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.
Synopsis : The colony needs to be broodless for effective oxalic acid treatment in winter. You might be surprised at how early in the winter this broodless period can be (if there is one). How can you easily determine whether the colony is broodless?
In late spring or early summer a broodless colony is a cause for concern. Has the colony swarmed? Have you killed the queen? Since worker brood takes 21 days from egg to emergence, a broodless colony has gone 3 weeks without any eggs being laid.
You’re right to be concerned about the queen.
Of course, since you’ve been inspecting the hive on a 7-10 day rotation, you noticed the absence of eggs a fortnight ago, so you’re well on your way to knowing what the problem is, and therefore being able to solve it 😉 .
But in late autumn or early winter a broodless colony is not a cause for concern.
It’s an opportunity.
Are they rearing brood? Probably by now … it’s mid-January
In my view it’s a highly desirable state for the colony to be in.
If the colony is broodless then the ectoparasitic Varroa mites cannot be hiding away under the cappings, gorging themselves on developing pupae and indulging in their – frankly repellent – incestuous reproduction.
Instead the mites will all be riding around the colony on relatively young workers (and in winter, physiologically all the workers in the hive are ‘young’, irrespective of their age) in what is incorrectly termed the phoretic stage of their life cycle.
This is incorrect as phoresy means “carried on the body of another organism without being parasitic” … and these mites are not just being carried around, they’re also feeding on the worker bees.
A broodless colony in the winter is an opportunity because phoretic mites (whether misnamed or not) are very easy to kill because they’re not protected by the wax capping covering the sealed brood.
Total mite numbers surviving OA treatment depends upon the proportion in capped cells
And today’s post is all about identifying when the colony is broodless.
Discard your calendar
I’ve said it before 1 … the activities of the colony (swarming, nectar gathering, broodlessness 2 ) are not determined by the calendar.
Instead they’re determined by the environment. This covers everything from the available forage to the climate and recent weather 3.
And the environment changes. It changes from year to year in a single location – an early spring, a late summer – and it differs between locations on the same calendar date.
All of which means that, although you can develop a pretty good idea of when you need to intervene or manage things – like adding supers, or conducting swarm control – these are reactive responses to the state of the colony, rather than proactive actions applied because it’s the 9th of May 4.
And exactly the same thing applies to determining when the colony is broodless in the winter. Over the last 6 years I’ve had colonies that are broodless sometime between between mid October and mid/late December. They’re not broodless for this entire period, but they are for some weeks starting from about mid-October and ending sometime around Christmas.
Actually, to be a little more precise, I generally know when they start to be broodless, but I rarely monitor when they stop being broodless, not least because it’s a more difficult thing to determine (as will become clear).
Don’t wait until Christmas
A broodless colony is an opportunity because the phoretic mites can easily be killed by a single application of oxalic acid.
Many beekeepers treat their colonies with oxalic acid between Christmas and New Year.
It was how they were taught when they started beekeeping, it’s convenient because it’s a holiday period, it’s a great excuse to escape to the apiary and avoid another bellyful of cold cuts followed by mince pies (or the inlaws 5 ) and because it’s ‘midwinter’.
But, my experience suggests this is generally too late in the year. The colony is often already rearing brood by the time you’ve eaten your first dozen mince pies.
If you’re going to go to the trouble of treating your colonies with oxalic acid, it’s worth making the effort to apply it to achieve maximum efficacy 6.
I’m probably treating my colonies with oxalic acid in 8-9 days time. The queens have stopped laying and there was very little sealed brood present in the colonies I briefly checked on Monday this week. The sealed brood will have all emerged by the end of next week.
It’s worth making plans now to determine when your colonies are broodless. Don’t just assume sometime between Christmas and New Year ’will be OK’.
But it’s too early now for them to be broodless … or to treat with oxalic acid
If your colonies are going to go through a broodless period this winter 7 it’s more likely to be earlier rather than later.
Because if the colonies had a long broodless period stretching into mid-January or later it’s unlikely they’ll build up strongly enough to swarm … and since swarming is honey bee reproduction, it’s a powerful evolutionary and selective pressure.
Colonies that start rearing brood early, perhaps as early as the winter solstice, are more likely to build up strongly, and therefore are more likely to swarm, so propagating the genes for early brood rearing.
