Synopsis : Apiguard activity is temperature-dependent but the instructions are vague on the minimum temperatures needed for high efficacy. Is Apiguard appropriate for the bees in your area? How might you determine this?
A few months ago I discussed the instructions supplied with some formic or oxalic acid-containing miticides beekeepers use to control Varroa. The post was titled ’Infernal contradictions’ reflecting the state of the documentation provided, either with the purchased products or via the Veterinary Medicines Directorate (VMD) database of approved treatments.
Are the instructions clear, unambiguous, meaningful, accurate and helpful?
Sometimes … but not always 🙁 .
This time I’m turning my attention to Apiguard, a thymol containing miticide that – used properly – is very effective at reducing Varroa levels. It’s an organic treatment and there is no evidence that mites can become resistant to the active ingredient.
Synopsis : Which are better, synthetic miticides or organic acid treatments? What about chemical-free? A large-scale, multi-year study of colony health and survival. Is a ‘healthy treatment-free’ colony an oxymoron?
In the gap between it being too cold for the oil seed rape to yield and too cold to start queen rearing I spotted this interesting large-scale, multi-year study of the comparison between three methods of managing honey bee colonies.
‘Managing’ in this context refers primarily, but not exclusively, to Varroa.
For example, it involves comparison between colonies treated with Apivar vs. Apiguard, but also between hives where open mesh floors were used or queen excluders were omitted.
It’s a US-based study 1, so some of the treatment regimes are not applicable in the UK 2 (this is not a “how to” guide; think of it more as an indication of what’s possible), but I think the results should be interesting and relevant to many beekeepers in temperate regions.
Apivar strip on wire hangar
The three approaches compared were termed ‘conventional’, ‘organic’ and ‘chemical-free’.
Inevitably, because of the huge variation in the ways beekeepers manage their colonies, these are rather simplistic terms applied to rather complex differences. Nevertheless, I think these define approaches that most beekeepers will recognise. The study emphasises some of the points I have previously made here but, more importantly, also investigates the relative contributions of the different management methods to colony survival and productivity.
The results demonstrate that management using organic or synthetic miticides are equally effective in promoting colony survival and productivity. In contrast, colonies managed using chemical-free approaches exhibited significantly elevated winter losses and markedly reduced honey yields.
Synopsis : Our recent study on landscape-scale coordinated Varroa control suggest there are benefits for colony health. I know it makes sense, but how many actually do it?
In the magnum opus last week I discussed how bees discriminate nestmates from non-nestmates at the hive entrance. Inevitably I had to discuss the processes of drifting and robbing as these activities, together with the peripatetic drones, largely account for the ‘foreign’ bees arriving at a hive entrance.
I described drifting as a short-range phenomenon, predominantly of bees with immature cuticular hydrocarbon profiles 1, on their first few orientation flights. In contrast, I described robbing as a potentially long-range event that could occur over at least one kilometre.
I should have re-read the literature and refreshed my memory of what others have already reported for these activities before writing the post 🙁 .
Synopsis : The manufacturer’s instructions for miticide use are often poorly worded, confusing or contradictory. Many beekeepers already struggle to control Varroa and this makes things worse.
How many beekeepers read the documentation that accompanies the miticides they use for Varroa control? How many understand what all the terms – including the pharmacological ones – mean?
What about the phrase “Withdrawal period”?
Can all miticides containing the same active ingredient be used in the same way? If not, why not?
What about repeat usage? Can you repeat a treatment (if needed) if the instructions do not explicitly state that repeat treatments are not allowed? 1 Or can you only administer a second application if the instructions explicitly state that it is allowed?
And if a you are allowed to apply a second treatment, can you administer a third? What about treating in November and the following January? Two different calendar years, but well under one year apart.
Don’t expect any answers to these or related questions in this post 😉 .
Out, damn’d mite …
The intention here is to highlight the slightly shambolic nature of the documentation that accompanies (and sometimes does not accompany, but which you are probably expected to read!) the miticides approved for use in the UK. I don’t have time to cover all the miticides in a single post so will restrict this post 2 to two containing formic acid and one that contains oxalic acid.
And … while we’re at it … which are the legally binding instructions? Those in teensy-weensy lettering on the purchased product or the ones listed in the Veterinary Medicines Directorate (VMD) database?
MAQS and FormicPro
MAQS (Mite Away Quick Strips) and FormicPro are very similar products.
