Midseason mite management

The Varroa mite and the potpourri of viruses it transmits are probably the greatest threat to our bees. The number of mites in the colony increases during the spring and summer, feeding and breeding on sealed brood.

Pupa (blue) and mite (red) numbers

In early/mid autumn mite levels reach their peak as the laying rate of the queen decreases. Consequently the number of mites per pupa increases significantly. The bees that are reared at this time of year are the overwintering workers, physiologically-adapted to get the colony through the winter.

The protection of these developing overwintering bees is critical and explains why an early autumn application of a suitable miticide is recommended … or usually essential.

And, although this might appear illogical, if you treat early enough to protect the winter bees you should also treat during a broodless period in midwinter. This is necessary because mite replication goes on into the autumn (while the colony continues to rear brood). If you omit the winter treatment the colony starts with a higher mite load the following season.

And you know what mites mean

Mites in midseason

Under certain circumstances mite levels can increase to dangerous levels 1 much earlier in the season than shown in the graph above.

What circumstances?

I can think of two major reasons 2. Firstly, if the colony starts the season with higher than desirable mite levels (this is why you treat midwinter). Secondly, if the mites are acquired by the colony from other colonies i.e. by infested bees drifting between colonies or by your bees robbing a mite infested colony.

Don’t underestimate the impact these events can have on mite levels. A strong colony robbing out a weak, heavily infested, collapsing colony can acquire dozens of mites a day.

The robbed colony may not be in your apiary. It could be a mile away across the fields in an apiary owned by a treatment-free 3 aficionado or from a pathogen-rich feral colony in the church tower.

How do you identify midseason mite problems?

You need to monitor mite levels, actively and/or passively. The latter includes periodic counts of mites that fall through an open mesh floor onto a Varroa board. The National Bee Unit has a handy – though not necessarily accurate – calculator to determine the total mite levels in the colony based on the Varroa drop.

Out, damn'd mite ...

Out, damn’d mite …

Don’t rely on the NBU calculator. A host of factors are likely to influence the natural Varroa drop. For example, if the laying rate of the queen is decreasing because there’s no nectar coming in there will be fewer larvae at the right stage to parasitise … consequently the natural drop (which originates from phoretic mites) will increase.

And vice versa.

Active monitoring includes uncapping drone brood or doing a sugar roll or alcohol wash to dislodge phoretic mites.

Overt disease

But in addition to looking for mites you should also keep a close eye on workers during routine inspections. If you see bees showing obvious signs of deformed wing virus (DWV) symptoms then you need to intervene to reduce mite levels.

High levels of DWV

High levels of DWV …

During our studies of DWV we have placed mite-free 4 colonies into a communal apiary. Infested drone cells were identified during routine uncapping within 2 weeks of our colony being introduced. Even more striking, symptomatic workers could be seen in the colony within 11 weeks.

Treatment options

Midseason mite management is more problematic than the late summer/early autumn and midwinter treatments.

Firstly, the colony will (or should) have good levels of sealed brood.

Secondly, there might be a nectar flow on and the colony is hopefully laden with supers.

The combination of these two factors is the issue.

If there is brood in the colony the majority (up to 90%) of mites will be hiding under the protective cappings feasting on sealed pupae.

Of course, exactly the same situation prevails in late summer/early autumn. This is why the majority of approved treatments – Apistan (don’t), Apivar, Apiguard etc. – need to be used for at least 4-6 weeks. This covers multiple brood cycles, so ensuring that the capped Varroa are released and (hopefully) slaughtered.

Which brings us to the second problem. All of those named treatments should not be used when there is a flow on or when there are supers on the hive. This is to avoid tainting (contaminating) the honey.

And, if you think about it, there’s unlikely to be a 4-6 week window between early May and late August during which there is not a nectar flow.


The only high-efficacy miticide approved for use when supers are present is MAQS 5.

The active ingredient in MAQS is formic acid which is the only miticide capable of penetrating the cappings to kill Varroa in sealed brood 6. Because MAQS penetrates the cappings the treatment window is only 7 days long.

I have not used MAQS and so cannot comment on its use. The reason I’ve not used it is because of the problems many beekeepers have reported with queen losses or increased bee mortality. The Veterinary Medicines Directorate MAQS Summary of the product characteristics provides advice on how to avoid these problems.

