Kick ’em when they’re down

Out, damn'd mite ...

Out, damn’d mite …

Why bother treating colonies in midwinter to reduce Varroa infestation? After all, you probably treated them with Apiguard or Apivar (or possibly even Apistan) in late summer or early autumn.

Is there any need to treat again in midwinter?

Yes. To cut a long story short, there are basically two reasons why a midwinter mite treatment almost always makes sense:

  1. Mites will be present. In addition, they’ll be present at a level higher than the minimum level achievable, particularly if you last treated your colonies in late summer, rather than early autumn.
  2. The majority of mites will be phoretic, rather than hiding away in sealed brood. They’re therefore easy to target.

I’ll deal with these in reverse order …

Know your enemy

DWV symptoms

DWV symptoms

The ectoparasite Varroa feeds on honey bee pupae and, while doing so, transmits viruses (in particular DWV) that can completely mess up the development of the adult bee. Varroa cannot replicate anywhere other than on developing pupae. It’s replication cycle, and the resulting mite levels in the colony, are therefore tightly linked to the numbers and availability of hosts … honey bee pupae.

If developing brood is available the mite can replicate. Under these conditions, newly emerged adult, mated, female Varroa spend a few days as phoretic mites, riding around the colony on young bees. They then select a cell with a late-stage larvae in, enter the cell and wait until pupation occurs. If developing worker brood is available each infested cell produces 1 – 2 new mites (drone cells produce 3+) and mite numbers increase very rapidly in the colony.

In contrast, if there’s no developing brood available, the mites have to hang around waiting for brood to become available. They do this as phoretic mites and can remain like this for weeks or months if necessary.

Therefore, when brood is in abundance and the queen in laying freely mites can replicate to very high levels. In contrast, when brood is limiting and the queen has reduced her egg laying to a   v  e  r  y     s  l  o  w     r  a  t  e     the mite cannot replicate and must be predominantly phoretic.

When does this happen?

Lay Lady Lay … or don’t

Ambient temperature, day length and the availability of nectar and pollen likely influence whether the queen lays eggs. When it’s cold, dark and there’s little or no pollen or nectar coming into the hive the queen slows down, or even stops, laying eggs.

About 8 days after she stops laying there will be no more unsealed brood in the colony. About 13 days after that all the sealed brood will have emerged (along with any Varroa). Therefore, after an extended cold period in midwinter, the colony will have the lowest level of sealed brood … and the highest proportion of the mite population will be phoretic.

Under normal (midsummer) circumstances about 10% of the mite population is phoretic. It’s probably unnecessary to state that, if there’s no brood available, 100% of the mites must be phoretic.

All licensed miticides work extremely well against phoretic mites.

Caveats, guesstimates, global warming and the Gulf Stream

Global warming

Global warming …

Whatever the cause, the globe is warming (irrespective of what Donald Trump tweets). Long, hard winters are getting less common (or perhaps even rarer, as they were never particularly common in the UK). In Central, Southern or Eastern Britain it’s possible that the colony will have some brood present all year. In parts of the West, warmed by the Gulf Stream, I’d be surprised if a colony was ever broodless. Only in the North is it likely that there will be a brood break in midwinter.

Most of the paragraph above is semi-informed guesswork. I don’t think anyone has systematically analysed colonies in the winter for the presence of sealed brood. Sure, many (including me) have opened colonies for a quick peek. Others will have peered intently at the Varroa board to search for shredded wax cappings that indicate emerging brood. The presence of brood will vary according to environmental conditions and the genetics of the bees, so it’s not possible to be dogmatic about these things.

However, it’s safe to say that in midwinter, sealed brood – within which the mites can escape decimation by miticides – is at a minimal level.

Reducing mite levels and minimal mite levels

Within reason, the earlier you apply late summer miticides, the better you protect the all-important overwintering bees from the ravages of viruses, particularly deformed wing virus. This is explained in excruciating detail in a previous post, so I won’t repeat the text here.