Actually, they are so similar that it’s rather difficult to tell them apart.
The packaging is similar – a cardboard box or plastic tub filled with sachets, each containing two strips impregnated with formic acid (and some other stuff – but what isn’t specified). Even the price is similar; two doses (by which they mean sufficient to treat two hives, or one hive twice, cost an eye-watering 3 £16.50. I’ve not checked other suppliers, but Thorne’s list the 2, 10 and 30 dose boxes of MAQS and FormicPro at identical price points 4.
If you bother to read the online documentation (which you should) you will see that both are marketed by NOD Apiary Ireland Limited, and that each strip contains 68.2 g of formic acid. Even the description of the individual gel strips is very similar:
Brown, semi-rigid to soft gel strip covered in a biodegradable laminated paper, which maintains form (FormicPro).
Each strip is an off-white to caramel coloured gel wrapped in white laminated biodegradable paper (MAQS).
So, we have the same active ingredient, formulated in the same way, packaged in a similar manner, with identical diagrams for how to apply two strips to the brood box. The temperature range recommended for use is identical and both have similar warnings about queen damage.
The same but different
But, although MAQS and FormicPro appear to be essentially the same, from a practical beekeeping standpoint they are very different.
MAQS can be used with honey supers on the hive but FormicPro cannot.
Of course, pedantically, that’s not exactly true.
You could use them ’any-damned-way’ you please, but you would probably be breaking the rules.
You are allowed to use MAQS when there are honey supers present, but you are not allowed to use FormicPro – in all other regards an identical product – when there are honey supers on the hive.
Here are the relevant words from the online SPC’s (Summary of Product Characteristics) 5:
Supers with honey must be removed from the hive prior to product application. See Section 4.5. Honey stored in super(s) put on for the treatment period must be removed and not used for human consumption. Spent strips must be removed before supers intended for harvest are placed on the hive (FormicPro – section 4.11 ‘Withdrawal period’).
The strips may be applied during honey flow; put on honey supers if honey flow is anticipated, to allow adequate space for colony expansion (MAQS – section 4.5 ‘Special precautions for use’).
There is one other difference as well … you can buy FormicPro whereas MAQS appears to be out of stock from all the suppliers I’ve checked.
Perhaps it has been withdrawn already by the manufacturer … ?
This is going to confuse a lot of beekeepers who have come to value MAQS as a short-term and effective treatment for Varroa management during the season.
Many will continue to use FormicPro in the same way that they used MAQS … which could be problematic if they are visited by a Seasonal Bee Inspector.
Summary of Product Characteristics (SPC)
Any miticides you purchase should be accompanied by a set of instructions – on the outside of the box, or the foil packet or wherever. These are often like ’ant tracks’ – illegibly small printing, almost impossible to read without the use of a binocular microscope 6.
Api-Bioxal … where’s my microscope?
Importantly, the packet will also carry a lot number and a use by date – you need to keep records of the former for several years 7 after use. I almost always forget to write this into my notes, but I always photograph the packet so have a dated copy on my ‘phone.
Use the VMD search facility to avoid the budgie treatments
A document prefixed SPC (the Summary of Product Characteristics).
A document prefixed QRD (for Quality Review of Documents), which is the Product Literature; essentially the labelling and text that is supplied when you purchase the product.
If you read these you will find a large amount of duplication. These documents are periodically revised – the MAQS and FormicPro paperwork is all dated June 2022, with MAQS being first authorised in 2013 and FormicPro in 2021.
Discrepancies and confusion
Aside from the ‘biggy’ (not being allowed to use FormicPro when there are supers on the hive) there are other discrepancies or confusing text in these documents.
I’ve already listed one example …
The MAQS SPC indicates the ability to use the product when supers are present under section 4.5 ’Special precautions for use’.
In contrast, the FormicPro SPC indicates that the product cannot be used when honey supers are present under section 4.11 ’Withdrawal period’, though they do refer to section 4.5 (where, perplexingly, only empty honey supers are mentioned).
Section 4.5 seems to me to be the logical place to mention the ever-so-slightly-critical matter of not being allowed to use FormicPro when there are honey supers present.
Does anyone proof read or sanity check these documents?
If so, why don’t they ever define the term withdrawal period?
