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.

You can read all about phoretic mites, their diet and their repulsive reproductive habits in previous posts.

What is the opportunity?

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.

Hive debris

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 ...

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.

Urgh! (again)

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.

It isn’t.

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 …



  1. And I’ll no doubt say it again … I’m nothing if not repetitive.
  2. Is that a word? Auto-correct wanted to make it bloodlessness, so I’ll assume it is. You know what I mean anyway.
  3. In addition, the strain of bees and age of the queen can also influence things.
  4. Or whenever …
  5. Visiting that is, not part of the diet.
  6. Of course, the same sentiment applies to any treatment applied to the colony … use them properly, at the right time, the right temperature and for the correct duration.
  7. And not all do, particularly in the more balmy southern parts of the UK.
  8. Hurrah!
  9. Abelo floors certainly used to be particularly poorly designed.
  10. Infuriatingly I’ve got about a dozen of these floors and not a single really good picture showing how well-fitting the tray is. Take my word for it … I’ve not seen better.
  11. Or grey for that matter
  12. Each time placing a clean tray in for sufficient time to see the cappings that fall through the OMF.
  13. Send me an email using the link in the footer.

30 thoughts on “Broodless?

    1. David Post author

      Hello Peter

      The instructions – at least those on the VMD website (I’ve not got a packet to hand to check, but those online are dated March ’22) state “Any unused veterinary medicinal product or waste materials derived from such
      veterinary medicinal products should be disposed of in accordance with local
      requirements. The veterinary medicinal product should not enter watercourses as this may be
      dangerous for fish and aquatic organisms.”

      … which, of course, is little help. Where do you find the ‘local requirements’ and do they differ between England and Scotland, or Fife and Midlothian?

      Most local councils don’t accept ‘hazardous waste’ (which these strips aren’t as far as I’m concerned) and their advice on disposal of unwanted ‘medicines’ is to return them to the chemist.

      I wrap the used strips in a plastic bag and dispose of them in waste going to landfill. It’s worth remembering that these are ‘slow release’ strips and much of the amitraz will have been released after 9-10 weeks in the hive. I don’t know how much remains, but I expect it is significantly reduced from that in a freshly opened packet.

      You probably shouldn’t take this as advice … instead, go and find the ‘local requirements’ for your area 😉


  1. Steve Donohoe

    Very much agree. The work done by Mobus in 1974 & 1975 showed that the time when the majority of colonies had no or low brood was in November. But , as you say, we can’t really go just by the calendar as each year is different. Also, it’s been warming up (doesn’t feel like that at this precise moment!) and I’m 300 miles south of where Mobus was.

    I don’t have mesh floors/inserts and I don’t want to disturb the bees, so I gamble. I try to find a prolonged cold period in November and then hope that she stops laying at that point. Two weeks later I zap them. Or I just guess – sometime between mid Nov to mid Dec. Last year it was 12th Dec, in 2020 it was 5th Dec, and the year before was 30th Nov. Seems to be getting later and later…

    Mine were piling in the pollen until a few days ago. I think it will be early December this year. All I really know is that Christmas is probably too late.

    1. David Post author

      Hi Steve

      Yours is the first of several comments that mention pollen. Some of my colonies were taking in a bit of pollen on Monday this week, but they were almost broodless. I’ll need to try and be more observant to see if there’s a direct relationship between pollen and brood rearing … or do the bees just collect pollen because it’s there and they will need it later?

      I guess there might be a way to slide a sheet of white Correx into a hive with a solid floor (but I bet there’s an entrance block in place 🙁 ) if you wanted to eyeball the debris falling from the cluster. I’m so used to the advantages (as I see them) of OMF’s that I’m not sure I’d cope using a solid floor again.


      1. Steve

        I spoke to somebody today who bought my queens this summer and lives near my bees. He looked in his hives and they had, on average, 2 frames of brood yesterday. Pollen still coming in (the research I’ve seen shows that the presence of eggs/young larvae greatly stimulates pollen foraging). So I’m going to do oxalic around 7th-10th December and hope for the best!

