Midwinter, no; mites, yes

There’s a certain irony that the more conscientious you are in protecting your winter bees from the ravages of Varroa in late summer, the more necessary it is to apply a miticide in the winter.

Winter bees are the ones that are in your hives now 1.

They have a very different physiology to the midsummer foragers that fill your supers with nectar. Winter bees have low levels of juvenile hormone and high levels of vitellogenin. They are long-lived – up to 8 months – and they form an efficient thermoregulating cluster when the external temperature plummets.

Winter bees production

In the temperate northern hemisphere, winter bees are reared from late summer/early autumn onwards. The combination of reductions in the photoperiod (day length), temperature and forage availability triggers changes in brood and forager pheromones.

Factors that influence winter bee production

Together these induce the production of winter bees.

For more details see Overwintering honey bees: biology and management by Döke et al., (2010).

Day length reduces predictably as summer changes to autumn. In contrast, temperature and forage availability (which itself is influenced by temperature and rainfall … and day length) are much more variable (so less predictable).

All of which means that you cannot be sure when the winter bees are produced.

If there’s an “Indian summer“, with warm temperatures stretching into late October, the bees will be out working the ivy and rearing good amounts of brood late into the year. The busy foragers and high(er) levels of brood pheromone will then delay the production of winter bees.

Conversely, low temperatures and early frosts reduce foraging and brood production, so bringing forward winter bee production.

It’s an inexact science.

You cannot be sure when the winter bees will be produced, but you can be sure that they will be reared.

Protect your winter bees

And if they are being reared, you must protect them from Varroa and the viral payload it delivers to developing pupae. Most important of these viruses is deformed wing virus (DWV).

Worker bee with DWV symptoms

Worker bee with DWV symptoms

Aside from “doing what it says on the tin” i.e. causing wing deformities and other developmental defects in some brood, DWV also reduces the longevity of winter bees.

And that’s a problem.

If they die sooner than they should they cannot help in thermoregulating the winter cluster.

And that results in the cluster having to work harder to keep warm as it gets smaller … and smaller … and smaller …

Until it’s so small it cannot reach its food reserves (isolation starvation) or freezes to death 2.

So, to protect your winter bees, you need to treat with an appropriate miticide in late summer. This reduces the mite load in the hive by up to 95% and so gives the winter bees a very good chance of leading a long and happy life 😉

Time of treatment and mite numbers

Time of treatment and mite numbers

I discussed this in excruciating detail in 2016 in a post titled When to treat?.

The figure above was taken from that post and is described more fully there. The arrow indicates when winter bees are produced and the variously coloured solid lines indicate mite numbers when treated in mid-July to mid-November.

The earlier you treat (indicated by the sudden drop in the mite count) the lower the peak mite numbers when the winter bees are being reared.

Note that the mite numbers indicated on the right hand vertical axis are not ‘real’ figures. They depend on the number present at the start of the year. In the figure above I “primed” the in silico modelled colony with just 20 mites. This will become very important in a few paragraphs.

Late season brood rearing

Compare the blue line (mid-August treatment) with the cyan line 3 (mid-October treatment) in the figure above.

The mid-October treatment really hammers the mite number down and they remain low until the end of the year 4.

The reason the mite numbers remain low after a mid-October treatment is that there is little or no brood being reared in the colony during this period.

Mites need brood, and specifically sealed brood, to reproduce on.

In the absence of brood the mites ‘colony surf‘, riding around as phoretic mites on nurse bees (or any bees if there aren’t the nurse bees they prefer).

And that late season brood rearing is the reason the end-of-year mite number for the colony treated in mid-August (the blue line) remains significantly higher.

Mites that survive the miticide in August simply carry on with their sordid little destructive lives, infesting the ample brood available (which could even include some highly mite-attractive and productive drone brood) and reproducing busily.

So, the earlier you treat, the more mites remain in the hive at the end of the year.

Weird, but true.

Early season brood rearing

The winter bees don’t ‘just’ get the colony through the winter.

As the day length increases and the temperature rises the colony starts rearing brood again. Depending upon your latitude it might never stop, but the rate at which it rears brood certainly increases in early spring.

Or, more correctly, in mid- to late-winter.

And it’s the winter bees that do this brood rearing. As Grozinger and colleagues state Once brood rearing re-initiates in late winter/early spring, the division of labor resumes among overwintered worker bees.”

Some winter bees revert to nurse bee activity, to rear the next generation of bees.

And this is another reason why strong colonies overwinter better … not because they (also) survive the cold better 5, but because there are more bees available to take on these brood rearing activities.

Strong, healthy colonies build up better in early spring.