If you do a search online for ’withdrawal period’ there are all sorts of things about hormonal birth control and legal contract cancellations, but you need to scroll down to the penultimate item on the first page to get the relevant meaning:
The time that must elapse between the last administration of a veterinary medicine and the slaughter or production of food from that animal, to ensure that the food does not contain levels of the medicine that exceed the maximum residue limit.
And that’s from the European Medicines Agency; it wasn’t until somewhere on the third page of results I could find anything from the VMD 9.
Helpful? Not 🙁 .
Of course, there’s an argument that if you’re applying the ‘medicine’ then you should understand all the paperwork and seek further advice if needed.
But I suspect many do not.
Whilst very specific in places e.g. duration of treatment, maximum temperature for use 29.5°C (Really? Does that 0.5°C make a difference? How many domestic thermometers are that accurate?), the documentation also carries other contradictory or vague instructions.
Both MAQS and FormicPro contain the following words under Section 4.4 (‘Special warnings for each target species ‘) of the SPC
Use according to local treatment recommendations, if available.
Who makes these local treatment recommendations? Are they legally binding? Can you just invent them? What can they cover or not cover? Could the local treatment recommendations state “Use five strips for a month”?
And what about disposal of the used, unused and waste products? Here you will find instructions in two separate places in the SPC.
When removed, dispose of by composting (FormicPro, Section 4.9 “Administration”).
The strips do not need to be removed from the hive after the application period of 7 days as the honey bees dispose of the spent strips. If they are removed, dispose of by composting (MAQS, Section 4.9 “Administration”).
And, confusingly …
Any unused veterinary medicinal product or waste materials derived from such veterinary medicinal products should be disposed of in accordance with local requirements (MAQS and FormicPro Section 6.6 ‘Special precautions for disposal’)
So can they be composted, or do ‘local requirements’ take precedence?
I can’t even be bothered to comment on section 4.6 ‘Adverse reactions’ which helpfully define very common, common, uncommon, rare etc events, but then only apply them to one adverse reaction, despite listing many others.
I’ve spent a career trying to make sense of badly worded, confusing, verbose, self-contradictory documents (until the arrival of ChatGPT this was the norm for both student essays and University administrative paperwork) but some of these instructions still baffle me.
The active ingredient in Api-Bioxal is oxalic acid (OA). I’ve discussed this extensively in previous posts. There are several other miticides listed on the VMD database that have OA as the active ingredient; Oxuvar, VarroMed (which also contains formic acid), Dany’s BienenWohl powder/solution and Oxybee. Of these, the last two may not be routinely available in the UK.
I’m going to restrict my (brief) discussion to Api-Bioxal as it’s the only one I’m familiar with and because it highlights a different form of internal infernal contradiction in the official instructions and paperwork.
The Api-Bioxal SPC and instructions clearly state (in section 4.5 ’Special precautions for use’ … or ‘the logical place’ as it should be known) that it should be administered when supers are not present on the hive.
In addition, it also clearly states that the withdrawal period is ‘Zero days’ 10.
Sublimox vaporiser … phoretic mites don’t stand a chance
The duration of application for MAQS and FormicPro is seven days and the formic acid permeates the cappings and kills mites in capped cells. In contrast, Api-Bioxal is a single shot treatment … it may (or may not) remain active in the hive for some time after administration, but you essentially apply it and walk away.
Job done 🙂 .
Oxalic acid does not penetrate capped cells and so is only effective if the colony is broodless. The instructions are clear on this point (to their credit).
A single shot used once … or twice?
The instructions describe two approved methods of administering Api-Bioxal. Trickling a 4.2% (w/v) solution made up in 1:1 (‘thin’) syrup onto the visible seams of bees, or vaporising a hive with up to 2.3 g of Api-Bioxal.
Administration by trickling … Up to two treatments per year (winter and/or spring-summer season in brood-free colonies). The treatment should be made in a single administration.
Administration by vaporisation … Maximal dose 2.3g per hive as a single administration. One treatment per year.
I think the ‘single administration’ means that you cannot split a treatment into two e.g. vaporise 1.15 g twice, or trickle 2.5 ml per seam and then repeat it the following day.
What’s odd is that trickling can be conducted twice per year, whereas vaporisation cannot. What about vaporising in December and January? i.e. once in each of two successive years … which could even be on successive days (31/12 and 1/1).
This is odd for two reasons – firstly it seems strange that the same compound can be administered a different number of times depending upon the route of administration.