However, I will re-present the graph that illustrates the modelled (using BEEHAVE) mite levels.

Time of treatment and mite numbers

Time of treatment and mite numbers

The gold arrow (days 240-300 i.e. September and October) indicates when the winter bees are being reared. These are the bees that need to be protected from mites (and their viruses). Mite numbers (starting with just 20 in the hive on day zero) are indicated by the solid coloured lines. The blue, black, red, cyan and green lines indicate modelled mite numbers when the colony is treated with a miticide (95% effective) in mid-July, August, September, October or November respectively.

The earlier you treat, the lower the mite levels are when the winter bees are being reared. Study the blue and black lines.

This is a good thing.

In contrast, by treating very late (the cyan and green lines) the highest mite numbers of the season occur at the same time as the winter bees are being reared. A bad thing.

But … look also at mite numbers after treatment

Look carefully at the mite numbers predicted to remain at the end of the year. Early treatment leaves higher mite levels at the start of the following year.

This is simply because mites escaping the treatment at the end of summer have had an opportunity to reproduce during the late autumn.

This is why the additional midwinter treatment is beneficial … it kills residual mites and gives the colony the best start to the new calendar year§.

Kick ’em when they’re down

Early treatment protects winter bees but risks exposing bees the following season to unnecessarily high mite numbers. However, in midwinter, these residual mites are much more likely to be phoretic due to a lack of brood in the colony. As I stated earlier, phoretic mites are relatively easy to target with miticides.

So, give the mites a hammering in late summer with an appropriate and effective miticide and then give those that remain another dose of the medicine in midwinter.

But not another dose of the same medicine

Since the majority of mites in a colony with little or no brood will be phoretic, you can easily reduce their numbers using a single treatment containing oxalic acid. This can be administered by sublimation (vaporisation) or by trickling (dribbling).

There’s no need to use any treatment that needs to applied for a month. Indeed, many (Apiguard etc.) are not recommended for use in winter because they work far less well on a largely inactive colony.

Trickle 2 - £1

Trickle 2 – £1

I’ve discussed sublimation previously. However, since this requires relatively expensive (£30 – £300) specialised delivery and personal protection equipment it may be inappropriate for the two hive owner. In contrast, trickling requires almost no expensive or special equipment and – reassuringly – has been successfully practised by UK beekeepers for many years. I did it for years before I bought my Sublimox vaporiser.

Therefore, in two further articles this autumn (well before you’ll need to treat your own colonies) I’ll describe the preparation and storage of oxalic acid solutions and its use.

Be prepared

If you want to be prepared you’ll need to beg, borrow or steal the following – sufficient oxalic acid (or Api-Bioxal), a Trickle 2 bottle sold by Thorne’s, a cheap vacuum flask (Tesco £2.50), granulated sugar and a pair of thin disposable gloves.

Do this soon. Don’t leave it until midwinter. You need to be ready to treat as soon as there’s a protracted cold spell (when brood will be at a minimum). Over the last few years my records show that this has been anywhere between the third week in November and the third week in January.

More soon …

† Only MAQS is effective against mites sealed in cells. This is why most miticides are used for extended periods in the late summer or early autumn … the miticide must be present as Varroa emerge from sealed cells.

‡ I’ll repeat the caveat that this is an in silico simulation of what happens in a beehive. Undoubtedly it’s not perfect, but it serves to illustrate the point well. It’s freely available, runs on PC and Mac computers, and is reasonably well-documented. In the simulations shown here the virtual colony was ‘primed’ with 20 mites at the beginning of the year. BEEHAVE was run using all the default settings – climate, forage etc. – with the additional application of a miticide (95% effective) in the middle of the months indicated. Full details of the modelling have already been posted.

