidence for me and is based on the known effective (and minimally toxic) concentration of the active ingredient (oxalic acid). Since the active ingredient in oxalic acid and Api-Bioxal is one and the same thing this makes sense.

      I’ll try and dig out some oxalic acid toxicity data sometime over the winter.


  18. vince poulin

    Interesting that here in Canada our OA is sourced from a pharmaceutical company. It is OA dihydrate 99.6% and a registered pharmaceutical miticide. None of our local suppliers to my knowledge have Api-Bioxal. In our area they offer the “pan” style vapourizer. It is locally manufactured and powered using a 12-volt car/motorcycle battery. I have found OA to condense occasionally on a Varroa screen but haven’t thought a problem. When first used I placed the tool directly into the lowest brood box with no adjustments in height. This resulted several times in dead bees and melted wax. The following spring I designed bases that allowed the unit to be inserted below a Varroa screen and into the area where the Varroa board normally sits. The single issue I have had with this method it the tool has to be cooled following use which can take a few minutes. I haven’t tried sticking it in a bucket of water but that’s an idea.

    1. David Post author

      Hi Vince

      Api-Bioxal is licensed and sold in Europe and the UK. At least you’re saved paying the mark up!

      It sounds like you have a solution to using a pan-type vaporiser under the OMF. I’ve never tried this as my vaporiser is a different design. Mine does not need cooling between hives as the OA is added to a cup and then the machine is inverted to drop the OA into the hot pan. As you’ll see from my response to Mark Haworth delivery is faster for an individual hive (~45 seconds) but slower when you take all the preparation time into account.

      But it’s the winter … what else would I be doing?


      1. vince poulin

        Well David – finally some zeros on mites, but frankly zero mites is also suggestive of few bees and near collapse of a hive. Still a few bees flying and some in the sugar feeder. It will be a miracle if this last hive survives. I removed the base yesterday (10c here) and cleaned out 300+ dead bees – DWV, dead larvae, black bees, fat bellies, also many normal etc. Cumulative mite kill in this remaining hive is 3,120. Many others not counted – flown away with bees, cleaned, etc. This means over 3,000 winter bees were likely host to a mite. Crazy bad. If you like I’ll email you the spreadsheet. I think you would find it interesting. Mite drops over treatment days. Sample size N = 7 hives. 6-collapsed. One remaining. Formic acid plus OA treatments – nothing worked. Still on the “learning curve!”.

        1. David Post author

          Hi Vince

          It’s always distressing to lose colonies when you suspect it’s for an avoidable reason. A cumulative mite kill of over 3000 is high, but not necessarily catastrophic. The key things are the strength of the winter colony and whether the winter bee population was exposed to that level of mites hen they were being reared. In our studies we see about 30% of infested pupae exposed dying or producing overt symptoms, the remaining 70% look ‘normal’. We’ve not measured the survival of the latter, but Dainat and Neumann have shown these are the bees with reduced longevity, and others have shown they have other impairments. Remember that 3000 mites will be produced from something like 3000/1.4 (which is the number of mites produced per worker cell) infested pupae.

          By all means email me the spreadsheet, but I’m snowed under with winter talks and writing so can’t promise to look at it immediately.

          Finally, we’re all on the “learning curve” … or should be, as there are always new things to learn 🙂


  19. James Axtell


    Thanks for your very informative post. Thanks too for your talk to the Chichester BKA recently. I recall that you suggested that the 2.3g recommended amount for vaporisation was unnecessarily high.

    What amount have you discovered is appropriate?



    1. David Post author

      Hello James

      In the dim and distant days when OA was allowed I used to use 1.6 g I think. Now the approved treatment is Api-Bioxal I just multiply up by the relevant amount of oxalic acid in Api-Bioxal (86%) which I think works out at around 1.9 g. Check the maths (this is top of my head stuff). I have a scoop that appears to weigh the correct amount. There were quite a few studies done on OA treatment and attempted overdosing many years ago. It’s a long time since I looked at the data (try Randy Oliver’s scientificbeekeeping.com which might have the details) but do remember there’s a large effective window, perhaps 1-4 grams.

      Delighted you enjoyed the talk.


      1. Sandra Wilson

        Hello David
        Thanks for the information. I am a bit nervous of using oxalic this year because of a colony that died last year . I didn’t know the packet makes up a slightly stronger mix than used traditionally in UK beekeeping, not that the loss of the colony was necessarily due to the treatment but it did come soon after. I have used it successfully in previous years. However you convince me of the necessity of it. So I do intend to treat with 3g to 59ml of 1:1 syrup. Can the Spring treatment with a miticide instead however be an alternative solution?
        Many thanks

        Sandra Wilson

        1. David Post author

          Hi Sandra

          You can use a Spring treatment but, since there will be brood present, the miticide might have to be present for longer (depending upon what you use – OA is not appropriate then) and it could clash with the early season nectar … all depends on your location. I’d be surprised if the OA/Api-Bioxal was responsible for the loss of the winter colony. Over the seasons I’ve treated 200+ colonies and am not aware of any losses directly attributable to OA. Post-mortems usually show failed queens or bl%@dy mites 🙁


  20. Sandra Wilson

    That’s reassuring…I also hadn’t considered the possibility of the presence of open brood or a failed queen (quite possibly) but had questioned my application. I think I’ll research a Spring treatment further though. Don’t mind them keeping their Spring honey.

    1. David Post author

      There’s very little you can get wrong with trickle treatments. Done at ~5C the bees are semi-torpid. You can be in and out of the hive in well under 2 minutes. One of those Trickle 2 containers from Thorne’s makes it a really easy task … infinitely better than a syringe.

