Contact killer

As the days get shorter and the beekeeping season becomes just a fading (happy) memory, visitor numbers to this site start to dwindle. Still healthy 1, but perhaps only 30% of the numbers in May and June.

This is partly because there seems to be less to do at this time of the season.

No swarming, no queen rearing, no honey to harvest … and for many, no real thoughts of beekeeping.

It’s also undoubtedly because the voracious ‘read all you can’ beginners now have several months beekeeping experience.

Some are likely to think they know it all already 2.

Others may have given up in disgust when their colony swarmed (again) in August and they ended the season dispirited, queenless and honey-less 🙁

Of course, some beekeepers will be aware that, although there are some winter (or late autumn) tasks, there is no rush … there’s a whole winter ahead to deal with these and everything should then be fine until the season starts.

Au contraire as we used to say before Brexit 🙁

The paradox of timing miticide treatments

There remains one critically ‘time sensitive’ task to complete before the bees are ready for the season ahead.

Or, as I shall show shortly, not time sensitive with regard to the calendar, but time sensitive with regard to the state of the colony.

Not feeding … that should already be complete and they should not need topping up (if at all) until brood rearing really starts to ramp up in the early spring.

What needs to be done is to kill the mites – or as many as you can of them – that survive after the late summer/early autumn miticide treatment.

There’s an interesting paradox in miticide treatment …

The earlier you treat for mites once the summer honey has been removed, the more mites are present in the hive at the end of the calendar year. If you think about this – or look at my crudely drawn diagram 3 – it should be obvious why this is:

Mite numbers at the end of the year and the influence of early treatment

If you treat early enough (red line) to protect the winter bees from the ravages of Varroa and viruses, the mites that survive treatment will continue to reproduce in the small amount of brood reared at the end of the season (red arrow).

In contrast, if you treat too late to protect the winter bees (blue line), the surviving mites will have nowhere to reproduce as brood rearing will have stopped (blue arrow).

And there will be surviving mites.

None of the approved miticides 4 will kill more than 95% of mites in the hive 5.


So, to start next season with the minimal mite load, you really need to kill as many of these surviving mites as possible.

The usual choice for a ‘midwinter’ mite treatment is oxalic acid. This can be trickled or vaporised and, under optimal conditions, kills 90-95% of the mites. You only need to administer it once and it is reasonably well tolerated by the bees 6

Let’s assume there are 200 mites remaining in your colony after the late summer miticide treatment and the last little flurry of mite hanky panky reproduction in the final round or two of brood reared by the colony.

If you can kill 90% of these mites with a single OA treatment there will only be 20 mites remaining at the beginning of the following season.

But can you kill 90% of them?

Oxalic acid is only effective against phoretic mites. Any mites lurking in capped brood cells will escape treatment. Therefore, if some, most or all of those 200 mites are in capped cells there will be significantly more remaining at the start of the following season.

During the active brood rearing season it has been determined that ~10% of mites are phoretic at any one time. Unfortunately, I’m not aware of any similar studies for the proportion of mites that are phoretic outside the spring and summer.

So, in the absence of any hard data let’s do some arbitrary arm waving calculations … 😉

The graph below shows the numbers of mites surviving a 90% 7 oxalic acid treatment where the percentage of phoretic mites ranges from 100% to 10% i.e. a range covering everything from a totally broodless colony to one with excess brood in all stages for the mites to parasitise.

Total mite numbers surviving OA treatment depends upon the proportion that are phoretic when treated

Unremarkably … the greater the percentage of mites that are phoretic, the fewer mites are left in the hive after the oxalic acid treatment.

And, equally unremarkably, for a contract contact killer 8 like oxalic acid, it is only when all the mites are phoretic that 90% of the original 200 mites can be killed. As you can see from the graph, if only 50% of the mites are phoretic, 55% of the total number of mites in the hive will survive treatment.

If you look at the 10% phoretic column you will understand why a single oxalic acid treatment in the height of the season – or for that matter dusting with icing sugar (which is even less effective) – has only a very limited impact on the overall mite numbers. If only 10% of the mites are phoretic then a whopping 91% of the mites (182) will survive.


Aren’t these are all quite small numbers?

20, 74, 146?

What’s a handful of mites between friends?

Does it really make a difference whether your hive contains 20 or 74 or 146 mites at the beginning of the following season?

Yes, it does.

It makes an enormous difference.

The mites present in early January will reproduce as brood rearing ramps up in spring. Therefore, at any particular time point in the season – assuming all other things are equal – there will be a significantly higher mite load in a colony that started the year with more mites, than one that started the year with fewer mites.

We know quite a bit about the reproduction of Varroa. For example, we know more progeny are reared when feasting on drone rather than worker pupae (because of the longer duration of pupation). There are a host of additional parameters that influence the reproduction rate of the mite population – the proportions of drone to worker brood, the availability of brood, the duration of the phoretic phase of the life cycle (in turn, likely influenced by the availability of suitably aged nurse bees) and so on …

All of which means that we can predict the number of mites present in a hive during the season based upon the number of mites at the start if we make a series of assumptions of hive strength, time of the season, rate of colony build up etc.