But surely it would be better to treat with oxalic acid towards the end of the winter?
Mites do not reproduce during the misnamed phoretic stage of the life cycle. Therefore, aside from those mites lost (hopefully through the open mesh floor) due to allogrooming, or that just die 8, there will be no more mites later in the broodless period than at the beginning.
Since the mites are going to be feeding on adult workers (which is probably detrimental to those workers), and because it’s easier to detect the onset of broodlessness (see below), it makes sense to treat earlier rather than later.
Your bees will thank you for it 😉 .
How to detect the absence of brood
Tricky … how do you detect if something is not present?
I think the only way you can be certain is to conduct a full hive inspection, checking each side of every frame for the presence of sealed brood.
Perhaps not the ideal conditions for a full hive inspection
But I’m not suggesting you do that.
It’s a highly intrusive thing to do to a colony in the winter. It involves cracking open the propolis seal to the crownboard, prising apart the frames and splitting up the winter cluster.
On a warm winter day that’s a disruptive process and the bees will show their appreciation 🙁 . On a cold winter day, particularly if you’re a bit slow checking the frames (remember, the bees will appear semi-torpid and will be tightly packed around any sealed brood present, making it difficult to see), it could threaten the survival of the colony.
And don’t even think about doing it if it’s snowing 🙁 .
Even after reassembling the hive the colony is likely to suffer … the broken propolis seals will let in draughts, the colony will have to use valuable energy to reposition themselves.
A quick peek
I have looked in colonies for brood in the winter. However, I don’t routinely do this.
Now, in mid/late autumn the temperature is a bit warmer and it’s less disruptive. I checked half a dozen on Sunday/Monday. It was about 11°C with rain threatening. I had to open the boxes to retrieve the Apivar strips anyway after the 9-10 week treatment period.
Recovered Apivar strips
I had repositioned the Apivar strips about a month ago, moving them in from the outside frames to the edges of the shrinking brood nest. By then – early October – most of the strips were separated by just 3 or 4 frames.
The flanking frames were all jam packed with stores. The fondant blocks were long-gone and the bees had probably also supplemented the stores with some nectar from the ivy.
Over the last month the brood nest continued to shrink, but it won’t have moved somewhere else in the hive … it will still be somewhere between the Apivar strips, and about half way is as good a place as any to start.
Apivar strip (red bars) placement and the shrinking brood nest
So, having removed the crownboard and the dummy board, I just prise apart the frames to release the Apivar strips and then quickly look at the central frame between them. If there’s no sealed brood there, and you can usually also have a look at the inner faces of the flanking frames down the ‘gap’ you’ve opened, then the colony is probably broodless.
It takes 45-60 seconds at most.
It’s worth noting that my diagram shows the broodnest located centrally in the hive. It usually isn’t. It’s often closer to the hive entrance and/or (in poly boxes) near the well insulated sidewall of the hive.
But you don’t need to go rummaging through the brood box to determine whether the colony is broodless (though – as noted earlier – it is the probably the only was you can be certain there’s no brood present).
The cappings on sealed brood are usually described as being ‘biscuit-coloured’.
Not this colour of biscuit
‘Biscuit-coloured’ is used because all beekeepers are very familiar with digestive biscuits (usually consumed in draughty church halls). If ‘biscuit-coloured’ made you instead think of Fox’s Party Rings then either your beekeeping association has too much money, or you have young children.
Sorry to disappoint you … think ‘digestives’ 😉 .
That’s more like it …
The cappings are that colour because the bees mix wax and pollen to make them air-permeable. If they weren’t the developing pupa wouldn’t be able to breathe.
And when the developed worker emerges from the cell the wax capping is nibbled away and the ‘crumbs’ (more biscuity references) drop down through the cluster to eventually land on the hive floor.
Where they’re totally invisible to the beekeeper 🙁 .
Unless it’s an open mesh floor … in which case the crumbs drop through the mesh to land on the ground where they’ll soon get lost in the grass, carried off by ants or blown away 🙁 .
It should therefore be obvious that if you want detect the presence of brood emerging you need to have a clean tray underneath the open mesh floor (OMF).
Open mesh floors and Correx boards
Most open mesh floors have a provision to insert a Correx (or similar) board underneath the mesh. There are good and bad implementations of this.