Colonies that are weak in spring and stagger through the first few months of the year, never getting close to swarming, are of little use for honey production, more likely to get robbed out and may not build up enough for the following winter.

Midwinter mite treatments

Which brings us back to the need for miticide treatment in midwinter.

The BEEHAVE modelled colony shown in the graph above was ‘primed’ at the beginning of the season with 20 mites. These reproduced and generated almost 800 mites over the next 10-11 months.

What do you think would happen if you start the year with 200 mites, rather than 20?

Like the 200 remaining at the year end when you treat in mid-August?

Lots of mites … probably approaching 8000 … that’s almost as many mites as bees by the end of the season.

So, one reason to treat in the middle of winter is to reduce mite levels later in the season. The smaller the number you start with, the less you have later.

Vapour leaks out ...

Vaporisation … oxalic acid vapour leaks out …

But at the beginning of the season these elevated levels of mites could cause problems. High levels of mites and low levels of brood is not a good mix.

There’s the potential for those tiny patches of brood to become mite-infested very early in the season … this helps the mites but hinders the bees.

Logically, the more mites present at the start of brood rearing, the more likely it is that colony build up will be retarded.

So that’s two reasons to treat with miticides – usually an oxalic-acid containing treatment – in midwinter.

Midwinter? Or earlier?

When does the colony start brood rearing again in earnest?

This is important as the ‘midwinter’ treatment should be timed for a period before this when the colony is broodless. This is to ensure that all the mites are phoretic and ‘easy to reach’ with a well-timed dribble of Api-Bioxal.

In studies over 30 years ago Seeley and Visscher demonstrated that colonies have to start brood rearing in midwinter to build up enough to have the opportunity to swarm in late spring. These were colonies in cold climates, but the conditions – and season length – aren’t dramatically different to much of the UK.

Low temperatures regularly extend into January or February. The temperature is also variable year on year. It therefore seems (to me) that the most likely trigger for new brood rearing is increasing day length 6.

The apiary in winter ...

The apiary in winter …

I therefore assume that colonies may well be rearing brood very soon after the winter solstice.

I’m also aware that my colonies are almost always broodless earlier in the winter … or even what is still technically late autumn.

This is from experience of both direct (opening hives) or indirect (fresh brood mappings on the Varroa tray) observation.

Hence the “Midwinter, no” title of this post.

Don’t delay

I therefore treat with a dribbled or vaporised oxalic acid-containing miticide in late November or early December. In 2016 and 2017 it was the first week in December. Last year it was a week  later because we had heavy snow.

This year it was today … the 28th of November. With another apiary destined for treatment this weekend.

If colonies are broodless there is nothing to be gained by delaying treatment until later in the winter.

Most beekeepers treat between Christmas and New Year. It’s convenient. They’re probably on holiday and it is a good excuse to escape the family/mince pies/rubbish on the TV (delete as appropriate).

But it might be too late … don’t delay.

If colonies are broodless treat them now.

If you don’t and they start rearing brood the mites will hide away and be unreachable … but their daughters and granddaughters will cause you and your bees problems later in the season.

Finally, it’s worth noting that there’s no need to coordinate winter treatments. The bees aren’t flying and the possibility of mites being transferred – through robbing or drifting – from treated to untreated colonies is minimal.



  1. Assuming you’re not one of the 3% of readers from the Antipodes.
  2. Or, as I’ll discuss shortly, it staggers through the winter but isn’t strong enough to build up properly in early spring … but I’m getting ahead of myself.
  3. At least, I think it’s cyan … I’m colourblind.
  4. However, although it’s difficult to tell from the figure, the mite numbers at the year end are actually a bit higher than it started the year with (I’ll return to this shortly).
  5. Because they’re better at thermoregulation and because they use food reserves more economically.
  6. I don’t know if this has formally been tested.

19 thoughts on “Midwinter, no; mites, yes

  1. Fred

    have been impatiently waiting on your oxalic green light! go! go! go! ) all week, 5 ° in Ireland this weekend and all mine getting done (used to go late December but followed your advice last year and defo successful)

    1. David Post author

      Hello Fred

      Thanks for the vote of confidence but remember that there are regional differences across these small islands. What works for me might not be right for you (or vice versa) and so it’s important to use your local experience. We’ve got 5 below forecast for the next couple of nights, with daytime temperatures in the low single figures. If that’s correct (and the car is already iced up, so I suspect it is) I’ll be delaying treatment for a few days until the cluster is a bit less tightly, er, clustered.

      I treated yesterday at about 8C and there were a few bees on cleansing flights today.