      It’s the closed brood in Spring that’s a problem. That’s where the mites will be hiding!


  21. John Cox

    My local bee club have decided to bring in a new policy of NOT TREATING FOR VARROA at the Club’s training apiary, either in the Autumn or in mid winter. They are going to rely on the bees to get rid of the varroa and become naturally resistant in accordance with Tom Seeley’s ideas of natural resistance as set out in his book Lives of Bees I find this very worrying for surrounding beekeepers and not a good instruction for new beekeepers. I have always found your blogs very helpful and treat my bees with Apivar in Autumn and Oxalic Acid in January. I have virtually no varroa and have healthy bees.

    Could I please have your comments (I think I know what you will say!)

    1. David Post author

      Hello John

      You’re in the Midlands going by your IP address. There are a lot of bees in the Midlands, so very little chance of your club apiary being in an isolated spot.

      Are they going to actually follow Seeley’s recommendations or just stop treating? For example, are individual hives 30-50 metres apart to minimise drifting? I thought not. Are they going to diligently monitor Varroa levels and cull any colonies in which mite levels are skyrocketing? I thought not. Are they going to manage the colonies with a single brood and one super, effectively ‘forcing’ small colonies which shed mites through enforced swarming? Again, I thought not.

      It’s likely to be a tough few years … you can expect very heavy colony losses in the next 2-3 years. The people I know who are treatment-free (or almost treatment-free) fall into four groups. The first three are not necessarily mutually exclusive:

      • Exceptionally fine beekeepers
      • Isolated
      • Willing to accept a very high level of colony losses (at least at the beginning)
      • Living in one of the few Varroa-free regions of the globe

      I think it’s a poor way to ‘train’ beekeepers. If I was an association member I’d ask them to:

      • Make sure they’ve informed trainees the likely consequences in advance
      • Justified the scientific rational to members – why is treating undesirable?
      • Contact beekeepers in the neighbourhood (<5 miles) to tell them what they're doing
      • Developed a policy of what to do with heavily infested hives (cull or permanently remove and treat?)
      • Keep good, accurate records … if it works it’ll be a good example of what to do
      • Keep honest records of colony losses, mite levels, swarming … if it doesn’t work it’ll be a good example of what not to do
      • Enforce a ban on moving bees off the site unless it was to a quarantine apiary for treatment
      • Developed a strategy to provide sufficient nucleus colonies to new beekeepers in light of changes to import regulations – can they meet demand if there are no hives on the association apiary (at all, or that do not have high mite and virus pathogen levels)?
      • Refund my membership fee 😉

      I’m not saying it can’t be done. It can. However, I’m not at all sure it’s compatible with beekeeping in an area densely populated with other beekeepers or with training new beekeepers.

      I’ve written about Darwinian beekeeping a few months ago.

      Good luck and post an update in a year or two 🙂

  22. Geoff

    I am more and more frequently hearing of non treating for varroa and claims of varroa resistance…..I would make the following comments….

    1. A lot of Beekeepers have swarms which will provide a brood free period … this is effectively a varroa treatment as the broodless period will knock back numbers. The swarm also contains a disproportionate number of young bees …..these work nearer the brood nest which is the perfect place for the varroa mite to inhabit, hence, a decent load of varroa will go. The older bees (who largely remain) do not have the high varroa numbers.
    2. if the colonies that survive varroa do so by swarming, then it follows that natural selection will lead to more and more swarmy stock.
    3. If everybody stopped treating for varroa then there would be massive losses but eventually a resistant strain of honeybee would probably emerge from somewhere…..however, this would need commitment from everyone……but…….the moment someone imports or introduces “foreign” queens without resistance, there will be pollution of the drone pool….there always seems to be someone locally who buys designer queens…..
    4. the most dangerous scenario within an apiary is the dwindling colony …….all sorts of other diseases will take a hold when a colony is struggling with health ….. one of the best defences against disease is to keep strong colonies ……and destroy anything struggling.

    1. David Post author

      Hello Geoff

      I am also seeing more claims of resistance and advocacy of not treating. Most are just jumping on a bandwagon and have not – as you clearly have – thought the consequences through.

      1. The swarms, which have a higher Varroa level, are likely to perish anyway … leaving the mites untreated will simply hasten this.
      2. Yes, and I think that’s what Seeley sees in the Arnott forest bees.
      3. The losses (>90%, based on Gotland and similar “Leave and Let Die” studies) would result in the destruction of the UK bee farming industry, and a massive reduction in honey production. Seeley’s work shows that small, swarmy colonies survive after ‘selection’ … probably too small for beekeeping as we know it. Amateur beekeeping would also be of less interest and crop pollination would suffer.
      4. I agree … and what I’d really like to see is pathogen loads in treatment free colonies, together with measurements of physiological markers of pathogen stress (elevated AMP levels, RNAi levels etc.) to be monitored over a season or two. I suspect the treatment free colonies would have elevated markers of pathogen stress, possibly with reduced resistance to other pathogens, or elevated susceptibility to environmental stressors like a nectar dearth.

      The ‘herd immunity’ to Covid 19 didn’t go well and no one seriously suggests not vaccinating against measles. If we did, resistance would undoubtedly develop, but at what cost?

      Other than the need to continue treating, it’s not clear to me what the downside of treating is … and it’s a very good way of helping develop strong colonies, which – as you say – are one of the best defences against disease (except mites 😉 ).


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