Well, OK, perhaps it’s really bad for the colony to be vaporised? In that case it would be understandable, though some explanation of the point would help.
The good old days … trickle treating colonies before Api-Bioxal
Trickling and vaporising do cause differential damage to colonies, but it is trickling that does more damage. Trickled OA damages open brood and studies from the LASI group in Sussex showed that colonies trickle-treated when brood was present were subsequently weaker than those that were vaporised (Al Toufailia et al., 2015).
Conversely, several studies of repeated vaporisation have shown that it is well tolerated by the colony.
So, in this instance the instructions are at odds with my understanding 11 of the current science.
If the withdrawal period for Api-Bioxal is zero days (it is), can you add a stack of supers to the colony the day after vaporising or trickle treating a colony?
Which is a little odd as the oxalic acid remains active in the colony for several days after it is added. If you apply Api-Bioxal and then monitor mite drop on a daily basis over about a week it often peaks a day or two after it is administered, but goes on at a reducing rate for ~5-6 days. Whilst it could just be taking its time killing the mites 13, I think it is more likely that residual activity remains for several days.
Perhaps the wording in the instructions on ‘honey flow’ precludes this, but you can certainly add supers before a honey flow and I’d argue that the wording isn’t completely clear cut.
I know almost nothing about the licensing of veterinary medicines. My understanding is that a license is applied for, supported by evidence of efficacy, toxicity etc. and that it is restricted in terms of the range of methods used to apply the miticide.
Therefore, if the manufacturer only applies for a license for trickling or vaporisation, then that’s what they get (if approved). Varromed (an OA solution) can be administered by trickling and spraying. When made up for spraying the OA solution has a long shelf life as there is no sugar present.
But that’s not an option for Api-Bioxal 🙁 .
Beekeepers are restricted in what they can (legally) do by what the manufacturer sought a licence for, even if there are better ways of administering the active compound, or even if the scientific evidence (sometimes preceding licensing, and certainly preceding updates of the documentation) indicates that – for example – repeat administration is both safe and effective.
Trying to make sense of it all
In Scotland a Working Group has been established to try and resolve some of these discrepancies and provide better advice to beekeepers on the use of the currently licensed miticides.
The Working Group involves representatives from a variety of interested parties including an acronym salad comprising SASA, VMD, BFA, FSS, SBA, SRUC, SEPA, APHA, DEFRA, DAERA, NBU and some academics and ex-academics with a particular interest in honey bee health.
I have written a lot about Varroa control on this site. In my view it is relatively straightforward to control mite numbers using the currently licensed miticides appropriately. In my experience it is easier to do this in Scotland, where we have lower winter temperatures and a greater chance of an extended broodless period.
However, Scotland – unlike the Midlands where I have also kept bees – offers some additional complications where Varroa control is concerned. Our most important (by £££) nectar source is heather which yields late in the year, too late in some years to subsequently protect the winter bees from mites and viruses.
Balancing the needs of the bees (low mites and viruses to overwinter successfully) with those of the beekeeper (hundreds of kilograms of heather honey) requires a careful balancing act and a good understanding of the benefits and limitations of the miticides available.
In turn, this needs good documentation and better advice that is both easily accessible and understandable by beekeepers.
And … to my surprise – and I look forward to it being confirmed or refuted – I’m told that the SPC is the legally binding document with regard to the use or misuse of licensed miticides.
I’ve (had to) read them all now … have you?
Al Toufailia, H., Scandian, L., and Ratnieks, F.L.W. (2015) Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid on the mortality of phoretic Varroa destructor mites and their honey bee hosts. Journal of Apicultural Research 54: 108–120 https://doi.org/10.1080/00218839.2015.1106777.
Synopsis : Does repeated oxalic acid vaporisation of colonies rearing brood work sufficiently well? Is it as useful a strategy as many beekeepers claim?
Oxalic acid is a simple chemical. A dicarboxylic acid that forms a white crystalline solid which dissolves readily in water to form a colourless solution. It was originally extracted from wood-sorrels, plants of the genus Oxalis, hence the name. In addition to the wood-sorrels it is present in a wide range of other plants including rhubarb leaves (0.5% oxalic acid 1 ), the berries and sap of Virginia creeper and some fruits, such as starfruit. Additionally, fungi excrete oxalic acid to increase the availability of soil nutrients.