  2. Roger Gill


    I am based in slightly sunnier climes that you – Herefordshire – and we had our first frost last night. Although I have seen liitle life from my hives recently I know they are not all dead as I have thermocouples in them (and clear acrylic crownboards). I do not , however, know if they are broodless and do not propose finding out. I have been contemplating doing three applications, with Sublimox, next month at five day intervals. It sounds like overkill, but can you have overkill where varroa are concerned?

    Unfortunately work takes me out of the country for the week before Christmas (something has to pay for my beekeeping, particualrly the Sublimox). I intend therefore to ‘vape’ them on 21st, 26th and 31st December.

    Is this a sign of paranoia on my part?

    1. David Post author

      Hello Roger

      If the colonies are broodless you only need to do one treatment, whether vaporised or trickled OA. Assuming the colony isn’t tightly clustered it’s not a problem to have a very quick peek at the centre of the cluster to see if they are broodless. Alternatively, slide in a cleaned Varroa tray and look for ‘stripes’ of biscuit-coloured cappings indicating bees emerging. If there are none over a 2-3 days period you can be reasonably confident they’re broodless.

      Whilst I’m a strong supporter of treatment where necessary, I think we should avoid over-treating if possible. I’m going to post something else in the next week or two on mite counts and repeat treatment (or hope to … I might get distracted).


  3. Andy Cameron

    A nice timely read David, thanks for that, if only to instil to me that I might be taking my bees into their first winter in good order🤞I carried out a 3 dose sublimation session completed on 26/11/19 so hopefully I have mitigated the marauding mites for the time being. Interestingly my 2 hives that are around 2 feet apart showed a remarked variation in mite drop after treating with the “ local black- ish” bees dropping 300 plus 24 hour post first vap and the purchased buckfast bees dropping 20 in same time scale.

    1. David Post author

      Hi Andy

      I’ve seen exactly the same in hives treated here – 7 in a row in the bee shed treated yesterday afternoon … one had ~150 the rest no more than 20. All of these are local or local(ish) mongrels of the Hienz (57 varieties) type.

      I suspect this reflects different rates of late season brood rearing, with more mites from colonies that reared more brood after the late summer treatment. Alternatively, it could reflect differences in the efficiency of the late summer treatment.

      Or something else entirely that escapes me at the end of another long week 😉


  4. David Post author

    Thanks Greg

    I’d missed that one. Interesting. That might well make it into a topic for a new post sometime in the New Year … once the days are getting longer and the temperature increases a bit 😉


  5. Vince poulin

    This link in Honeybee Suite has been helpful. We went into fall with low mite numbers but haven’t done our broodless window yet. Thanks for the reminder. It will be completed by the weekend (first week of Dec). We haven’t spared OA treatments this year and so far have retained good numbers. No sign of DWV. Three years straight we lost all colonies. Number one issue has been wasps. But frirst year was due to mites and associated viruses. I run OA on all packages despite being told the bees have been treated. Two years ago 375 mites dropped out of our replacement package. This year less than 100, but one can imagine had those phoretic mites remained in the colony. All our spring OA treatments have worked out very well. No queen loses, no apparent impact on population growths. Lots of splitting despite starting from packages. Thanks.

    1. David Post author

      Hi Vince

      It must be devastating losing all your colonies. If wasps are a major problem it almost certainly means your colonies aren’t strong enough. A strong colony with a small entrance will always be able to defend itself against wasps. It’s only weak or unbalanced colonies that are at risk. If you are splitting colonies late in the season perhaps they don’t have a chance to develop enough to defend themselves?

      I also treat all new bees brought into my apiaries – packages or swarms. A third of all the mites in a colony leave with the swarm and, being broodless, it’s an ideal time to treat.

      A package dropping 375 mites either hadn’t been treated at all or had been made up from colonies with a stratospheric mite population. Either way, I’d have been looking for a full refund.

      It’s also worth remembering that there’s no need to over treat colonies. I read a lot on Beesource about US beekeepers using treatment almost all season. If you appreciate the mite replication cycle and treat at the right time with an effective treatment (used properly) there should be no reason to treat more than twice. Late summer and midwinter.

      Of course, if you’re in an area with lots of untreated or poorly managed colonies then you’ll be acquiring mites through robbery and drifting. Time to educate your neighbours and consider coordinated treatment.