I used to use the BEEHAVE software to do this type of colony modelling. However, recent changes to the programming language 9) means BEEHAVE now barfs a slew of error messages back at me when I use it. Since I’m not keen to try and patch up something that is based on outdated or deprecated libraries I’ve instead been dabbling with Randy Oliver’s Varroa Model which is Excel-based.

Randy Oliver’s Varroa Model

Many of you will know Randy as a regular contributor to the American Bee Journal, a commercial beekeeper and the author of

Modelling mite numbers

Using this mite calculator you can easily predict how mite levels build up over the season.

Assuming there was an excess of brood available throughout the season (there isn’t as I shall explain shortly) you could expect mite numbers to increase 154-fold between January and September.

Unrestricted mite replication – the more you start with the (many) more you get

Therefore, if you started the season with just 20 mites there would be ~3000 in the hive by the time the colony is rearing the winter brood in September.

Conversely, 182 mites in January would multiply to over 28,000 by September 🙁

Of course, there is not an excess of brood available throughout the season. For example, in the early spring brood is limiting. However, we can factor brood availability by modifying the calculations to take account of colony strength and build up, reinfestation rates and the proportion of drone brood being reared in the hive.

All of which has conveniently been included in the Varroa Model … thanks Randy 🙂

Mite numbers in September predicted with Randy’s Varroa Model

These various limitations inevitably restrict mite reproduction and the fold-increase between January and September is ‘only’ about 100. This means that a colony that started the season with 20 mites will contain just over 2,000 by September, whereas a colony that started with 182 mites will end up with over 18,000 by the end of the summer.

18,000 is a lot less than 28,000 … but it’s still a humungous number of mites.

Or, more scientifically, it’s an infestation level that the colony is unlikely to survive. 18,000 mites is probably well over one mite for every two adult bees in the colony. With that level of mites you can expect every pupa to be parasitised.

The colony is doomed.

You can check these numbers if you want. The Varroa model is freely available from and is well documented. I used V19 for the calculations above. I also used the model with almost all of the default settings unchanged 10 – specifically this was the colony type Randy designates ‘D’ meaning Default colony in temperate climate, managed to prevent swarming (slight fall brood buildup). The only change I made was to set mite immigration (drifting) to 0.

Are you now convinced of the need to treat in ‘midwinter’?

The ‘midwinter’ mite treatment needs to be applied to minimise the mite levels the colony starts the season with the following year.

However, to be maximally effective, this ‘midwinter’ treatment needs to be applied when all of the mites in the colony are phoretic. That means that winter oxalic acid trickling (or vaporisation) needs to be done when the colony is broodless.

Not when it’s convenient for the beekeeper because s/he is getting over an excess of mince pies and port in the now almost universal holidays between Christmas and New Year’s Day.

‘Midwinter’ is not in the middle of winter … beekeepers should (mis)use the term in the same way they (mis)use phoretici.e. not literally.

Brood rearing – if it ever stops (which I’ll return to at the end) – probably restarts around the winter solstice. That means that there will be sealed brood in the colony early in the New Year. I don’t know how much of that brood is likely to be infested, but I do know that any that is infested will inevitably mean that I’ll be killing fewer mites than I could … and therefore that I’ll be risking exposing the colony to much higher mite levels later in the season.

We’re now in mid-November. Almost all beekeepers should have completed their late summer miticide treatment by now. My Apivar strips were removed almost a month ago.

The precise timing of the ‘midwinter’ mite treatment is irrelevant as long as it coincides with a broodless period in the colony.

I therefore monitor brood production in my colony from late October onwards. As soon as the colonies are broodless I treat with oxalic acid.

There is nothing to be gained by waiting until later in the year. A phoretic mite is a phoretic mite … once they’re unable to hide away I’ve got a 95% chance of killing them.

Those are my sort of odds 😉

I’ve previously discussed how to monitor for a broodless period. If you don’t want to open the hive then learn how to read the debris on a Varroa tray. It’s not witchcraft or rocket science.

Biscuit coloured (or a bit darker) cappings indicating brood rearing in this colony

I expect my colonies to be broodless next week. It’s a little later than last season, but we had warm weather through much of the early autumn. If they’re not broodless yet I’ll hold off treatment for a fortnight or so. Past experience has taught me that the colonies (here in Scotland) are almost inevitably broodless for at least 2-3 weeks between late October and mid-December.

And, if your colonies are never broodless in the winter, all of the above still applies … except you have the slightly more difficult task of identifying when there is the minimal level of sealed brood in the colony.

Why the minimal level?

Because, unless there are weird things like multiple mites infesting each cell, it is logical to assume that when the brood level is at a minimum the phoretic mite level will be at a maximum.

Global warming

As we reach the end of a not-altogether-convincing COP26 conference I thought I’d also mention a recent paper by Giles Budge and colleagues in Newcastle.

I have found it is easier to manage mite infestation levels in Scotland than when I lived in the Midlands. I have a lot more flexibility in the timing of the winter treatment now as the colonies are broodless for longer.