        1. David Post author

          Hi Steve

          Sounds like a plan. It remains unseasonably warm and even here they’re still bring in a little bit of pollen in between the rain showers. I also noticed today that the gorse that usually flowers in February/March is starting to flower now. Weird. I’m off to check my Fife hives on Sunday/Monday and will treat then if, as I expect, they are broodless.


  2. claxby pluckacre

    Am I correct in thinking that as general rule if there is pollen coming in,then there is brood ..or am I confused with a queen being present…
    many thanks

    1. David Post author

      Hello Claxby … in the summer it’s not a definite sign that there’s brood rearing occurring as colonies containing a virgin queen will still collect some pollen. In that instance I interpret it as being an indicator that the colony is queenright and expects to need pollen for brood rearing in the near future. However, in the winter I’m really not sure how good an indicator it is. Perhaps the colony collects pollen (e.g. ivy) because it’s available and fresher pollen is better for brood rearing when they restart?

      I’m guessing … it would be good to know.


  3. Chris Tomlins

    Living in the Channel Isles and enjoying a fairly mild climate my bees are never broodless, however I always treat them by sublimation in December after two treatments with Formic Acid as an August and September treatment using the Liebig dispenser.
    I am seeing articles referring to an OA treatment 3 or even 4 times at 5 day intervals (not sure why it’s not 7 days). This I think is for an autumn treatment and seems to be a USA practice
    What are your thoughts on this as a mid-winter treatment.

    1. David Post author

      Hi Chris

      Repeated OA vaporisation is well tolerated by the colony. However, according to the VMD database it’s not an approved method. Weirdly (and it’s not the only inconsistent or nonsensical ‘rule’) you can treat a colony more than once by trickling with an approved OA-containing miticide, but you are only allowed to treat once by vaporisation. Idiotic of course, because trickled OA damages open brood whereas vaporised OA does not.

      I might be wrong but I think the first empirical test of the optimal repeat cycle for multiple OA administration was by the late Pete Little. There has been at least one scientific study of repeat treatment efficacy (by Jennifer Berry) and I’m intending to discuss it in a post sometime before the end of the year (it’s less good than you might expect).

      I’d recommend choosing the period with minimal brood (which may well before December, but I don’t now for your area) and treating once … but it’s worth noting that there may well be better ways to minimise your mite levels, but they’re not allowed (so I cannot recommend them).


      1. Chris Tomlins

        Hi David,
        thank you for your comments on my observation regarding three consecutive varroa treatments.
        Yesterday I saw three very healthy drones on the landing board of one of my Poly hives unmolested they all took of for a flight and whilst I was observing a drone returned and entered the hive again unchallenged. I immediately thought queenless although the behaviour of the bees didn’t indicate that, very busy with lots of pollen going in.
        Today the temperature reached 16/17°C never before known in November even this far south. With the idea that they might be queenless, not that I could do anything about it with a nice warm afternoon forecast I decided to have a look.
        I overwinter my bees on a brood and a half, the super was full of honey and below in the brood although I only removed one frame (feeling a bit guilty disturbing them) both sides were half full of capped brood with open brood and eggs there as well. With my other hives just as busy I expect this will be the situation with them all with more than one frame of brood in each hive.
        Not a good situation to be in if I want my winter OA treatment to be successful with only one OA treatment.
        I think I have to consider three consecutive treatments if I am to reduce my varroa count unless you have another suggestion!


        1. David Post author

          Hi Chris

          No other suggestions I’m afraid. I’m pretty sure ‘late’ brooding is likely to be a consequence of temperature, forage availability (though I bet it’s pretty limited now apart from ivy and perhaps some fill-in stuff in the fields) and the strain of bees.

          Of the dozen or so hives I checked yesterday all but two looked broodless based upon debris (and my last check a fortnight ago). I didn’t open them. Two were questionable … again, I didn’t open them. My bees are not very prolific and tend to shut down for the long cool winters we usually get here. It’s not that cool yet – they were flying well at 13°C when I arrived at the apiary at midday, but stopped as soon as the sun disappeared – which is unusual, but the bees appear to be behaving as they usually would if the temperature was lower.

          I think it’s easier to keep bees (or, at least keep them with low Varroa levels) in Scotland, where we have distinct seasons. The cool winters help.

          I guess with climate change we’re all going to have to get used to these sorts of problems.