Kill and cure isn’t the option I choose 😉 7

Of course, many beekeepers have used MAQS without problems.

So, what other strategies are available?

Oxalic acid Api-Bioxal

Many beekeepers these days – if you read the online forums – would recommend oxalic acid 8.

I’ve already discussed the oxalic acid-containing treatments extensively.

Importantly, these treatments only target phoretic mites, not those within capped cells.

Trickled oxalic acid is toxic to unsealed brood and so is a poor choice for a brood-rearing colony.

Varroa counts

In contrast, sublimated (vaporised) oxalic acid is tolerated well by the colony and does not harm open brood. Thomas Radetzki demonstrated it continued to be effective for about a week after administration, presumably due to its deposition on all internal surfaces of the hive. My daily mite counts of treated colonies support this conclusion.

Consequently beekeepers have empirically developed methods to treat brooding colonies multiple times with vaporised oxalic acid Api-Bioxal to kill mites released from capped cells.

The first method I’m aware of published for this was by Hivemaker on the Beekeeping Forum. There may well be earlier reports. Hivemaker recommended three or four doses at five day intervals if there is brood present.

This works well 9 but is it compatible with supers on the hive and a honey flow?

What do you mean by compatible?

The VMD Api-Bioxal Summary of product characteristics 10 specifically states “Don’t treat hives with super in position or during honey flow”.

That is about as definitive as possible.

Another one for the extractor ...

Another one for the extractor …

Some vapoholics (correctly) would argue that honey naturally contains oxalic acid. Untreated honey contains variable amounts of oxalic acid; 8-119 mg/kg in one study 11 or up to 400 mg/kg in a large sample of Italian honeys according to Franco Mutinelli 12.

It should be noted that these levels are significantly less than many vegetables.

In addition, Thomas Radetzki demonstrated that oxalic acid levels in spring honey from OA vaporised colonies (the previous autumn) were not different from those in untreated colonies. 

Therefore surely it’s OK to treat when the supers are present?

Absence of evidence is not evidence of absence

There are a few additional studies that have shown no marked rise in OA concentrations in honey post treatment. One of the problems with these studies is that the delay between treatment and honey testing is not clear and is often not stated 13.

Consider what the minimum potential delay between treatment and honey harvesting would be if it were allowed or recommended.

One day 14.

No one has (yet) tested OA concentrations in honey immediately following treatment, or the (presumable) decline in OA levels in the days, weeks and months after treatment. Is it linear over time? Does it flatline and then drop precipitously or does it drop precipitously and then remain at a very low (background) level?

Oxalic acid levels over time post treatment … it’s anyones guess

How does temperature influence this? What about colony strength and activity?

Frankly, without this information we’re just guessing.

Why risk it?

I try and produce the very best quality honey possible for friends, family and customers.

The last thing I would want to risk is inadvertently producing OA-contaminated honey.

Do I know what this tastes like? 15

No, and I’d prefer not to find out.

Formic acid and thymol have been shown to taint honey and my contention is that thorough studies to properly test this have yet to be conducted for oxalic acid.

Until they are – and unless they are statistically compelling – I will not treat colonies with supers present … and I think those that recommend you do are unwise.

What are the options?

Other than MAQS there are no treatments suitable for use when the honey supers are on. If there’s a good nectar flow and a mite-infested colony you have to make a judgement call.

Will the colony be seriously damaged if you delay treatment further?

Quite possibly.

Which is more valuable 16, the honey or the bees?

One option is to treat, hopefully save the colony and feed the honey back to the bees for winter (nothing wrong with this approach … make sure you label the supers clearly!).

Another approach might be to clear then remove the supers to another colony, then treat the original one.

However, if you choose to delay treatment consider the other colonies in your own or neighbouring apiaries. They are at risk as well.

Finally, prevention is better than cure. Timely application of an effective treatment in late summer and midwinter should be sufficient, particularly if all colonies in a geographic area are coordinately treated to minimise the impact of robbing and drifting.

I’ve got two more articles planned on midseason mite management for when the colony is broodless, or can be engineered to be broodless 17.