§ The National Bee Unit recommend Varroa levels are maintained below 1000 throughout the season. Without treatment, 20 mites at the start of the season can easily replicate to ~750 in the autumn. If you start the season with 200 mites then levels are predicted to reach ~5000 in the following summer. The colony will almost certainly die that season or the next. There’s a more detailed account of the consequence of winter brood rearing and the level of mite infestation written by Eric McArthur and reproduced on the Montgomeryshire BKA website that’s worth reading.

¶ The cumulative (year upon year) effect of late summer treatment with no midwinter treatment has been discussed previously. I’ll simply re-post the relevant figure here – 5 years of bee (in blue, left axis) and mite (in red, right axis) numbers with only one treatment per season applied in late September. Within two years the higher mite numbers that are present at the start of the year reproduce to dangerously high levels.

Mid September

Mid September

14 thoughts on “Kick ’em when they’re down

  1. Liz

    Why not the same medicine if the medicine in question is sublimated oxalic (applied in warm weather 3 times with 5 day spacing as per ‘hivemaker’ advice – and just once in winter?

    1. David Post author

      Fair point Liz … 😉
      I think it’s generally good practice to not use the same treatment repeatedly. Resistance to oxalic acid is not known to exist (though there are a number of people still reporting big mite drops after 3, 4 or 5 vaporisations this autumn on the forums) and it’s difficult to see how it could develop. However, if it could/did then treating with the same thing repeatedly is not ideal.

      Midwinter sublimation has a lot to offer, though I prefer to do it when it’s warmer and the cluster is dispersed. Getting these conditions and broodlessness (is that a word?) might be tricky. Not having to open the hive is a big bonus. The speed benefits – as I shall shown in a couple of weeks are specious.

      This and the next two posts are really about trickling/dribbling. ‘Vapers’ (a method I’m happy to use) are likely to carry on vaping. However, if you don’t currently treat in the winter and don’t own the gear to treat like this, then trickling is cheap and very effective.

      You can even do it with Api-Bioxal … though you probably shouldn’t follow the recipe that’s provided.

      Which I discuss next week.


  2. H. Bond

    A while back you mentioned you’d be writing about CBPV. I’d be interested in hearing your thoughts about it. Might be more widespread than people think.

    1. David Post author

      It is … if not ‘widespread’, then certainly a much bigger problem to some beekeepers than DWV.
      In my day job we’re shortly starting a big research project with collaborators in the National Bee Unit on the biology of CBPV. It’s a very interesting virus and we’ve been working on it for a year or so already.
      However, it’s a midsummer, peak-of-the-season virus … not something that is an issue at all as we slide gracefully into winter. The priority now is to be prepared to tackle Varroa in a month or two.

      If you’re registered to receive updates to the site you’ll receive an email when the CBPV posts are published.


  3. Larry

    Thanks for an excellent article!

    Kick ’em when they’re up
    Kick ’em when they’re down
    Kick ’em when they’re up
    Kick ’em all around

    1. David Post author

      Many thanks Larry … and for the reference to Don Henley’s Dirty Laundry (you must have read an earlier post Peaceful Easy Feeling). While checking the lyrics, which is a criticism of journalism and mass media, I came across a reference to narcotizing dysfunction. This term was new to me and refers to the saturating press coverage of events, which people are well informed by … but that they then get confused between knowing about an issue and doing something about. For example, we have near-saturation political coverage in the press, but political participation (voting, for example) is dropping.

      So, thanks also for introducing me indirectly to narcotizing dysfunction … which, in due course, will make a great title for a future post 😉


      1. Larry

        You’re miles ahead of me with the quote. While an Eagles and Henley fan I hardly know the origins of their lyrics. It was the current post with the lines, “Kick ’em when they’re down” that caused me to think of the Don Henley song. Isn’t it appropriate that we kick those nasty Varroa when they’re up and also kick them when they’re down for optimal effect. Just seemed to work for me.