  4. John Eaden

    As always David your analysis is clear and easy to understand.
    Following your post last year on this topic I did assess the extent of brood in my colonies last December and found 3 or 4 frames of brood in each one! We’re a lot further south and saw pollen being brought in right up till the beginning of November – which I use as evidence that brood is still being reared. Your point about assessing the situation rather than using the calendar is well made.
    In the end the oxalic acid treatment was done by a set of three vaping treatments during January. This was very effective and resulted in daily mite drop counts of 0.16 0 0.3 and 0.3 at the spring equinox.
    Vaping three times over a brood cycle would I suppose allow the oxalic acid to get at more mites even if there were some brood being reared. ( I prefer this to trickling as it doesn’t require opening up the colonies). We certainly had low Varroa levels throughout the summer.
    I shall this year attempt more systematic assessment of brood rearing and try to establish when brood rearing ceases – if indeed we do have a brood break and not just a minimum.
    Would you think that a single vaping treatment would work in a brood less colony? The queens didn’t appear to be adversely affected by repeated vaping exposure.

    1. David Post author

      Hello John

      A single vaporised treatment on a broodless colony is very effective. It’s what I do.

      As I commented in response to Chris Tomlins (above) I’m going to discuss some science on repeat treatments shortly. However, I can’t recommend it because it’s not an approved administration method.

      Yours is the third comment that makes the point about pollen collection and brood rearing … I’m going to have to be more observant to determine if the two are invariably linked.


  5. John Bolger

    Oxalic acid is difficult to administer, especially for someone with just a few colonies.

    Could we eliminate most of the mite with a treatment of e.g. Apivar?

    1. David Post author

      Hi John

      I’d argue that trickled oxalic acid is just about the cheapest and easiest miticide to apply to a small number of colonies. It’s faster than vaporising, you need no specialised tools to administer it and it costs about £3 per colony (assuming you buy the approved Api-Bioxal). If you use unadulterated (and not approved) oxalic acid – for example, purchased in bulk from eBay – it probably costs about 3p per treatment.

      Typically something like Apivar is used in late summer/early autumn to reduce the mite levels and protect the winter bees that are developing. The strips need to be in for several weeks. If you treat early enough to protect the winter bees there will still be some mite reproduction in the brood reared late in the year. That’s why a ‘midwinter’ (which actually should be ‘broodless’) treatment is also needed. I’ve discussed this in a post last year on rational Varroa control. I also discussed irrational Varroa control, which outlines the wrong way to do things.


  6. Helen

    Hey David

    Great info as always but many better-and more efficient-ways to do what you’ve suggested:

    1- use plastic corrugated sheets-bought for pennies-in bulk online~just use box-cutters to cut down to size. I’m sure the correx boards are reasonably priced but would soon add up. (I place a thin piece of insulation (bought as a larger piece and used for several hives). A simple piece of heavy duct tape, doubled over, with another piece as a tab, works well to keep draughts out.

    2- why revisit the apiary so many times to do nothing but look at the tray for detritus when you could be doing a repeated OA sublimation to actually kill the mites? Yes, you’ve mentioned you’re a rule follower, but who’s going to know if you keep it on the down-low? Repeated applications (2+, 4-7 days apart) of sublimated OA are so incredibly effective at killing mites when the colony is practically broodless. Plus, you’re not dribbling that nasty, cold-and toxic when consumed-dribble. I’m sure your bees would prefer not having their inner home exposed to the cold air when this liquid is dripping down and chilling them to the bone, so to speak.

    Kind regards, Helen.