With global warming we can expect warmer winters and therefore it’s probable that colonies may have sealed brood for more of the calendar year.

That will make mite management more difficult.

Certainly not impossible though … particularly if you learn now 😉


A major power outage has meant this was written by candlelight and hot-spotted mobile phone connection. Once power is restored I’ll go back and tidy some of the text and add the keywords. In the meantime I’ll fire up my trusty Ghillie kettle to make another brew 😉

Ghillie kettle


  1. I’d like to thank you both for being so loyal.
  2. Spoiler … you don’t. I’m well aware that I don’t and I already have several successful seasons under my beesuit belt.
  3. Very crudely, but this was being written during a major power outage and I didn’t have the luxury of getting it anything like perfect.
  4. And none of the homegrown recipes or various ’Snake oils’ or ‘magic cures’ offered via social media.
  5. Why not? One of the reasons is that the miticides are also detrimental to the bees … if you used them at a high enough concentration, or for long enough, to kill 100% of the mites you would also probably damage the colony.
  6. Only ‘reasonably’, as dribbled OA damages unsealed brood, but the benefits to the colony – in terms of mites slaughtered – probably still far outweighs the brood damage caused.
  7. I’m being conservative here … I could have chosen 95% but it would have just changed the overall numbers a bit, not the point I’m trying to make.
  8. Apologies … one of my poorer titles.
  9. An unpleasant brew of Java and NetLogo … urgh!
  10. It’s worth noting that some of these defaults may be wrong for UK beekeepers … the volume of the hive, the size of the colony, the duration of the foraging season etc.. However, none of these differences alter the basic principles of mite reproduction. The final numbers might be slightly different, but that’s all.

53 thoughts on “Contact killer

  1. Frazer Munro

    Was just out in the garden judging whether the wind was in the right direction, the air was damp enough and whether the berries on the holly tree were sufficiently red enough to indicate it was time to subliminate OA… Fortunately I retreated to the house and there was the answer…!

    I no longer need to wait for the slugs all to face East and bow down for me to subliminate. 🙂

    1. David Post author

      Hi Frazer

      The berries on the holly is an interesting comment … the rowan have all been stripped bare by redwing here, except one tree. What’s all that about. Perhaps if I keep records for several seasons of the date on which the tree is cleaned out and the presence/absence of brood I’ll be able to suggest something a little less mundane than “look at the cappings on the Varroa tray” 😉


  2. James Gordon

    Phew I was just wondering when to do my oxalic acid vaporisation. Now I know. 2 weeks time after next cold few days.

    1. David Post author

      Hi James

      I used to find that after the first protracted cold period – say 2-3 weeks – of late autumn/winter the colonies would usually be broodless. Typically this would be mid/late November. This year it is notably warmer than the last few years, so I’m going to be having a quick peek on Monday/Tuesday and expect they’ll probably be ready to treat. Ideally I would always look for some form of physical evidence they were broodless rather than trusting the weather (or the slugs or berries as Frazer – above – does 😉 ).


  3. Tim Masters

    Hi David.

    As a new-bee (sic) its a bit daunting as to how and when to attempt the winter OA treatments. I hopefully have a strong well resourced colony that had Apivar in what I hope was the right window of opportunity.

    I suspect that is a matter of being confident to open up the hive in “darkest” winter here in not so sunny Ladybank when all common senses say don’t!

    Any advice as to approach this rather daunting challenge would be greatly appreciated – apart from a supply of confidence pills!!!

    Many thanks for you supply of Friday informative dissertations – they are greatly appreciated.

    Tim Masters

    1. David Post author

      Hello Tim

      No need to open them … place a Varroa tray underneath for 3-4 days and look for the biscuit-coloured cappings. If they’re present then brood is being uncapped and they are – by definition – not broodless. If all that are present are white cappings (from stores) then there should be no sealed brood present.

      But Ladybank isn’t exactly polar at the moment! Temperatures are in low double figures (at best) most days. You can open the colony if you want to do it that way. Just aim for the centre of the brood nest. There’s not need to look anywhere else. If there’s brood present, that’s where it will be.

      But, personally, I almost always check the Varroa tray first … much easier, though not absolutely dependable (for obvious reasons … if they’ve ended the broodless period, but the newly sealed brood isn’t yet emerging … I think we need another graph/diagram).


      1. Jude

        Varroa treatment was left for recommended 6 weeks, and removed but then If there is brood and the drop is in excess of 10 per day over a 7 day period, should I treat more than once.? I live in Sussex, we are expecting our first snow/sleet this week end.

        1. David Post author

          Hello Jude

          That’s an impossible question to answer without writing an essay.

          It’s late November. Six weeks ago was mid-October. That’s really too late to apply your autumn treatment and be sure of protecting your winter bees. All might be OK, but if your mite drop is still high (which it is) it suggests the infestation level was very high to start with.

          You don’t say what you treated with (if Apivar, then 6 weeks is too little if there is brood present). If you used Apistan (the minimum treatment for which is 6 weeks) then you need to be sure that the mites in your area are not already resistant (and they may well be).