Oxalic acid is inexpensive to produce by a variety of processes and was possibly the first synthesised natural product. About 120,000 tonnes are produced annually and it is mainly used for bleaching wood (and often sold as ‘wood bleach’) and cleaning products – including teeth. It chelates iron and so is used for rust removal and is used as a dye fixative (or mordant 2 ).
Oxalic acid and API-Bioxal … the same but different
It is also, when used properly, devastatingly effective against the ectoparasitic mite Varroa destructor.
And, even more importantly, when used properly it is extremely well-tolerated by honey bees.
Not so fast …
Unfortunately for beekeepers, some of the commercially available i.e. licensed and approved, oxalic acid-containing treatments either contain unnecessary additives and/or have limitations in their approved modes of administration that reduces their efficiency and use in real world beekeeping situations.
Oxalic acid-containing miticides and their use
A quick search of the UK’s 3 Veterinary Medicines Directorate snappily titled Product Information Database for ‘target species = bees’ and ‘active ingredient = oxalic acid’ yields three products :
Varromed (BeeVital GmbH) which is a solution containing formic acid and oxalic acid
Oxybee (DANY Bienenwohl GmbH) which is an oxalic acid solution PLUS a separate powder containing essential oils and sugar. As far as I can tell, Oxybee looks to be the same product as Dany’s BienenWohl powder and solution, which – although listed and licensed – I cannot find for sale 4 in the UK
API-Bioxal (Chemicals Laif S.P.A) which is purchased as a powder composed of 88% oxalic acid dihydrate together with silica and glucose
I’m going to largely ignore Varromed and Oxybee for the rest of this post. I’m sure they’re perfectly good products but I’ve not used either of them so cannot comment from personal experience.
Keeping your powder dry
More relevant to this post, Oxybee and Varromed are both liquids, and this post is about vaporising (aka sublimating) oxalic acid.
And vaporisation involves using the powdered form of oxalic acid.
Which neatly brings me to the methods of application of oxalic acid-containing treatments to kill mites.
I’m sure there are some weird and wonderful ones, but I’ll be limiting any comments to just three which – from my reading of the instructions – are the only ones approved (and then not for all of the products listed above) : 5
Spraying a solution onto the surface of the bee-covered frames
Dribbling or trickling a solution onto each seam of bees between the frames
Vaporisation or sublimation of powdered oxalic acid by heating it in a metal pan to convert it to a gas. This permeates the hive, settling on all the surfaces – woodwork, comb, bees – and remains active against mites for a period after administration
Broodless is best
Oxalic acid, however it is administered, does not penetrate brood cappings. Therefore all of the approved products are recommended for use when the colony is broodless.
Typically – though not exclusively – this happens in the winter, but the beekeeper can engineer it at other times of the season.
If the colony is broodless you can expect any oxalic acid-containing miticide to reduce the mite population by 90% or more. There are numerous studies that support this level of efficacy and it’s what you should be aiming for to give the colony the best start to the season.
I discussed at length how to determine whether a winter colony is broodless a fortnight ago in Broodless?
This post is a more extensive response to several comments (made to that Broodless? article) that recommended repeated vaporisation of oxalic acid at, either 4, 5 or 7 day intervals.
The idea is that this kills the phoretic mites present when the colony is first treated and the mites subsequently released as brood emerges.
How many repeats?
I’ve seen anything from two to seven recommended online.
I’ll discuss this further below, but I’d note that the very fact that there’s such variation in the recommended repeat treatments – perhaps anything from two, fours days apart to seven at weekly intervals (i.e. spanning anything from 8 days to 49 days) – suggests to me that we don’t know the optimal treatment schedule.
Which is a little weird as, a) Varroa is a globally-distributed problem for beekeepers and is more or less invariant (as is the brood cycle of the host honey bee), and b) repeated treatment regimes have been used for over 20 years.
Which brings me back to a crude comparison of vaporisation vs dribbling, or …
Sublimation vs. trickling
A hive can be sublimated with oxalic acid without opening the hive. The vaporiser alone is introduced through the hive entrance or – in the case of certain models – the vapour is squirted through a hole in the floor, brood box or eke. In contrast, trickling oxalic acid requires the removal of the crownboard.
In the video above I’m using a Sublimox vaporiser. The hive entrance is sealed with foam and the open mesh floor is covered with a tightly fitting slide-in tray. As you can see, very little vapour escapes.