    1. David Post author

      Hi Helen

      1. I suspect your ‘corrugated sheets’ are essentially the same as the Correx I use. I either make these from used house For Sale signs (if they are white on the reverse) when they cost £0 or as purchased sheets. If the latter (and my memory is correct) I paid £3.50 per square metre last time, which is at least 4 boards for my floors. I use the purchased stuff for roofs as well where they work out at about £1.75 per roof.
      2. Partly it’s to ‘follow the rules’, but there’s more to it than that. It minimises the amount of chemicals added to the hive and it minimises the number of times I need to get all dressed up in the mask and goggles, and to get the generator out etc, for the vaporiser. It also minimises the disturbance to the bees; if you listen to a colony after vaporising they remain agitated for a long time afterwards. I’d prefer to make relatively infrequent trips to the apiary and not do all that, then treat only when ideal. Multiple treatments when brood is present must – mathematically – need more than two as the efficacy of vaporised OA drops quite rapidly after delivery and brood is capped for 12 days. Finally, I only trickle treat with OA solution if it’s very cold (<7°C) as the cluster is packed in tightly and I don't think the OA vapour works as well (though have done no tests to check if this is actually the case ;) ). When I do trickle treat I pre-warm the OA solution to about 24°C so it's not too much of a shock to the colony. Trickling when the colony is broodless is quicker than vaporising (there's a video somewhere on the website) when you take into consideration the sealing and unsealing of the colony and the PPE gear. I believe it's also only toxic to open brood ... and, by definition, there isn't any.

      The way I describe it does work. The way you describe it works as well, but I’d argue it’s actually more work and might be less good for the colony because of the additional disturbance. I also do it the way I describe because I can hardly write one thing and actually do something completely different 😉 Not least because I have a title and association with a University that might be problematic if I promoted an unapproved activity.

      I’m also planning to discuss some evidence that multiple OA treatments are less effective than some think, in work published relatively recently. Inevitably, it raises as many questions as it answers, but it makes for interesting reading.

      With thanks for the coffee 🙂 … I’ll drink it when writing on repeat OA treatments.


  7. Mandy

    I just have to say that I love your blog and I’ve learned SO MUCH from you! Thank you – I always look forward to your name in my inbox!!

  8. Helen

    You’re so welcome, David, and I look forward to your future thoughts. Those are the same boards, too.

    I guess it’s location, location, etc. Luckily, in TN, we have days around Thanksgiving where the cluster is loose (I shoot for 55°) where the OAS can penetrate. I installed wifi hive scales on one hive a couple of months ago. The sensors really didn’t indicate an extended disturbance of the kind that you mention.

    I’m using the Instantvap now which is absolutely spectacular and incredibly fast. It, literally, takes me less than a minute, in total, to treat each hive. Also, I load a solid 4 grams in the chamber which seems particularly effective.

    I didn’t use Apivar for the first time this summer. I’m really not sure if I’ll resume it next year. I’m definitely in two minds about it. I did appreciate your research where you made the colonies broodless before putting the strips in. That’s not something I’ll ever do, due to the amount of disturbance and time constraints but your research was interesting.

    Thank you, once again, for your kind and thoughtful reply.


    1. David Post author

      Hi Helen

      The InstantVap looks impressive. I wasn’t aware of that model. My vaporizer (a Sublimox) is similarly fast … it’s the sealing and unsealing the hive that takes the time.

      I’m afraid I’m not sure what you’re referring to about using Apivar on broodless colonies? I only treat with Apivar in August/September, leaving the strips in the hive for the full 9-10 weeks as recommended for a colony rearing brood. At the beginning of treatment lots of my colonies have 8+ frames of brood and the strips are well-separate on opposite sides of the box. I reposition the strips midway through the treatment period, placing them at the edge of the shrinking broodnest. By the end of treatment the colony often has little sealed brood left.

      My comment re. colony disturbance was based upon the noise the colony makes. Before treatment – in winter – the colony is largely silent. After treatment there’s quite a bit of noise – even if there are no bees flying – for an extended period afterwards. I can’t remember how long, but will try and remember to time it when I treat in the next days/weeks.


  9. Jeremy Quinlan

    Another excellent blog!
    As broodlessness is so important, would it be a good idea to remove all insulation until after the winter oxalic acid treatment?

    1. David Post author

      Hi Jeremy

      A good question, and something that I’ve wondered about. Logic would suggest that the colony would cluster earlier, would reduce stores usage and would reduce and/or stop brood rearing (earlier). However, I’d want to see some evidence that this is actually what happens and that the behaviour was exhibited by the variety of different strains of bees that are managed.

      Unfortunately, I suspect it’s not as straightforward as that.

      We already know that young queens rear brood later into the autumn than ageing queens. Would removing the insulation curtail this behaviour, in which case the colony would be smaller due to a lesser number of winter bees (and we already know that strong colonies overwinter better)? Alternatively, perhaps a very early cold snap would force the uninsulated colony to abandon the brood it would otherwise have continued rearing and instead cluster tightly, so ‘wasting’ the resources already invested in the brood?