Poor designs have a large gap between the mesh and the Correx board, with no sealing around the edges 9. Consequently, it’s draughty and stuff that lands on the board gets blown about (or even blown away).
Good designs – like the outstanding cedar floors Pete Little used to make – have a close-fitting wooden tray on which the Correx board is placed. The tray slides underneath the open mesh floor and seals the area from draughts 10.
Open mesh floor and close-fitting Varroa tray by Pete Little
Not only does this mean that the biscuity-coloured crumbs stay where they fall, it also means that this type of floor is perfect when treating the colony with vaporised oxalic acid. Almost none escapes, meaning less chance of being exposed to the unpleasant vapours if you’re the beekeeper, and more chance of being exposed to the unpleasant vapours if you’re a mite 😉 .
Since the primary purpose of these Correx trays is to determine the numbers of mites that drop from the colony, either naturally or during treatment, it makes sense if they are pale coloured. It’s also helpful if they are gridded as this makes counting mites easier.
Easy counting …
And, with a tray in situ for a 2-3 days you can quickly get an idea whether there is brood being uncapped.
Reading the runes
The diagram below shows a schematic of the colony (top row) and the general appearance of debris on the Varroa tray (bottom row).
It’s all rather stylised.
The brood nest – the grey central circle is unlikely to be circular, or central 11.
The shrinking broodnest (top) and the resulting pattern on the Varroa tray (bottom)
Imagine that the lower row of images represent the pattern of the cappings that have fallen onto the tray over at least 2-3 days.
Biscuit-coloured cappings on Varroa tray
As the brood nest shrinks, the area covered by the biscuit-coloured cappings is reduced. At some point it is probably little more than one rather short stripe, indicating small amounts of brood emerging on two facing frames.
With just one observation highlighted should you plan to treat next week?
Let’s assume you place the tray under the open mesh floor and see that single, short bar of biscuity crumbs (highlighted above). There’s almost nothing there.
Do you assume that it will be OK to treat them with oxalic acid the following week?
Not so fast!
With just a single observation there’s a danger that you could be seeing the first brood emerging when there’s lots more still capped on adjacent frames.
It’s unlikely – particularly in winter – but it is a possibility.
Far better is to make a series of observations and record the trajectory of cappings production. Is it decreasing or is it increasing?
Multiple observations allows the expanding or contracting brood nest to be monitored
With a couple of observations 10-12 days apart you’ll have a much better idea of whether the brood area is decreasing over time, or increasing. Repeated observations every 10-12 days will give you a much better idea of what’s going on.
Developing brood is sealed for ~12 days. Therefore, if brood rearing is starting, the first cappings that appear on the Varroa tray are only a small proportion of the total sealed brood in the colony.
Very little cappings but certainly not broodless
Of course, in winter, the laying rate of the queen is much reduced. Let’s assume she’s steadily laying just 50 eggs per day i.e. about 12.5 cm2. By the time the first cappings appear on the Varroa tray (as the first 50 workers emerge) there will be another 600 developing workers occupying capped cells … and the worry is that they’re occupying those cells with a Varroa mite.
The cessation of brood rearing
In contrast, if there’s brood in the colony but the queen is slowing down and eventually stops egg laying, with repeated observations 12 the amount and coverage of the biscuit-coloured cappings will reduce and eventually disappear.
At that point you can be reasonably confident that there is no more sealed brood in the colony and, therefore, that it’s an appropriate time to treat with oxalic acid.
In this instance – and unusually – absence of evidence is evidence of absence 🙂 .
But my bees are never broodless in the winter
All of the above still applies, with the caveat that rather than looking for the absence of any yummy-looking biscuity crumbs on the tray, you are instead looking for the time that they cover the minimal area.
If the colony is never broodless in winter it still makes sense to treat with oxalic acid when the brood is at the lowest level (refer back to the first graph in this post).
At that time the smallest number of mites are likely to be occupying capped cells.
However, this assumption is incorrect if the small number of cells are very heavily parasitised, with multiple mites occupying a single sealed cell. This can happen – at least in summer – in heavily mite infested hives. I’ve seen 12-16 mites in some cells and Vincent Poulin reported seeing 26 in one cell in a recent comment.
I’m not aware of any data on infestation levels of cells in winter when brood levels are low, though I suspect this type of multiple occupancy is unlikely to occur (assuming viable mite numbers are correspondingly low). I’d be delighted if any readers have measured mites per cell in the winter, or know of a publication in which it’s reported 13.