          Read up the links embedded above. This post (above) is on the importance of treating when brood levels are at a minimum over the winter. I suspect that’s unlikely to be with your first snow/cold period, but I’d be guessing as I’ve not lived in Sussex (for decades anyway).

          Mite treatment is easy if you do it at the right time and with the right miticide. The former depends where you live and changes from year to year. The latter depends on where you live and how you keep your bees.


  4. Archie McLellan

    All I can say is, congratulations on producing this against the odds – and still making it as worthwhile as ever! Hope you get power back soon. It can be quite miserable.

    1. David Post author

      Hi Archie

      It was touch and go … I had most of the text and the photos, but they were all in the wrong place. Not clear why we lost power for so long (wrong sort of rain on the electricity wires?) but it wasn’t an issue – the house is well-insulated and I could always build a huge conflagration from old frames to huddle around if needed …


  5. Josh

    We’re never truely broodless here in New Zealand (with exception of some far south beeks) so midwinter OA dribble isn’t something you will find many doing.

    As you may, or not, be aware OA has a resurgence of popularity here. But as a strip made with glycerine solutions range in from 30-50% OA (most around 40%) based on the evolution of Randy’s Shop Towel Treatments.

    All the evidence here is at best anecdotal at present, and hope that some bigger researchers take this idea and test it properly.

    1. David Post author

      Hi Josh

      I’ve read the stuff that Randy has published on the ‘shop towel’ treatments. I’ve not tried it. Even if I had, I probably wouldn’t write about it as it’s not an approved treatment regime here. It was clearly showing promise, but it needs testing in other environments as well.

      Oxalic acid is an excellent miticide when used properly. It has a lot going for it … inexpensive, no long-term residues, no resistance and very effective.

      I’m always conscious that my writing can only be based upon my experience keeping bees in a few locations in the UK. Bees are bees, but the climate/forage is so variable in other places that what I discuss may not be relevant at all 🙁

      Though, of course, I hope some of it is.



  6. Patricia Haslehurst

    Thank you for a readable, encouraging email.
    So often, us ‘new’ bee keepers are daunted by fact, which often turns out to be fiction in our personal experience.
    Why don’t bees read the same books as us? Don’t bother replying to that as a post card isn’t big enough.
    let’s see what the winter brings… hopefully a better spring and more predictable bee behavior won’t make us lose heart!!

    1. David Post author

      Hi Patricia

      There’s a whole lot of facts that are actually fiction, or at least wishful thinking. My draft book of Beekeeping Myths is almost three volumes long now. However, I also think it’s good that bees haven’t read the books (other than mine 😉 ) … if they did the same thing all the time it might get a bit dull.

      Of course, when you’re starting out this variation/wilfulness can be extremely frustrating … the secret is to try and learn something every time you visit the apiary. Before long you’ll suddenly realise you’re understanding things that would previously have perplexed you.

      Though, if my experience is anything to go by, there will still be things that leave you scratching your head 🙁


  7. Meriet Duncan

    Fantastic article -as always – I’ve been a beekeeper for 12 years and always done it at the same time every year. So thank you for this… I’ll be checking for a brood less colony before applying the oxalis treatment.

    1. David Post author

      Hi Meriet

      I think one of the most important lessons I have learned is that you can’t use the calendar to decide when to do anything with the bees – adding the first super, removing the honey crop, either of the annual treatments etc. There’s just too much variation year-to-year. I try and keep an eye on what’s happening in the environment – the migrant birds appearing, the flowering of particular forage plants – in the hope/expectation that it will help me get the timing a little better when I’m dealing with the bees.

      The broodless winter treatment is important. I’ve managed my colonies like this for years and generally have very low mite levels (and, in the research colonies which are managed in the same way, very low virus levels).


  8. Joss Bartlett

    Hi David,
    I’ve got used over the last decade or so to the first frost, if we have any at all, not coming until well after Christmas. I assume that the broodless period will get a bit later going south from where you are, but would you expect there to be a kind of tipping point so that, if they haven’t been broodless before, say mid-January, they won’t have a broodless period at all and you may as well just chuck in the oxalic acid and kill whatever you can ? As you say, I suppose then it becomes a question of trying to identify the minimum brood point.
    And did you see the reports this week of the bees in Blenheim Palace that they claimed were resistant to Varroa ?
    Thanks, Joss

    1. David Post author

      Hi Joss

      If a colony is going to be broodless I’d expect it to be so before Christmas. If the colony were to wait until mid-January I think there’s a risk they wouldn’t be strong enough to exploit the early forage when it became available.

      However, once I’ve treated in the late autumn I tend not to check them very much, so usually miss when they start brood rearing again. It’s also worth noting that I only have experience of keeping bees in the Midlands and Scotland. Things might be completely different in the South East or West … though, as before, I’d be surprised if they were broodless in mid-January for the reasons given above.


      PS I’ve read the stuff about the Blenheim “Varroa resistant” bees … watch this space.

  9. Gnome Liston

    Hi David,

    Thank you for all your articles, they are really helpful. Having convinced me of the need for the mid-winter treatment can you point us to a link that tells us HOW to do it? Thanks.