      Dark/native bees are known to be very responsive to environmental conditions – stopping brood rearing if there’s a dearth of nectar, or if the weather turns nasty. In contrast, some other strains seem to be hell-bent on rearing more brood, even if the conditions are wholly unsuitable. My expectation would be that these different strains would also respond differently to an uninsulated hive before midwinter.

      The LASI team at the University of Sussex once suggested that the beekeeper should fork out sealed brood prior to treating the colony with oxalic acid in midwinter … that seems a bit harsh and it isn’t something I would do.

      Considering the variation in bees and the climatic differences across even somewhere as small as the UK, determining that an insulation-free period was beneficial before the midwinter OA treatment is not easy. What works for one beekeeper in one location might not work for another. I wouldn’t want to try and do the experiment with a hope of producing statistically-meaningful results 😉


  10. James Gordon

    David – I find your blog of great practical use and what’s more entertaining. I am feeling completely unsettled by the current weather and not sure what it will mean for the bees and my oxalic acid treatment. We have been below freezing for one night so far this year. In the Scottish Borders -a cold spot. The temperatures are ranging between 8 and 19 + centigrade. My bees are collecting from ivy and gorse when it is not wet and windy. One hive seems close to broodless -the other not. I think I am going to treat next week – simply because the situation is not going to improve/ simplify and I think there now will always be some capped brood around. Do winter eggs always produce winter bees? No matter what the conditions are. . I anticipate more disruptive extreme fluctuations in temperatures. 🙁

    1. David Post author

      Hello James

      Interesting Q about the types of bees produced from eggs laid in the winter. I’m not sure of the answer, but would expect – at least until brood rearing starts in earnest – that the bees would be ‘long lived’ winter bees. However, I don’t think the eggs are any different, it’s how the larvae are reared. The definition of bees as winter or summer depends upon a number of factors including Juvenile hormone and vitellogenin levels, which vary depending upon environmental criteria.

      There have been time course studies of JH and vitellogenin levels over the winter but I don’t think they took account of when the eggs were laid, they simply harvested workers over the winter.

      I’ve written more about winter bees a year or two ago in a post entitled Diutinus bees, where diutinus means ‘long lived’.

      The way Cop27 appears to be going I expect more disruptive unseasonal weather I’m afraid to say 🙁


  11. Archie McLellan

    Thanks for this, David. Your mention of Pete Little’s tight fitting boards, and a conversation on the subject with a friend, has shown that one can have and eat cake with solid-mesh floors.

    I’m going to modify my floors (which are mesh with under floor entrances) to copy Pete Little’s. I hope that will give me the advantages (?) of solid floors, and not lose the option to monitor debris and mites.

    The bees won’t be able to keep the floor clean, so the beekeeper must keep up with that. It’s horrible to see debris fill the gap between the varroa board and the mesh. The bees do what they can to keep the place tidy, but they rely on the moths and ants to hoover up below the mesh.

    1. David Post author

      Hi Archie

      Yesterday’s post has a video showing how effective those floors are when vaporising (one or two of mine are a little too tightly fitting and need a bit of sandpapering). On my floors there’s a 2-3 cm gap between the bottom of the mesh and the tray, ample space to accommodate a sheet of Correx laid on the tray.

      Varroa trays in the bee shed

      I only leave the trays in place when monitoring mite drop – after treating and periodically during the season.

      And when checking for broodlessness of course 😉


  12. John Eaden

    Am I alone in thinking that counting mites is much easier if you draw a grid of black lines in the Correx insert? Counting mites in each square helps to avoid counting the same ones twice!
    I suppose you could instead use a grid drawn on a thin sheet of Perspex and overlay it on the varroa insert.
    I am sure my mite counts are more accurate since I adopted the grids

    1. David Post author

      Hello John

      You’re not alone … many of my Correx trays have lines on, though some are so old now they’ve faded badly. I suspect this is because I’ve used a thin blue marker so that I can better see any Varroa lying on one of the lines.

      Counting mites

      Thanks for the reminder … another task for the winter once they’re out, dried and scrubbed clean.



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