Although oxalic acid is well tolerated by bees, and it has no effect upon sealed brood, a solution of oxalic acid is detrimental to open brood. Therefore, trickled oxalic acid weakens the colony – because the acidity kills some or all of the open brood – and repeated trickling of oxalic acid is likely to compound this (see Al Toufailia et al., 2015). In contrast, repeated oxalic acid vaporisations appear not to be detrimental to the colony (caveat … I’m not aware of any long-term studies of this, or for the impact on the queen).
API-Bioxal approved methods of administration
The instructions for API-Bioxal clearly state that only a single treatment by vaporisation is approved per year. The exact wording is:
Maximal dose 2.3g per hive as a single administration. One treatment per year.
In contrast, when used as a solution for trickling the instructions state:
Up to two treatments per year (winter and/or spring-summer season in brood-free colonies).
This seems nonsensical to me considering what we now know about oxalic acid – remember, API-Bioxal was licensed in the same year (2015) that Al Toufailia et al., demonstrated it was detrimental to open brood, and I’m reasonably sure this had been shown previously (but can’t currently find the reference).
But, it gets worse …
API-Bioxal contains oxalic acid with powdered silica and glucose. I presume the silica is to keep it free-running. I’m not aware that powdered silica kills mites and I’m damned certain that glucose has no miticidal activity 😉 .
Neither of these two additives – which I’ve previously called cutting agents – are there to increase the activity of the oxalic acid … and the presence of the glucose is a real problem when vaporising.
Caramel coated Sublimox vaporiser pan
When glucose is heated to 160°-230°C it caramelises (actually, this happens at 150°C 6 ), coating the inside of the vaporising pan. This needs to be cleaned out afterwards 7. The instructions state:
Cool down and clean the vaporizer after use to remove possible residue (max 6%, around 0.140 g).
However, I don’t want to focus on what I consider to be a very effective but decidedly sub-optimal product … instead I want to discuss whether repeat treatment with oxalic acid actually works when there is brood present.
Why is repeat treatment recommended?
Remember, it’s not recommended or approved by the manufacturers of API-Bioxal or the Veterinary Medicines Directorate. I really should have titled this section ’Why is repeat treatment recommended by those who advocate it?’
But that wouldn’t fit on a single line 😉 .
When you sublimate oxalic acid, the gas cools and the oxalic acid crystals settle out on every surface within the hive – the walls, the frames, the comb, the bees etc.. For this reason, I prefer to vaporise oxalic acid when the colony is not tightly clustered. I want everything to be coated with oxalic acid, and I particularly want every bee to be coated because that’s where most of the mites are.
Unless they’re in capped cells 🙁 .
And if they’re in capped cells, the only way the Varroa (released when the brood emerges) will come into contact with oxalic acid is if it remains present and active within the hive. Unfortunately, it’s unclear to me exactly how long the oxalic acid does remain active, or what accounts for a drop in its activity.
But it does drop.
If you treat a colony with brood present and count the mites that appear on the Varroa tray every day it looks something like this:
Mite drop per day before and after treatment
’Something like’ because it depends upon the phoretic mite levels and the amount and rate of brood uncapping. For example, you often see higher mite drops from 24-48 hours than 0-24 hours after treatment.
I know not why.
The drop in the first 48 hours – presumably almost all phoretic mites – can be very much higher than the drop from day three onwards 8.
The duration of activity after vaporisation
Some studies claim oxalic acid remains active for 2-3 weeks after administration. I’m a little sceptical that it’s effective for that long and my own rather crude observations of post-treatment mite drop (of brooding colonies) suggests it returns to background levels within 5-7 days.
I could rabbit on about this for paragraphs as I’ve given it a reasonable amount of thought, but fortunately the late Pete Little did the experiment and showed that:
The recommended dose for colonies with brood is three or four doses seven days apart, however I found out that this is not effective enough, and treated 7, 6, 5 4, 3, 2 days apart to find out the most effective which is 5.
It therefore makes sense that three treatments at five day intervals should be sufficient. This period comfortably covers a complete capped brood cycle (assuming there is no drone brood in the colony) which is 12 days long.
If there is drone brood present you would theoretically need four treatments at 5 day intervals to be sure of covering the 15 day capped brood cycle of drones.
But it turns out there are some additional complications to consider.