    1. David Post author

      Hi Gnome

      There are two links in the text to some additional guidance on trickle treating and sublimation/vaporisation. However, it’s worth noting that my vaporiser (a Sublimox) is not the type most beekeepers use. I can’t provide advice on using a Varrox-type vaporiser as I’ve never used one.


  10. Maccon Keane

    Dear David
    As ever a really helpful piece. Given the winter decline in readership and hence questions I am taking the liberty of asking 3 if possible.
    1. I had thought the broodless period related to temperature related cessation of egg laying rather than ‘time of year’ duration of sunlight etc so I have tried to be opportunist and treated colonies as best possible after a week + of frosty / hard weather albeit global warming has made this less common. Am I misguided?
    2. Should one consider a second Oxalic acid treatment at day 13-14 as all capped brood at time of application #1 will have hatched and hence now be phoretic and all bees will have been treated. ( This question also applies to question of considering using Oxalic acid in autumn management of Varroa – rather than other agents that can raise resistance and leave ‘permanent’ residues’ ).
    3. Should the practice of hive insulation in winter be abandoned be abandoned to allow / guarantee a broodless period to allow better Varroa management.
    Apologies for hogging the questions section. And yes I love your Friday posts

    1. David Post author

      Hi Maccon

      Some interesting Q’s …

      1. I discuss this elsewhere in these comments … I used to rely upon the timing of the first really cold snap, but am not sure that’s as reliable as checking the colony. We’ve had no frosts yet on the east coast and so it will be interesting to see what’s happening in the colonies. I’ve got some very good weather data available now and hope to do some analysis of this over the next couple of years.
      2. Multiple OA treatments are recommended by some. It might help mop up a few stragglers. However, it should be noted that it is not actually approved if you go by the precise wording on the side of the Api-Bioxal packet. Personally, I’d prefer to get my treatment timed exactly right rather than having to apply it twice. From the limited data I’ve seen (including some of my own) I’m not convinced that repeated autumn treatments with OA are as effective as Apivar. My current beekeeping setup (150 miles from my Varroa infested colonies) means the logistics prevent me from investigating this further.
      3. Derek Mitchell is the person to ask about hive insulation. I use poly hives and have a 50 mm block of Kingspan over my crownboards all season. I certainly don’t think they need more insulation than that. However, I’ve also got colonies in cedar boxes and don’t see any obvious differences in broodlessness or overwinter survival. However, it’s only a relatively small number of hives, so not statistically relevant.


  11. Sarah Haynes

    It would appear the ‘other’ reader this week is Tim, so as the ‘other other’ reader 😉 I’d like to say I cannot understand why anyone DOESN’T read your blogs every week! After all, this luxurious excess of ‘bee-free’ leisure time just allows additional time to browse indulgently through all your archives; plotting, planning and scheming how to improve for next season. I can live in eternal hope, even if I die in despair!
    So keep up the fantastic blogs and maybe the numbers will, one day, double 🙂

    1. David Post author

      Hi Sarah … going by the comments accumulating today it looks like the two regular readers have told a few of their friends to check out the article as well 😉

      Many thanks for the kind words

  12. Helen

    Hi David
    In Chattanooga, Tn, we are mostly broodless now. I am sublimating oxalic acid using a band heater. I have never dribbled and love the ease of this tool and not having to open up the hives. The bees respond really well in that there is minimal, if any, brood and bee damage. If my varroa tray has more than single digit numbers I repeat every 3 days until it does. I’ve never lost a colony overwinter in 8 years due to using this method (even when it wasn’t legalized here) on 23 colonies. I always wear my respirator with organic acid cartridges. Each colony takes 60 seconds and I get them all done in less than an hour and a half. I also use Apivar from August 1st through October 1st though my observations show that this miticide may be losing its effectiveness.
    Thank you for your posts. I really enjoy reading them.

    1. David Post author

      Hi Helen

      That’s the great advantage of vaporising … the fact you don’t need to open the hive at all. All my floors have a 7 mm hole drilled (and plugged) through the rear wall so I can just poke the spout of my vaporiser through and give them a blast.

      However, I’ve done a side-by-side comparison and it takes me longer overall to vaporise than to trickle treat. That’s if you take into account the setting up, getting the generator working, sealing the hives properly, donning the PPE, waiting to unseal the hive etc.

      I usually vaporise if it’s warmer (say >8°C … ~47°F) but trickle if it’s colder. Perhaps wrongly I’m not convinced the vapour permeates a tightly clustered colony.

      You make an interesting comment about Apivar. I’me still convinced of its effectiveness here, but know that some US beekeepers are reporting problems. I’ve previously written about resistance (which is a bit weird going by the data).

      Pleased you enjoy the posts.

  13. Helen

    I drill the hole at the top of the box that has the brood. The vapor tends to descend with our relative humidity so I feel this permeates the cluster more efficiently.

    I use an extension cord where possible or an 800 watt inverter hooked up to my running car battery so there’s no time wasted.

    I sublimate at 55 degrees where the cluster is loose on a cloudy day when most of the foragers are home.