This isn’t an exact science
What I’ve described above sounds all rather clinical and precise.
Draughts blow the cappings about on the tray. The queen’s egg laying varies from day to day, and can stop and start in response to low temperatures or goodness-knows-what-else. The pattern of cappings is sometimes rather difficult to discern. Some uncapped stores can have confoundingly dark cappings etc.
But it is worth trying to work out what’s going on in the box to maximise the chances that the winter oxalic acid treatment is applied at the time when it will have the greatest effect on the mite population.
By minimising your mite levels in winter you’re giving your bees the very best start to the season ahead.
Unrestricted mite replication – the more you start with the more you end up with (click image for more details)
The fewer mites you have at the start of the season, the longer it takes for dangerously high mite levels (i.e. over 1000 according to the National Bee Unit) to develop. Therefore, by reducing your mite levels in the next few weeks you are increasing your chances that the colony will be able to rear large numbers of healthy winter bees for next winter.
That sounds to me like a good return on the effort of making a few trips to the apiary in November and early December …
Synopsis : A recent study shows increased overwinter colony survival of ‘covered’ hives wrapped in Correx and with insulation under the roof. What provides the most benefit, and are the results as clear cut as they seem?
A recent talk by Andrew Abrahams to the Scottish Native Honey Bee Society coincided with me catching up my 1 backlog of scientific papers on honey bees. I’d been reading a paper on the benefits of wrapping hives in the winter and Andrew commented that he did exactly that to fend off the worst of the wet weather. Andrew lives on the island of Colonsay about 75 km south of me and we both ‘benefit’ from the damp Atlantic climate.
The paper extolled the virtues of ‘covered’ hives and the data the researchers present looks, at first glance, compelling.
For example, <5% of covered hives perished overwinter in contrast to >27% of the uncovered control hives.
Why doesn’t everyone wrap their hives?
However, a closer look at the paper raises a number of questions about what is actually benefitting (or killing) the colonies.
Nevertheless, the results are interesting. I think the paper poses rather more questions than it answers, but I do think the results show the benefits of hive insulation and these are worth discussing.
Bees don’t hibernate
Hibernation is a physiological state in which the metabolic processes of the body are significantly reduced. The animal becomes torpid, exhibiting a reduced heart rate, low body temperature and reduced breathing. Food reserves e.g. stored fat, are conserved and the animal waits out the winter until environmental conditions improve.
However, bees don’t hibernate.
Winter cluster 3/1/21 3°C (insulation block removed from the crownboard)
If you lift the lift the roof from a hive on a cold midwinter day you’ll find the bees clustered tightly together. But, look closely and you’ll see that the bees are moving. Remove the crownboard and some bees will probably fly.
The cluster conserves warmth and there is a temperature gradient from the outside – termed the mantle – to the middle (the core).
If chilled below ~5.5°C a bee becomes semi-comatose 2 and unable to warm herself up again. The mantle temperature of the cluster never drops below ~8°C, but the core is maintained at 18-20°C when broodless or ~35°C if they are rearing brood. I’ve discussed the winter cluster in lots more detail a couple of years ago.
The metabolic activity of the clustered winter bees is ‘powered’ by their consumption of the stores they laid down in the autumn. It seems logical to assume that it will take more energy (i.e. stores) to maintain a particular cluster temperature if the ambient temperature is lower.
Therefore, logic would also suggest that the greater the insulation properties of the hive – for a particular difference in ambient to cluster temperature – the less stores would be consumed.
Since winter starvation is bad for bees (!) it makes sense to be thinking about this now, before the temperatures plummet in the winter.
Cedar and poly hives
I’m not aware of many comparative studies of the insulation properties of hives made from the two most frequently used materials – wood and polystyrene. However, Alburaki and Corona (2021) have investigated this and shown a small (but statistically significant) difference in the inner temperature of poly Langstroth hives when compared to wooden ones.
Poly hives were ~0.5°C warmer and, perhaps more importantly, exhibited much less variation in temperature over a 24 hour period.
Temperature and humidity in poly and wood hives
In addition to the slight temperature difference, the humidity within the wooden hives was significantly higher than that of poly.