  1. Which, for convenience, we’ll define as the 1000 mite limit/colony suggested by the National Bee Unit.
  2. In addiition, mite numbers will be influenced by the laying rate of the queen, the available pupae and the proportion of drone brood in the colony.
  3. leave and let die beekeeper.
  4. Formally we cannot prove these were mite-free but they originated in an apiary with very tightly managed mite levels in which an early autumn drop – during treatment – of 20-30 mites in total was considered high.
  5. Mite Away Quick Strips
  6. Fries I. (1991). Treatment of sealed honey bee brood with formic acid for control of Varroa jacobsoni. American Bee Journal, 131, 313–314.
  7. I’ve also not used MAQS because I rarely see worryingly high midseason mite levels.
  8. And a few would suggest rhubarb leaves.
  9. I’ve used this many times as my sole Varroa control method in late summer.
  10. Of course, oxalic acid comes with no instructions and is not an approved treatment so there’s no documentation to consult … but the active ingredient is exactly the same so you can assume that the same applies to oxalic acid.
  11. Bogdanov et al., (2002) Determination of residues in honey after treatments with formic and oxalic acid under field conditions. Apidologie 33, 399–409.
  12. F. Mutinelli et al., (1997) L’acido ossalico nella lotta alla varroasi, L’ape 4/1997, Istituto Zooprofilattico, Legnaro, Italy … I’ll admit to not being able to read this in the original Italian.
  13. The Enzo et al., 2004 study sometimes quoted could be interpreted as having treatment and testing at least 6-8 weeks apart. However, it is not specifically stated and the authors are rather vague in the paper …
  14. You have to exclude common sense when drawing up rules … if it were allowed/recommended it is inevitable that some would treat very close to the time the supers were cleared for extraction (3 to 4 treatments at 5 day intervals takes 15-20 days).
  15. And are my taste buds – dulled by years of strong curry, red wine and fine cigars – sufficiently sensitive to detect any off flavours anyway?
  16. This is perhaps an ethical and financial decision. Just because they are insects does not mean they should not be managed in the best way we possibly can.
  17. But they won’t necessarily appear in the next two weeks as I need time to prepare some more high quality graphs like the one above …

25 thoughts on “Midseason mite management

  1. Jonny Lavin

    Dear Apiarist. Thank you for another informative article. I’ve been struggling with this since I started last year and treated with apivar in late August and then vaped just before xmas. All good coming out of the winter. The thing is I’m advised to vary treatment and like you mentioned MAQS seems ruthless. Therefore I studied and employed essential oils. Using lemongrass for early season feeding and intend using thyme oil later for autumn feed. My question is do you have any experience of using the oils? I should add that I’m ultra careful with the amounts and they seem to wolf it down with no detrimental effects.

    1. David Post author

      Hi Jonny
      I’ve not tried any of the essential oils.
      The advice to alternate treatments is to avoid resistance developing. Apivar and OA both have multiple targets and so the chance of resistance is reduced anyway. Apivar resistance is known, but rather poorly documented and not under stood at the molecular level. Resistance to OA has not been described.
      If used according to the instructions you should have no problems.

        1. David Post author

          Hello Peter
          If you mean HopGuard then Vita appear to have withdrawn the application to the VMD that sought approval for its use. You can read the letter from Vita here. There appear to have been problems with toxicity and the risk/benefit analysis. If you read the European Medicines Agency report it appears that there were some reasonably significant shortcomings in the data submitted for assessment or the way the data had been obtained.
          You might mean a different product, but there’s currently nothing designated HopGuard approved for use.

  2. Daire wWinston

    Another excellent and timely blog, thank you. I’ve been mulling over this problem recently as I’ve one colony with DWV and a super on. Taking the supr(s) off and vapourising has to be the way forward, at least with this colony.

    Thank you.

    1. David Post author

      Hi Daire
      Do you just remove the supers, treat and return the supers? I’d always give a day or two for the OA levels to reduce … alternatively, put the super onto another hive perhaps?

  3. Cris Reeves

    Feeding thymolated/thymolised syrup in the Autumn feed on top of an Apiguard treatment seems to help with mite (as well as Nosema) over the Winter months for us. That, and regularly applying swarm prevention splits during early May to break the brood cycle go a long way towards helping avoid a heavy mite load.

    1. David Post author

      Hi Chris
      Splits are certainly effective and can be combined with OA vaporisation if needed. I’ve not used thymolised syrup – I just feed fondant and very rarely see Nosema – but I know others who swear by it. I’ve almost abandoned Apiguard and didn’t use it at all last season. It’s generally not warm enough here in Scotland and I’ve never liked the way it puts many/most queens off laying, just when you need the winter bees to be reared. It was more useful when I lived in the Midlands and the seasons were longer.