    The thought of dribbling fluid on the cluster is not appealing to be at all, but maybe I’m anthropomorphizing!

    1. David Post author

      Hi Helen

      Think of dribbling as taking a warm shower in a delicious tasting syrup … I pre-warm the OA solution to mid-30’s Centigrade before using it.

      The inverter would be a good solution, but when I got the Sublimox a couple of my apiaries were a long way from the car … a portable generator was cheaper than a 4×4 😉


  14. vince poulin

    Helen and Maccon’s comments and questions to me are spot on as is your tenacity in treading back into Varroa issues. I’m envious of those who can be methodical with their treatments. The time it takes to understand how and when to treat thwarts some of us. Sometimes failure comes just by chance, treatments simply don’t work, application timing did not catch early mites, and other unknown and uncontrolled factors such as lack of uniformity in treatment within a brood box. A great post David. Those curves do not lie, 10 mites in spring = 2000+ mites in September. This year, with two mite-free colonies (packages from Australia) both went to 2000+ mites. One split from those packages went to 2000+ mites. Other late season splits (4) due to brood breaks produced just few mites or up to about 400. One colony with high a high mite count gave a 3 million Nosema spores/bee result. That test had me inspect the hive 2-weeks ago where I had a 5″ patch of capped brood on both sides of one frame. 3-other hives were also inspected and lacked brood. I’m just now charging a treatment battery. I do not fully trust my equipment. It vapourizes well but mite counts can sometimes not reach single digit numbers. This is expected if any brood are present but during a broodless period I want Zero’s after some number of days following treatment. If they don’t come I have no hesitancy in repeating an OA treatment. As your graphs show – 10 mites = 2000. Helen is spot on. Don’t stop until those numbers confirm your not missing many. Not all dead mites fall out immediately – some get stuck in a crack, on a slatted rack and only later shoved along by bees. It can take a few days but I agree fully – get them to near Zero. I could not commence Apivar early enough this year. My timing was off by factors beyond my control. I was stuck commencing Apivar on September 2 – data shows – it was too late for 2 of my hives. Those went 2000+ mites and have issues with viruses. I do not think they will make it. By comparison I have 4, strong – mid-to-late season NUCs with few mites and no symptoms of DWV or treatable Nosema. I’d say “lucky” but it does show on my small-scale splits and broodless windows play an important role in Varroa control. My approach next season will be a combination of tactics:1. early August Apivar for primary hives – those that overwinter, 2. Mite trapping (queen sequestering on drone comb for 24-days, includes replacement drone comb as needed – thank you David!), 3. OA – a post-trapping treatment to kill persistent phoretic mites not captured in drone traps, and 4. OA mid-winter clean-up. Monitoring will include alcohol washes, drone uncapping and mite board counts for every treatment.

    1. David Post author

      Hi Vince

      A thorough and informed approach to mite management is certainly best. I’m not sure how long dribbled OA remains effective in the hive, though – as you say – mite drop continues for a few days. With vaporised OA I think there are studies which shows mite drop continues for 5-7 days at most. In my experience it’s perhaps 4-5 days. I suspect some of this is freshly killed mites because the vaporised OA is deposited on every surface within the hive.

      Randy’s Varroa Model is quite good because you can investigate the influence of a variety of different treatments and timings. Give it a go … something to do as you’re planning for the season ahead.


      PS. I know there’s another post from you I’ve yet to get to … 🙂

  15. Juhani Lundén

    According to my studies about 28 ml of 3% oxalic acid sugar solution is needed to keep mite numbers steady, that is in our climate 90% efficiency. The situation stays stable. More than 28 ml is unnecessary, less will cause higher mortality.

    The common advice is to kill mites as well as you can.
    In order to breed better bees we need to kill mites as poorly as you can afford.

    I have suggested 15 ml oxalic acid treatment for a “soft bond breeding system”.
    It will definitely cause higher mortality, but your best bees will survive.

    More in my blog:

    1. David Post author

      Thanks Juhani

      I suspect 90% of the readers of this site will not understand the term “soft bond breeding system”. I think a priority is that new beekeepers learn to manage mites effectively (using chemical intervention) before they start to dabble with treatment-free or reduced-treatment options. Every year I see new beekeepers proudly saying they are going to be treatment free and that evolution will solve everything for their two colonies. I don’t think it ever does.

      My recommendations for oxalic acid strength and treatment is here.


  16. Helen Howarth

    reading this and having left varroa behind in Lancashire where in the days when I could dribble oxalic acid on a cold day after a long spell of frosty weather and the snow was piled high, I am sooooooooooo glad to be varroa free ………… Please please may we stay varroa free here on the west coast this vaporisation, generators and car batteries and sublimation is baffling me…..and not being complacent, it would be so lovely for a permanent solution to this mite instead of having to treat with these chemicals.