The hives used in this study were occupied by bees and the temperature and humidity were recorded from sensors placed in a modified frame in the ‘centre of the brood box’. The external ambient temperature averaged 0°C, but fluctuated over a wide range (-10°C to 20°C) during the four month study 3.
Whilst I’m not surprised that the poly hives were marginally warmer, I was surprised how low the internal hive temperatures were. The authors don’t comment on whether the ‘central’ frame was covered with bees, or whether the bees were rearing brood.
The longitudinal temperature traces (not reproduced here – check the paper) don’t help much either as they drop in mid-February when I would expect brood rearing to be really gearing up … Illogical, Captain.
The authors avoid any discussion on why the average internal temperature was at least 5-8°C cooler than the expected temperature of the core of a clustered broodless colony, and ~25°C cooler than a clustered colony that was rearing brood.
My guess is that the frame with the sensors was outside the cluster. For example, perhaps it was in the lower brood box 4 with the bees clustered in the upper box?
We’ll never know, but let’s just accept that poly hives – big surprise 😉 – are better insulated. Therefore the bees should need to use less stores to maintain a particular internal temperature.
And, although Alburaki and Corona (2021) didn’t measure this, it did form part of a recent study by Ashley St. Clair and colleagues from the University of Illinois (St. Clair et al., 2022).
Hive covers reduce food consumption and colony mortality
This section heading repeats the two key points in the title of this second paper.
I’ll first outline what was done and describe these headline claims in more detail. After that I’ll discuss the experiments in a bit more detail and some caveats I have of the methodology and the claims.
I’ll also make clear what the authors mean by a ‘hive cover’.
The study was conducted in central Illinois and involved 43 hives in 8 apiaries. Hives were randomly assigned to ‘covered’ or ‘uncovered’ i.e. control – groups (both were present in every apiary) and the study lasted from mid-November to the end of the following March.
Ambient (blue), covered (black) and control (dashed) hive temperatures
There were no significant differences in internal hive temperature between the two groups and – notably – the temperatures were much higher (15°-34°C) than those recorded by Alburaki and Corona (2021).
All colonies, whether covered or uncovered, got lighter through the winter, but the uncovered colonies lost significantly more weight once brood rearing started February. The authors supplemented all colonies with sugar cakes in February and the control colonies used ~15% more of these additional stores before the study concluded.
I don’t think any of these results are particularly surprising – colonies with additional insulation get lighter more slowly and need less supplemental feeding.
The surprising result was colony survival.
Less than 5% (1/22) of the covered hives perished during the winter but over 27% (6/21) of the control hives didn’t make it through to the following spring.
To put these last figures into context the authors quote a BeeI Informed Partnership survey where respondents gave a figure of 23.3% as being ’acceptable’ for winter colony losses.
That seems a depressingly high figure to me.
However, look – and weep – at the percentage losses across the USA in the ’20/’21 winter from that same survey 5.
These hive covers are available commercially in the USA (and may be here, I’ve not looked). At $33 each (Yikes) they’re not cheap, but how much is a colony worth?
Significantly more than $33.
I’ve not bothered to make the conversion of Langstroth Deep dimensions (always quoted in inches 🙁 ) to metric and then compared the area of Correx to the current sheet price of ~£13 … but I suspect there are savings to be made by the interested DIYer 7.
However, knowing (and loving) Correx, what strikes me is that it seems unlikely to provide much insulation. At only 4 mm thick and enclosing an even thinner air gap, it’s not the first thing I’d think of to reduce heat loss 8.
4 mm Correx sheet
Thermal resistance is the (or a) measure of the insulating properties of materials. It’s measured in the instantly forgettable units of square metre kelvin per watt m2.K/W.
I couldn’t find a figure for 4 mm Correx, but I did manage to find some numbers for air.
A 5 mm air gap – greater than separates the inner and outer walls of a 4 mm Correx hive cover – has a thermal resistance of 0.11 m2.K/W.
It’s not possible to directly compare this with anything meaningful, but there is data available for larger ‘thicknesses’ of air, and other forms of insulation.
An air gap of 100 mm has a thermal resistance of about 0.17 m2.K/W. For comparison, the same thickness of Kingspan (blown phenolic foam wall insulation, available from almost any building site skip) has a thermal resistance of 5, almost 30 times greater.