  4. Andrew Darcy-Evans

    Thanks for another informative article and a pertinent one for me as I have noticed some DWV in one of my colonies. I know that one shouldn’t use Apivar with supers on but on the Apivar documentation has the note “Withdrawal Period: Honey: Zero days” How do I interpret that?

    1. David Post author

      Hi Andrew
      The term ‘withdrawal period’ is used on veterinary medicines to indicate the time that must elapse between ending treatment and when the food production activity (in our case honey production) starts again. This means you can remove the Apivar strips and add supers to the colony on the same day … but you cannot overlap them.

  5. Neil

    Hi David,
    Another informative post 👍
    I’ve carried out a shook swarm, destroyed the brood and then vaped the colony before brood is sealed. Only one treatment required this way. The supers can be given to another colony as you say.

    1. David Post author

      Hi Neil
      This is very effective and we’re looking at the influence on the virus population … does it stay high or does it fall as the mites are removed?

      1. Neil

        A good question! I’d imagine that it would stay high in the existing bee population but fall in subsequent brood cycles as the effect of virus vectoring has been removed. Until, of course the mite population builds again. Let us know what you find

        1. David Post author

          Will do … but we need the weather to improve first. I spent the afternoon removing supers in the rain.
          Not a lot of fun.

  6. calum

    good old total closed brood removal as a treatment? The Nucs you build can be treated once the brood emerges… Some beekeepers practice total brood removal as a yearly treatment for all their colonies (Demeter beekeepers do it very often I understand).
    If the varroa infestation is that bad that a treatment is unavoidable, you can also just plonk the supers on top of a colony(ies) without the varroa issues…

    1. David Post author

      Hi (again!)
      I’ve done quite a bit on this from a research point of view and will be able to write about it shortly. One of the problems here in Scotland is that it’s sometimes seen as overly harsh if the weather is poor. We’ve had a lot of success in using it (from a research point of view … which is a bit different from practical beekeeping).

      I’m assuming you mean the cleared supers? That’s what I do if needed.

  7. vince poulin

    David – last response here is July, 2019 – hope the thread is still active. I checked 2-hives today for varroa using the uncapping method. I saw thought this post your reference to the National Bee Base varroa calculator which i found especially interesting given the method bases its outcomes on # of uncapped cells sampled to number of infested cells. I did estimates in 3-hives. I believe I may have inflated the number of mites because I counted # of mites not # of infested cells. Some cells had up to 4 mites and many having 2-3. Results: Hive 1 – #mites (181) #cells uncapped (584) = 31% infestation with 400 mites estimated in the colony. Recommendation >25% infestation – treat as soon as possible. With this hive I sampled 3-different frames containing drone cells. Looking at each frame separately the infestation rate varies 62%, 10%, 25%. Using fame 3 the recommendation is identical to when all 3-frames are combined. That being treat now or in 1 month. I will not wait – I’ll be treating ASAP using OA and likely doing several treatments. Hive 2 I only uncapped 1 Frame. The results a little better #mites 24, #cells uncapped 235 for a 9% infestation. Recommendation is to treat in 1-month. Both of these hives are 2019 wintered hives. I just placed several varroa boards in the hives with one in Hive 1. Results there showed very few mites – <1 mite/day for HIve 1 (no treatment). I've long learned varroa boards are useful but their best use comes in monitoring OA treatment intervals and not as much of a general indicator. Drone uncapping is a far more quantitative method and to me a very useful method for estimating mite densities. Hive 1 results are also useful in that they do help to show the kind of variability we can get in any method – 62%, 10%, 25%. Had say I only uncapped frame 2 – and got a 10% result I'd not be doing an OA treatment this early. A week ago I did examine one other hive by uncapping. Result – 4 mites in 317 uncapped cells for a 2% infestation and only 35 in the colony. That sample came from a single frame. Today's results suggest I needed to have examined more frames. I'm not not confident a single frame is best given the variability I saw in today's Hive 1. Uncapping is a super slow method. Not everyone would have the time but I think a good technique. I also see from the results that had I relied on an alcohol wash where I would scoop 300 bees off a frame – it too could be highly variable. Are both equally random?????? Probably not – I did find places where mite numbers were higher than in other parts of a frame. Is there any older posts where you discuss this method in greater detail?