    1. David Post author

      Hi Helen

      I agree about being mite free. However, mites are manageable. Whether a permanent – meaning chemical-free – solution will/can be found that is acceptable and workable i.e. probably not involving GM, robust enough to cope with open mating, able to generate bees that are suitable for both amateur and commercial beekeeping etc. is not clear to me. I remain pretty unconvinced by all the claimed Varroa-resistance I’ve currently seen. Either they are not resistant, or it is in isolated environments and/or with exceptional beekeeping skills. In none of these instances have I seen a detailed analysis of pathogen loads and statistically relevant overwintering colony numbers … but there are lots of anecdotes of course 🙁

      For those of us fortunate enough to live in mite-free areas we have our work cut out persuading new beekeepers in the region that they should also start with mite-free bees …


      1. Juhani Lundén

        Treatment free beekeeping is here to stay. It takes 10 years hard-bond (no treatments, big losses) to get varroa resistant bees if one has the possibility to control matings, either isolation mating stations or insemination. Examples are many Kefuss, Gotland bees, myself, Reinertsen etc.)

        Reinertsen did it actually with free mating, and is scientifically proven, that his bees are resistant.

        Soft-Bond is a milder method. Treatments are done according to need and the mite infestation levels are measured.

        1. David Post author

          I’ve been meaning to discuss the studies by Melissa Oddie and hope to get round to it soon. However, my understanding is both the Gotland bees and the Kefuss stocks had eventually to be treated to ‘save’ the remnant colonies … this was discussed on Bee-L earlier this year. To be useful, ‘resistant’ colonies need to survive the challenge from genetically and geographically divergent mite and virus populations. Many of the resistant stocks reported over the years have been locally isolated and genetic adapted mixes of hosts and parasites.


  17. Helen

    Haha…thanks, David. Now that almost sounds erotic! Saying that, I still don’t like the idea of opening up the hive and allowing all that moist, carbon-dioxide-rich warm cluster air from escaping, even if just for a minute. It sounds draughty🥶

    1. David Post author

      Hi Helen

      It probably is draughty … but do the cost/benefit analysis … you can dribble OA quickly and safely, but to vaporise you need a vaporiser and PPE (the amount of the latter depending upon the risks you are willing to take) and possibly an inverter and a truck 😉 Both methods will be hugely beneficial to the bees … done at the right time.

      There’s been a big increase in the use of vaporising in the UK (or, at least, an upsurge in people talking about it online). Far more important than how it is delivered though is the timing of delivery. If you do either at the wrong time of the season it’s of little benefit to the bees.

      One final real benefit of vaporising is that you can do it at night … I’ve done this several times very successfully when either I couldn’t get to the apiary in short daylight hours due to work commitments, or it was raining hard all day.

      Killing mites at night


  18. Helen

    David, all good points, however, after the initial small investment, vaporization is incredibly quick and easy and much more friendly to our bees. For example, if I vaporize and see a high mite drop after 24 hours I can do it again 3-5 days later, again without opening up the hive. My PPE is a respirator with organic acid cartridges and is 100% safe for me. Personally, I would not have an apiary that was inaccessible to my car. That would be a deal-breaker. Vaporizers, inverters and PPE are not very expensive and are one-time investments. They have made varroa management extremely easy for me. Like I mentioned before, over the 8 winters I have kept bees I have never, I repeat, never lost a colony. I attribute this to consistent and timely (mostly broodless) applications of OAS (oxalic acid sublimation) with 3-5 day follow ups until varroa drop is in the single digits.

    1. David Post author

      Hi Helen

      I don’t disagree with the points you make. My own vaporiser was not inexpensive, but it has been an excellent investment.

      It’s worth noting that the VMD-approved OA-containing powdered treatments (just about) suitable for vaporisation are not approved for multiple/repeat treatment. This is one of the many confusing and, frankly, daft consequences of the regulation of OA which although previously not approved, was routinely used. It’s not surprising that many beekeepers get confused about this. There are some plans to try and streamline the advice provided, but the way regulation works and real, practical beekeeping, appears to be incompatible.

      And I’d better stop there or I’ll start ranting 🙂


  19. Helen

    David, you’re an absolute gem and I have loved our conversation!
    Sometimes, you just have to go with your gut and s*** the regulations😂 I’m a master beekeeper, mentor and a club state inspector but I still advocate what is best to keep the bees alive and healthy. Our bees make an insane amount of honey (not my primary reason for keeping bees) because they come through winter thriving and, more importantly, with a low mite load. My vaporizer cost €150 and it has been one of the best beekeeping investments I’ve made. The inverter and PPE cost way less.
    I’m being bull-headed about this, I realize, but I just don’t think people realize how easy it can be to have 100% winter survival year after year, assuming adequate food supplies, of course.
    I also use the purest wood bleach (savogran) which costs pennies. Who is going to know unless you broadcast it?

    1. David Post author

      Hi Helen

      Good winter survival (I don’t get 100% because of queen failure) is within the reach of all beekeepers if they feed and treat appropriately. The fact that national surveys – ours by the BBKA and SBA, yours by BIP, and the Coloss studies – show such consistently high winter losses (routinely 20-30%) proves that we are a long way from achieving anything close to ‘good winter survival’ I’m afraid.

      I’ve emailed you on a couple of separate points.