In the UK the recommended i.e. approved, maximum dose of API-Bioxal is 2.3 g by vaporisation. Remember my comments about the other rubbish stuff API-Bioxal contains, 2.3 g of API-Bioxal actually contains a fraction over 2 g of oxalic acid dihydrate.
This is the active ingredient.
When comparing different experiments where some have used ‘plain’ oxalic acid dihydrate and others have used – or will use – API-Bioxal, it’s important to consider the amount of the active ingredient only 9 .
In the US, oxalic acid was registered as an approved treatment for Varroa in 2015. By vaporisation, the approved dosage is 1 g of oxalic acid dihydrate per brood box i.e. half that approved in the UK.
Remember also that a deep Langstroth is 5% larger (by volume) than a National brood box.
And Jennifer Berry and colleagues in the University of Georgia have recently determined whether repeated administration of vaporised oxalic acid to a colony rearing brood is an effective way of controlling and reducing Varroa infestations (Berry et al., 2021).
And the answer is … decidedly underwhelming
Here are the experimental details.
The paper doesn’t state 10when the experiment was done but they measured honey production in the treated colonies and were definitely brood rearing, so I’m assuming late summer.
Colonies were treated with 1 g / box (double Langstroth deeps) vaporised oxalic acid every five days for a total of 35 days i.e. 7 applications. Mite infestation levels (percent of workers carrying phoretic mites) were measured before and after treatment. Almost 100 colonies were used in the experiment, in three apiaries, randomly split into treated and control groups.
Let’s get the easy bit out of the way first … there was no difference in brood levels, adult bees or food stores at the end of the study. The treated hives were not disadvantaged by being treated … but they didn’t gain an advantage either 🙁 .
Mite levels after treatment normalised to pre-treatment levels (dotted line = no change)
During the experiment the percent mite infestation (PMI) levels in the untreated control colonies increased (as expected) by ~4.4. This is an average and there was quite a bit of variation, but it means that an initial mite infestation level of 4 (average) increased to 8.4 i.e. over 8 mites on every 100 adult workers in the hive.
3% is often considered the cutoff above which treatment is necessary.
Overall, the PMI of treated colonies reduced over the duration of the experiment … but only by 0.7.
From a colony health perspective this is a meaningless reduction.
Seven treatments with the recommended (in the US) dose of oxalic acid stopped the mite levels increasing, but did not reduce them.
Repeated administration of the US-approved oxalic acid dose by vaporisation does not reduce mite levels in a way that seems likely to significantly benefit the colony.
I’m not sure the primary data used to justify the US approved 1 g / box dosage. Early studies by Thomas Radetzki (PDF) showed a 95% reduction in mite levels using a dose of 1.4 g. This was a large study involving ~1500 colonies and a dose of 2.8 g was not significantly more effective. I’m quoting the figures for broodless colonies 11.
The Berry results were similar to two smaller previous studies by Jamie Ellis and colleagues (Jack et al., 2020, 2021) who demonstrated that 1 g oxalic acid vaporised three times at weekly intervals was ineffective in controlling mite levels.
However Jack et al., (2021) also applied a similar treatment schedule using different doses of oxalic acid.
Data from Jack et al., 2021 using different repeat doses of oxalic acid
Ignore the intermediate values in panel A, just look at the pretreatment and ‘3 weeks’ mite infestation values.
Mite levels increased in untreated controls and decreased in all treated colonies. However, there was a clear dose response where the more oxalic acid used the greater the impact on the mite levels.
Four grams of oxalic acid reduced the mite infestation rate significantly … from ~5% to ~2% (I’ll return to this). However, the intermediate levels of oxalic acid, whilst reducing mite levels, did not do so significantly from the next closest amount of oxalic acid. For example, 1 g wasn’t significantly more effective than no treatment (as already stated), 2 g was not significantly more effective than 1 g and 4 g was not significantly more effective than 2 g.
But wait … there’s more
I’m familiar with two other studies that look at dose and/or repetition and efficacy (there are more, but this isn’t meant to be an exhaustive review, more a ”Do we know enough?” overview).
Gregoric et al., (2016) published a 12 study that appeared to use combinations of treatments in multiple apiaries. The abstract claims 97% reduction using three 1 g vaporisations, though these are spread over a 57 day period (!) stretching from mid-August to late-November. Mite drop in November following treatment was ~75% (presumably broodless) , but only 10-20% in August. Interestingly I can’t find the figure 97% anywhere in the results …
Finally, Al Toufailia et al., (2015) investigated the dose response to vaporised oxalic acid, showing an 80% reduction in infestation at 0.56 g and 93-98% who using 1.125, 2.25 and 4 g of oxalic acid. All of these studies were determined using broodless colonies.