And, it turns out, St. Clair and colleagues also added a foam insulation board on top of the hive crownboard (or ‘inner cover’ as they call it in the USA). This board was 3.8 cm thick and has somewhat lower thermal resistance than the Kingspan I discussed above.
It might provide less insulation than Kingspan, but it’s a whole lot better than Correx.
This additional insulation is only briefly mentioned in the Materials and Methods and barely gets another mention in the paper.
A pity, as I suspect it’s very important.
Perspex crownboard with integrated 50 mm Kingspan insulation
I’m very familiar with Kingspan insulation for hives. All my colonies have a 5 cm thick block present all year – either placed over the crownboard, built into the crownboard or integrated into the hive roof.
Two variables … and woodpeckers
Unfortunately, St. Clair and colleagues didn’t compare the weight loss and survival of hives ‘covered’ by either wrapping them in Correx or having an insulated roof.
It’s therefore not possible to determine which of these two forms of protection is most beneficial for the hive.
For reasons described above I think the Correx sleeve is unlikely to provide much direct thermal insulation.
However, that doesn’t mean it’s not beneficial.
At the start of this post I explained that Andrew Abrahams wraps his hives for the winter. He appears to use something like black DPM (damp proof membrane).
Hive wrapped in black DPM (to prevent woodpecker damage)
Andrew uses it to keep the rain off the hives … I’ve used exactly the same stuff to prevent woodpecker damage to hives during the winter.
It’s only green woodpeckers (Picus viridis) that damage hives. It’s a learned activity; not all green woodpeckers appear to know that beehives are full of protein-rich goodies in the depths of winter. If they can’t grip on the side of the hive they can’t chisel their way in.
When I lived in the Midlands the hives always needed winter woodpecker protection, but the Fife Yaffles 9 don’t appear to attack hives.
Here on the west coast, and on Colonsay, there are no green woodpeckers … and I know nothing about the hive-eating woodpeckers of Illinois.
So, let’s forget the woodpeckers and return to other benefits that might arise from wrapping the hive in some form of black sheeting during the winter.
Solar gain and tar paper
Solar gain is the increase in thermal energy (or temperature as people other than physicists with freakishly large foreheads call it) of something – like a bee hive – as it absorbs solar radiation.
On sunny days a black DPM-wrapped hive (or one sleeved in a $33 Correx/Coroplast hive ‘cover’) will benefit from solar gain. The black surface will warm up and some of that heat should transfer to the hive.
And – in the USA at least – there’s a long history of wrapping hives for the winter. If you do an internet search for ‘winterizing hives’ or something similar 10 you’ll find loads of descriptions (and videos) on what this involves.
Rather than use DPM, many of these descriptions use ‘tar paper’ … which, here in the UK, we’d call roofing felt 11.
Roofing felt – at least the stuff I have left over from re-roofing sheds – is pretty beastly stuff to work with. However, perhaps importantly, it has a rough matt finish, so is likely to provide significantly more solar gain than a covering of shiny black DPM.
I haven’t wrapped hives in winter since I moved back to Scotland in 2015. However, the comments by Andrew – who shares the similarly warm and damp Atlantic coastal environment – this recent paper and some reading on solar gain are making me wonder whether I should.
Fortunately, I never throw anything away, so should still have the DPM 😉
Illinois has a temperate climate and the ambient temperature during the study was at or below 0°C for about 11 weeks. However, these sorts of temperatures are readily tolerated by overwintering colonies. It seems unlikely that colonies that perished were killed by the cold.
So what did kill them?
Unfortunately there’s no information on this in the paper by St. Clair and colleagues.
Perhaps the authors are saving this for later … ’slicing and dicing’ the results into MPU’s (minimal publishable units) to eke out the maximum number of papers from their funding 12, but I doubt it.
I suspect they either didn’t check, checked but couldn’t determine the cause, or – most likely – determined the cause(s) but that there was no consistent pattern so making it an inconclusive story.
There were some oddities in their preparation of the colonies and late-season Varroa treatment.
Prior to ‘winterizing’ colonies they treated them with Apivar (early August) and then equalised the strength of the colonies. This involves shuffling brood frames to ensure all the colonies in the study were of broadly the same strength (remember, strong colonies overwinter better).