  20. David Henderson

    Hi David
    I have kept bees for over 30 years, yes that was prevarroa, in the early years in East Lothian. Just a week or so before Christmas used to be the best time to trickle OA, but when I moved to Shropshire the hives were never brood free. It then just meant you trickled OA during a longish cold spell before the queen started laying??. That seemed to work and once I had Varroa floors on all my hives it was possible to carry out “counts” to verify the effectiveness of the treatment. I sold most of my hives before returning north to Berwickshire but unfortunately the two hives I returned with did not survive as they could not handle Scottish winters. That was the only year I had no bees, or so I thought, one hive I had left behind in East Lothian was buzzing and when I got into it with a hammer and chisel it was solid. That was five years ago and they were left to it. In total that is eighteen years they have survived without any human intervention, also they swarmed regularly the smallholding owners informed me. There is a great resource of beekeeping knowledge in the East Scottish/English border area although sadly old Fred Mitchell from Coldstream has left us joining George Hood from Ormiston.

    It is always great to read through the various discussions on the interweb regarding beekeeping, but it would be so good to get back to prevarroa days when supers were stacked high with bees going about their business unhindered by this horrendous infliction.

    Many thanks for your postings … REgards… Dave Henderson


    1. David Post author

      Hello Dave

      Although I’m sure Varroa has to share some of the responsibility, I suspect the main reason the skyscraper hives of stacked supers are no longer prevalent is the changes in our environment. Intensive farming and land use has dramatically altered the forage available. It’s boom and bust, with relatively little boom these days. There are similar declines in the population of farmland birds … so it’s not just bees that are suffering I’m afraid.

      Whether it will ever be possible to recapture those pre-mite days is unclear.

      The frequent swarming you discuss for your East Lothian colony reminds me of the reports from Tom Seeley about his Arnott Forest bees that survive/tolerate Varroa, at least in part by being swarmy. When a colony swarms it loses about a third of the mite population because of the high proportion of young bees in the swarmm.


  21. ian Robinson

    Great post thanks which set me thinking. With my locally adapted bees in Northumberland’s Tyne valley, I tend to find that my queens go off lay in September coming back into lay sometime in October. I have wondered whether this marks a more dramatic transition between summer and winter bees albeit quite a late one. The last few years have seen very mild weather in October and November so I presume this allows the production of a reasonable number of winter bees. I have not checked for broodless periods in December/January by looking for brood cappings but plan to do so this year to see when it occurs. It will be interesting to see if and when a second broodless period occurs.

    1. David Post author

      Hello Ian

      I’d be surprised if it was a switch from summer to winter brood rearing which I’ve always understood was a gradual transition, though it could also coincide of course. If the summer nectar dries up suddenly the queens are likely to stop laying as well.

      Mine – as predicted – appear broodless. I opened two colonies yesterday for a quick look.


  22. sean

    Does the cold weather slow the larvae development, or are cells capped at day 8 throughout the year?
    My thinking is that there are queen cages available to contain the queen on a single frame.
    If she is contained for 7 days, then the frame she is on is removed from the hive it will be uncapped and could be put into another hive to preserve the larvae and the queen’s hive will be completely broodless in 14 days. Ready for a dose of OA vapour to wreak havoc on the mites.
    The cage could be used on the next hive and in 7 days the frame of larvae from this hive placed into the first to restore it’s population.
    Obviously it’s a bit of a faf but could be done throughout the year

    1. David Post author

      Hello Sean

      The cold weather does not slow down larval development … they need to be maintained at about 34°C to develop properly, so the colony thermoregulates the cluster to achieve this.

      I’m sometimes criticised for suggesting you can open the hives to check for broodlessness … goodness know what people would say if I was rummaging about trying to cage the Q! I think this would probably be detrimental to the colony. In addition, it’s almost always that the Q lays on facing frames, so you’d have to be moving two frames between colonies.

      As I said in the post, I’m not sure we know the proportion of mites that are phoretic in winter if there is brood present. However, we can be pretty sure that if the amount of brood is minimal, then the capped mites will also be at a low level. Therefore, even if the colony doesn’t truly become broodless the timing of treatment to a ‘minimal brood’ period is important.


  23. Frazer Munro

    Hi David, …spurred on by your assurances I had a (very brief) keek inside my brood boxes before the cold set in last week and found ….a full frames worth of brood in all stages including eggs !
    If these are the last, and go full term, they will not hatch until mid December, near as damn it Solstice ..
    I’m feeling I need to just get on and vape a couple of times…
    Doon in Staff London/Kent

    1. David Post author

      Hi Frazer

      Those softy southerner bees might well go on brooding through the winter which certainly makes managing the Varroa a bit more of a headache. To cover a full brood cycle I think you’ll need three or four repeats suitably spaced. Vaporised OA appears to ‘linger’ a bit in the hive – presumably on surfaces – and the mite drop can continue for at least 5 days (or possibly even 7 if you read some of the literature).

      If you know which frame it was you should be able to work out the ‘stripe’ on the Varroa tray and determine when they’re emerging by the pattern of crumbs that appear.

      Cheers (and good luck)

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