    1. David Post author

      Hi Vince

      You’ve identified the problems inherent with mite monitoring. If you just measure phoretic mites (alcohol wash or mite drop) it can be variable, dependent upon the laying rate of the queen and the availability of brood of a suitable age for the mites to parasitise. I don’t think there’s a really thorough study of phoretic mite numbers over the season and late-stage larval numbers … perhaps Randy Oliver has done one? He’s certainly done some studies of alcohol wash efficacy.

      Your infestation levels are high. There are probably more effective (and certainly less trouble) ways of treating than multiple OA treatments. Remember that most treatments are incompatible with honey supers … with the exception of formic acid/MAQS.


      1. vince poulin

        David – one additional question – here 10-pads of Formic Proc (5-hives) run $80.00, but 1L of 65% liquid Formic Acid $18 (33-hives). A quick look suggests the latter can be applied by saturating two strips of paper towel and placing those towels in the hive in the same manner as FPro. The towels would be placed in small zip-locks with vents to allow the fumigation. Would use of the liquid acid be as effective as the commercial strips? Same number of treatment days?

        1. David Post author

          Hi Vince

          I’m not the person to ask. I know of commercials who use DIY solutions like this, or have in the past. I’d be very wary about dosage. Actually … I wouldn’t do a DIY treatment. It’s not a treatment I’m familiar with in its approved form, so am singularly ill-equipped to know how to DIY.

          As an aside, the prices are daft. MAQS here costs £55 for 10 hives (20 strips). 3 litres of 85% (not 65%) formic acid shipped is about £42 for comparison.


  8. vince poulin

    David – I was considering either removing the honey supers OR isolating them using a solid piece of plywood while OA was administered but necessitates getting the bees out of the supers and down into the lower boxes. Lots of work as you said given the need for multiple treatments. I haven’t used Formic Acid before but understand it can result in some queen mortality but I can re-queen if necessary. In all previous years wasps have been a serious issue but this season not a serious problem so can leave the hives open and well ventilated. Just now watched Formic Pro’s application video and see it allows honey supers to remain on the hive. A much less arduous treatment option – greatly appreciate the recommendation. I’ll go with it. Completed drone uncapping on an additional strong (and healthy) hive – same result – #mites 80, #uncapped 235 for a 35% infestation. Treatment recommendation >25% – treat ASAP. Left unchecked all of the hives sampled would have had serious mite issues. It is especially important to nip this in bud, winter bees are just around the corner. All the hives contain lots of new brood – eggs, larvae and capped. May be a bit early for winter bees, timing is good. Thanks greatly.

    1. David Post author

      Hi Vince … the MAQS/formic acid was a factual comment rather than an out and out recommendation. It’s one of the treatments I’ve never used as I’ve never needed to. I know it has a reputation for being hard on queens, partly because they got the dosage wrong when they released the stuff in the UK. I’d recommend you follow the instructions very carefully, in particular the maximum acceptable temperature for its use. I gave a talk two evenings ago and someone commented that the upper temp for usage is well below 30 centigrade.

      1. vince poulin

        I understand David – I’ll chat this up with some local people as well. These were overwintered hives that exploded in spring. In years past I always had to start with a new package which enabled me to treat the bees using OA before the queens got going. Those spring treatments worked extremely well with no adverse affects that I could see. Only next treatments required were in fall and winter but no significant infestations. I did not do the same this year as all the wintered hives had abundant eggs, larvae and brood. Also never any significant drop on screened varroa boards – under 10 all season. The uncapping numbers have come as a bit of a surprise. Vancouver is never “hot” but we have had a few warmish days.

        1. David Post author

          Hi Vince

          Unless you’re picking up lots of mites through drifting or robbing during the season then treatment in late summer and midwinter should be sufficient. The first treatment protects the winter bees (so the colony gets through the winter), the midwinter one ensures you start the year with low mite numbers, so they don’t get out of hand before the late summer treatment is needed.

          Of course, it doesn’t always work like that … however, if you get the timing right and the treatments are effective, this schedule works well. We monitor virus levels in our research hives (that are treated exactly as described here) and, just last week, they were at a very low level even relatively late in the season. Virus levels are essentially the gold standard … all of those mite treatments are solely aimed at reducing the virus load in the hive.



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