A follow-up Varroa check in mid-October showed that mite levels were still at 3.5% (i.e. 10.5 phoretic mites/300 bees) and so all colonies were treated with vaporised oxalic acid (OA).
Sublimox vaporiser … phoretic mites don’t stand a chance
In early November, mite levels were down to a more acceptable 0.7%. Colonies received a second OA treatment in early January.
For whatever reason, the Apivar treatment appears to have been ineffective.
When colonies are treated for 6-10 weeks with Apivar (e.g. early August to mid-October) mite levels should be reduced by >90%.
Mite infestation levels of 3.5% suggest to me that the Apivar treatment did not work very well. That being the case, the winter bees being reared through August, September and early October would have been exposed to high mite levels, and so acquired high levels of DWV.
OA treatment in mid-October would kill these remaining mites … but the damage had already been done to the ’diutinus’winter bees.
That’s my guess anyway.
An informed guess, but a guess nevertheless, based upon the data in the paper and my understanding of winter bee production, DWV and rational Varroa management.
In support of this conclusion it’s notable that colonies died from about week 8, suggesting they were running out of winter bees due to their reduced longevity.
If I’m right …
It raises the interesting question of why the losses were predominantly (6 vs 1) of the control colonies?
Unfortunately the authors only provide average mite numbers per apiary, and each apiary contained a mix of covered and control hives. However, based upon the error bars on the graph (Supporting Information Fig S1 [PDF] if you’re following this) I’m assuming there wasn’t a marked difference between covered and control hives.
I’ve run out of informed guesses … I don’t know the answer to the question. There’s insufficient data in the paper.
Let’s briefly revisit hive temperatures
Unusually, I’m going to present the same hive temperature graph shown above to save you scrolling back up the page 13.
Ambient (blue), covered (black) and control (dashed) hive temperatures
There was no overall significant difference in hive temperature between the control and covered colonies. However, after the coldest weeks of the winter (7 and 8 i.e. the end of February), hive temperatures started to rise and the covered colonies were consistently marginally warmer. By this time in the season the colonies should be rearing increasing amounts of brood.
I’ve not presented the hive weight changes. These diverged most significantly from week 8. The control colonies used more stores to maintain a similar (actually – as stated above – marginally lower) temperature. As the authors state:
… covered colonies appeared to be able to maintain normal thermoregulatory temperatures, while consuming significantly less stored food, suggesting that hive covers may reduce the energetic cost of nest thermoregulation.
I should add that there was no difference in colony strength (of those that survived) between covered and control colonies; it’s not as though those marginally warmer temperatures from week 9 resulted in greater brood rearing.
Are lower hive temperatures ever beneficial in winter?
Varroa management is much easier if colonies experience a broodless period in the winter.
A single oxalic acid treatment during this broodless period should kill 95% of mites – as all are phoretic – leaving the colony in a very good state for the coming season.
I’m therefore a big fan of cold winters. The colony is more likely to be broodless at some point.
I was therefore reassured by the similarity in the temperatures of covered and control colonies from weeks 48 until the cold snap at the end of February. Covered hives should still experience a broodless period.
I’m off for a rummage in the back of the shed to find some rolls of DPM for the winter.
I don’t expect it will increase my winter survival rates (which are pretty good) and I’m not going to conduct a controlled experiment to see if it does.
If I can find the DPM I’ll wrap a few hives to protect them from the winter weather. With luck I should be able to rescue an additional frame or two of unused stores in the spring (I often can anyway). I stack this away safely and then use it when I’m making up nucs for queen mating.
I suspect that the insulation over the crownboard provides more benefit than the hive ‘wrap’. As stated before, all my colonies are insulated like this year round as I’m convinced it benefits the colony, reducing condensation over the cluster and keeping valuable warmth from escaping. However, wrapping the hive for solar gain and/or weather protection is also worth considering.
Alburaki, M. and Corona, M. (2022) ‘Polyurethane honey bee hives provide better winter insulation than wooden hives’, Journal of Apicultural Research, 61(2), pp. 190–196. Available at: https://doi.org/10.1080/00218839.2021.1999578.
St. Clair, A.L., Beach, N.J. and Dolezal, A.G. (2022) ‘Honey bee hive covers reduce food consumption and colony mortality during overwintering’, PLOS ONE, 17(4), p. e0266219. Available at: https://doi.org/10.1371/journal.pone.0266219.