  6. Jane

    Thanks very much for your really informative blog. I might be pre-empting the future post you mentioned, but I have a question about multiple treatments. I treated my single hive with Apiguard at the end of August but, with hindsight, that was too late and levels of mite fall were still very high (>20 per day) in October. I therefore did an emergency treatment with sublimated Apibioxal in mid-Nov. My packet of Apibioxal says I should only treat by sublimation once per year and multiple treatments may harm the bees. Although I knew some mites would be protected in brood at the time I treated, and I knew 5-day repeat treatments of generic oxalic acid are commonly applied in summer/autumn, I intended to stick to the packet instructions. However, after 12 days the fall was still high (having peaked at 300), so I treated again using only 1.6g Apibioxal. Another 12 days later and I’m now down to single figures per day, but I think that’s still higher than it should be. My intention is to treat again when I can see the colony is broodless, as indicated by debris on the mite board.
    My question is, if I don’t get a clear broodless period (in Yorkshire), can I do multiple (3?) treatments in quick succession (5 days?) to get mites as they emerge from sealed cells, or am I risking doing more harm than good? Whether I do one or several treatments, should I use the packet dose of 2.3g or try a lower one again given that I’m treating more times than recommended (my 1.6g was based on a paper I read which suggested this could be as effective)?
    Thanks in anticipation,

    1. David Post author

      Hello Jane

      Oxalic acid (which, as far as we’re concerned here, is all that Api-Bioxal is … plus some glucose and powdered silica) can be vaporised repeatedly with no apparent adverse effects. This was demonstrated by Radetzki who treated on a weekly basis for about 3 months. 2.3g is more than you need to use. Studies have showed no difference in efficacy between 1.4g and 2.8g. That was for oxalic acid, but something like 88% of Api-Bioxal is OA, so it’s probably close enough.

      Pete Little empirically determined that treatments at 5 day intervals were most effective. This is what I do if there’s brood present.

      So the answer to your question (“… can I do multiple (3?) treatments in quick succession (5 days?) to get mites as they emerge from sealed cells … ?”) is yes. The instructions on Api-Bioxal are poor for both vaporising and trickling (which it’s much more suited to). I posted instructions on the correct concentrations for trickling a couple of years ago.

      I hope the mite levels drop to something more acceptable 🙂

      1. Jane

        Thanks very much for that very quick and useful reply. I think it’s due to get a bit warmer at the end of the week so, based on the point you made somewhere that sublimation might not be so effective with a tight cluster, I’ll start a sequence of treatments then rather than hoping for a broodless period which might not come.
        I don’t understand how the mite population got so high as the colony was a swarm (from a beekeeper who does treat) in late May and didn’t have brood until July because it turned out to be a virgin queen. Whatever criticisms people have of the Beebase graphs for mite drop and treatment, I’m embarrassed to have ended up with a daily count that is off the scale. A bee inspector visited in August and suggested MAQs as a routine treatment but I didn’t do that because it said the entrance should be unrestricted and at the time I had a wasp problem. I though the Apiguard later would have done the job but I now think it was too cold by the time the second tray went in.
        Thanks again.

        1. David Post author

          Lots of people use Apiguard too late in the year to be really effective. I used to use it when I lived in the Midlands but here in Scotland it’s almost always not dependably warm enough for the month needed for treatment. The other issue I had with Apiguard is that it tends to stop many queens laying, preventing the colony from rearing the all-important winter bees.

          I’ve just done a second treatment on some colonies with higher-than-desirable mite counts … it may not have been necessary and I’ll know in a day or two. However, the beauty of vaporising is that it is so well tolerated by the colony.

          Good luck with the follow-up treatments

  7. Janet Wilson

    Excellent post David, thanks! As is the post you linked to as well. I attended a lecture last winter given by Les Eccles of the Ontario Tech Transfer Program (Canada):

    He demonstrated their latest findings on mite population dynamics in the late summer/fall/winter, where they confirmed that the last rounds of brood emerging, often after the last mite treatment of the late summer, infest the hive with a fresh crop of Varroa just as the bee population goes into steep decline, triggering hive death. He emphasized that this causes a phenomenon I had puzzled over in my apiary: your best and biggest hives, the ones you are SURE will be ok all winter, crash and die in the first cold snap of winter.

    Based on this lecture, and after fellow club members reported extreme mite counts in Sept/Oct from mite infested colonies nearby crashing and being robbed out, I treated in September and mid October to ensure I had low mite levels entering winter. Fingers crossed.

    1. David Post author

      Hi Janet

      Busy week … only just got to this and not yet found the 45 minutes to watch the video. I will with interest and will then post an additional comment.

      I think there’s still a lot we don’t understand about the dynamics of the Varroa lifecycle. For example, I would have expected that the very earliest rounds of new brood rearing in an infested hive would get completely inundated with mites. I don’t think that’s the case and so I expect that mite viability drops markedly over the winter in a broodless colony.