        As for narcotizing dysfunction I’ll have to work that into beekeeping as well. Actually I can already: As beekeeping educators we deliver the message over and over on mites. It’s in our monthly newsletters, our subscription magazines, our new beekeeper classes, and group meetings. Yet, ironically, for all this saturation of information, the greatest failure with new beekeepers (at least here in the US) is the inability to translate the information into action. There! A perfect fit for narcotizing dysfunction.

        Thanks again for the post and the reply. Your articles are always excellent and thought stimulating. Now, off to read Peaceful Easy Feeling.

        1. David Post author

          Thanks Larry
          I agree entirely about the narcotizing dysfunction … unfortunately there’s a subset of beekeepers who don’t even read the newsletters, attend the talks or listen to the advice. Most of the time when I give evening talks I think I’m preaching to the choir … it’s the association members who didn’t attend the talk, or the lone beekeeper (who never joined an association in the first place) with a completely ignored couple of hives on his/her property that have the really bad mite problems.

          That they generously share with everyone within range 8-(



    2. MerryBee


      ” I don’t think anyone has systematically analysed colonies in the winter for the presence of sealed brood. ”

      LASI has done some work on this. When I went to a seminar there last year they told us of their research, over 3 winters, showing that in mid December 10-50% of colonies had sealed brood, whereas by mid January nearly 100% had sealed brood. They therefore recommend treating in mid December. and also you will know they recommend opening the colony quickly the day before treatment and forking out any sealed brood present. Controversial.
      This is what I did last year , in the second week of December, and indeed only 1 colony in 6 had any sealed brood. I am also in Sussex, and presumably further north there would be less. So maybe there are longer broodless periods than you suggest.

      1. David Post author

        I’m aware of the stuff from LASI. It’s not quite what I’d call systematic, but it’s certainly a good indication of what was happening in those years in that part of Sussex. How that relates to people living in Anglesey or Norwich of Thurso I’m not sure.

        When I was being taught about OA treatment it was something done between Christmas and New Year. Subsequent experience – and something that’s clearly backed up by the LASI work – is that there’s usually a broodless period earlier, rather than later, in the winter.

        Last season I treated at the end of the third week of November, after nearly 3 weeks of low temperatures. As it turned out, it was probably the longest sustained cold period last winter in Fife. I didn’t check for brood, but suspected from the Varroa tray that there was little or none. Friends in Angus just a few miles North did check the following weekend and their colonies were broodless.

        I understand the logic behind forking out sealed brood the day before treatment. It feels wrong, but makes scientific sense … though needs to be weighed against potential damage caused by disrupting the cluster.


  4. Emily

    Annoying that you can’t buy pre-mixed Apibioxal for trickling as you could in the past with oxalic acid (I am lazy).

    1. David Post author

      Hi Emily

      I agree. Aside from the price hike and the questionable way it was introduced, the absence of a pre-mixed solution is a big disappointment.

      I suspect this has as much to do with producing a single product for Europe when the recommended concentration it’s used at varies in different countries. You’ll be aware that the instructions on the packet make up a much more concentrated mix than is usually used in the UK.

      You might be lazy (!) but other beekeepers may not be able to confidently make up the solution at the right concentration, or store it properly once prepared. They therefore don’t bother, despite the benefits (outlined above) for the colony. This – together with the mess it leaves when used in a vaporiser – is why I think the introduction of Api-Bioxal is, on balance, a retrograde step for bee health.

      Fortunately, Api-Bioxal is easy to prepare at the right concentration and easy to store safely. I’ll be discussing this soon.


  5. Mike

    Great stuff – I’m looking forward to measuring out exactly 308.6g of sugar and 308.6ml of rain water – then throwing away half a £10 packet because it expires before next winter.

    1. David Post author

      Ha! As you’ll see in the next post, you’ll actually need more syrup than that unless you want to treat your bees with a higher concentration of OA than is usually recommended in the UK. There will also be recipes for smaller numbers of hives.

      Use by dates? What is there to go off? Oxalic acid dihydrate, powdered silica and glucose … all pretty stable I would think.


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