The Al Toufailia and Jack studies – as well as the Berry study – also reported on adverse effects on the colony. With certain exceptions vaporisation was well tolerated. Some colonies went queenless. Where the queen was caged in late summer to render it broodless (Jack et al.,) some colonies subsequently failed to overwinter successfully (though, look on the bright side, mite levels were reduced 😉 ).
Don’t do that at home … I presume they impacted the production of winter bees.
I’m not sure there’s a compelling, peer-reviewed study that definitively shows that repeat treatments of vaporised oxalic acid administered to a brood rearing colony reduces mite levels sufficiently.
Yes, the Jack et al., (2020) showed a significant reduction in the infestation rate (using 4 g three times at seven day intervals), but it was still around 2%.
In late summer, with 20-30,000 bees in the box and 6 frames of brood, that’s still ~600 mites (and potentially more in the capped brood).
In midwinter with about 10,000 workers and much smaller amounts of brood in the hive a 2% infestation rate is still 200 mites.
That’s still a lot of mites for a nearly broodless colony … I treat my colonies when broodless (and assume I’m killing ~90% of the mites present) and am disappointed if there are 45 mites on the Varroa tray. 50 mites on 10,000 workers is an infestation rate of 0.5%.
I’ve waffled on for too long.
All those advocating – or using – repeated oxalic acid vaporisation on brood rearing colonies in late autumn/winter need to think about:
dosage … 1 g is clearly too little (at a 5-7 day interval, but what if it was at a 4 day interval?), 2 g is better and 4 g is well-tolerated and certainly more effective
frequency … which I suspect is related to dosage. The goal must be to repeat sufficiently frequently that there is never a period when oxalic acid levels fall below a certain amount (and I don’t know what that amount is). 1 g on a daily basis might work well … who knows?
duration … you must cover a full capped brood cycle with the repeats
adverse effects … inevitable, but can be minimised with a rational treatment schedule
Broodless is best
It really is.
But, if your colonies are never broodless 13 then I wouldn’t be confident that repeat treatment was controlling Varroa levels sufficiently.
I have treated repeatedly with oxalic acid. In the good old days before API-Bioxal appeared. It certainly reduced Varroa levels, but not as well as my chosen Apivar does these days.
Repeated oxalic acid vaporisation is regularly proposed as the solution to Varroa but I’m certainly not confident that the data is there to support this claim.
Take care out there 😉
In a future post I’ll revisit this … I’ve got a pretty clear idea of how I’d go about demonstrating whether repeated oxalic acid treatments are effective in meaningfully reducing mite levels i.e. sufficient to protect the colony overwinter and through to the following late summer.
Al Toufailia, H., Scandian, L. and Ratnieks, F.L.W. (2015) ‘Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid on the mortality of phoretic Varroa destructor mites and their honey bee hosts’, Journal of Apicultural Research, 54(2), pp. 108–120. Available at: https://doi.org/10.1080/00218839.2015.1106777.
Berry, J.A. et al. (2022) ‘Assessing Repeated Oxalic Acid Vaporization in Honey Bee (Hymenoptera: Apidae) Colonies for Control of the Ectoparasitic Mite Varroa destructor’, Journal of Insect Science, 22(1), p. 15. Available at: https://doi.org/10.1093/jisesa/ieab089.
Gregorc, A. et al. (2016) ‘Integrated varroa control in honey bee (Apis mellifera carnica) colonies with or without brood’, Journal of Apicultural Research, 55(3), pp. 253–258. Available at: https://doi.org/10.1080/00218839.2016.1222700.
Jack, C.J., van Santen, E. and Ellis, J.D. (2020) ‘Evaluating the Efficacy of Oxalic Acid Vaporization and Brood Interruption in Controlling the Honey Bee Pest Varroa destructor (Acari: Varroidae)’, Journal of Economic Entomology, 113(2), pp. 582–588. Available at: https://doi.org/10.1093/jee/toz358.
Jack, C.J., van Santen, E. and Ellis, J.D. (2021) ‘Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa destructor’, Journal of Apicultural Research, 60(3), pp. 414–420. Available at: https://doi.org/10.1080/00218839.2021.1877447.