      I also don’t think we understand the detrimental effect of phoretic mite transfer of viruses. We know this happens and we can recapitulate it in the lab, but since the bees are already fully developed they are (apparently) symptomless. Are these bees ‘dead ends’ as far as virus transmission goes, or can they transmit viruses to the developing brood?

      On the subject of very large colonies going into the winter … I much prefer to overwinter on a single brood box. The few that go into the winter on double brood are more susceptible to getting blocked entrances, simply because of the normal attrition rate (and corpse build-up) from a very much larger cluster. Having learnt this lesson the hard way I now check them more frequently.

      I’ve got a post on winter mite counts in preparation … but need to go out in the rain today to count a few more 🙁


  8. Andrew

    Hi David,

    Thanks for this and your other posts, always very interesting.

    I’m a little puzzled by your graph, and the kill rate of a mid August treatment. Looking at the article its from, it seems you’ve modelled a 95% kill of phoretic mites – but surely a treatment in late summer is done to catch all mites? Eg MAQS (penetrates brood cappings), 2 doses of Apiguard, Apivar. You’re after a *total* kill of 90-95%. If you hit that, the number of mites you go into winter with is roughly the same as if you’ve treated in October (less mites to breed in the last rounds of brood) – but of course you’ve protected your winter bees which is the most crucial thing.

    One insight from Randy Oliver’s mite model seems to be that as long as you treat (without good total efficacy) before winter bees are developing, the number of mites is more or less the same once you hit that critical period. Assuming the rate of increase is constant (doubling every fortnight), getting a 90% total kill rate in June (with the surviving mites breeding until mid August) leaves you with the same mite numbers as leaving all mites to breed until mid August and then killing 90%.

    Anyway, I could have this all wrong so would be interested in your thoughts.


    1. David Post author

      Hello Andrew

      The graph was of a 95% kill of all mites in the hive, not just the phoretics. In BEEHAVE I seem to remember I modelled it as a 95% kill over a one month treatment cycle e.g. Apiguard. The idea is that you reduce the mite load before the winter bees are being reared. However, since the kill isn’t total, the small number of remaining mites will replicate in late season brood being reared after the late summer miticide is taken off. That’s why you have to treat in midwinter as well.

      The problem with an early season (June) treatment for me is that it’s more difficult to predict the onset of the summer nectar flows. There’s also a lot more brood in the colony at that time and – at least some years – there will be supers on. All these things cause problems with treatment. For me late summer (as soon as the summer honey comes off) and midwinter (now) treatment works really well, with very low mite levels when it’s really important (winter bees being reared and during early season brood rearing). In colonies we monitor virus levels this also works well.

      I’ve not looked at Randy’s mite model in detail. The Californian climate is a bit different from Eastern Scotland. However, he’s done a lot of studies – including practical ones – so is well placed to comment knowledgeably.

      I’ll be writing more about the kinetics of mite replication and consequences for treatment sometime in 2020.


      1. Andrew

        Thanks for the quick reply David, appreciated.

        Your post from which the graph is taken states:

        In each case the miticide is considered to be 95% effective at killing phoretic mites.

        Which is where I got the idea it was not modelling a total kill. Perhaps I’m not up to speed with the latest but my understanding is that an Apiguard treatment is expected to kill around 90% of total mites over the course of the 4 week treatment, ie if you have 100 mites on day 0 you’ll have 10 on day 28. The blue line on your graph never drops anywhere near your stated 95%, its more like 50% from 450 – 225. Why is that?

        (Not meaning in any way to criticise your treatment suggestions by the way: the main reason I need to treat earlier is because heather is a big part of what I do, and I have taken your advice re: protecting winter bees to heart! If I don’t treat before they go up to the moor it can be too late when they come back.)


        1. David Post author

          Hi Andrew

          The miticide only kills phoretic mites, but they keep on appearing as brood ecloses. However, I need to go back and look at the parameters used to program BEEHAVE to generate that graph. I think it was a 95% kill over the duration of treatment. It will mean burrowing through some old and dusty Excel spreadsheets. Some of the phoretic mites that escape treatment will reinvest cells and successfully reproduce. In August there should be ample brood available for them to do this, including drone brood which is both more attractive to the mites and significantly more productive.

          I don’t take mine to the heather and I’m aware that’s an issue with the general advice I produce. If I did – and I may in the future – I think I’d engineer a brood break during June and treat then, using a single hit treatment like OA which can be used to exploit very short windows of broodlessness. This is not dissimilar to the strategies you can use to treat midseason during splitting and/or swarming.


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