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 (PDF) 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 …

42 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?

      1. Daire Winston

        Thanks David, I’ll leave the super off for a couple of days as you suggest.

  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.


          1. vince poulin

            David – I almost bought 10-pads of Formic Pro – but discussing it with our local supplier made the decision to go with straight Formic – DIY approach. The reason being I can administer doses better by applying a known amount of Formic (67%) to a pad and applying a known number of pads to a hive. I’m now 8-days into treatments and things looking very good. I’ve based dosage on manufacturers recommendation of 15-20 ml per single deep Lang. That gave me a 7.5 ml dose per Warre Brood box. I started with a single, 3-brood box hive using 2-pads (a total 15 ml application – low dose). Mite drops were 62, 13, 12, 5, and 4 (first 5-days). At day 5 the bees had removed all of the pads – chewed into trash and thrown out the hive. I am now into day-3 of Treatment 2 for that hive. Mite drops 26, 46, 29 over the 3 days. Both dead and live mites were observed during the first treatment but now all dead. I’ve seen no dead bees inside the hive or outside on landing board or ground. The hive looks to be tolerating the 7.5 ml/pad dosage well. A hive check confirmed eggs still present. Looks like no obvious negative impacts (to date). Interesting that pads were being removed by bees which limits the time straight formic acid can work in the hive. I had a second hive with high drone uncapping counts. I did not treat immediately. I wanted to test T1 first. That hive had high mite board loads prior to treatment as well. I treated the hive similarly 5-days ago but this time using 3-pads of 7.5 ml (one pad per Warre Brood box). Mite counts were 186, 143, 54, 75, and 68 today. Drone uncapping was spot-on – treat ASAP. Bees have not trashed the pads in this hive so I’ve held off applying a second treatment. I’ll monitor and re-treat when numbers come off. As in the first hive I observed live mites within the first few days but all recent counts contained no live mites. I’m finding Formic to be as straight forward a treatment method as Oxalic Acid, actually easier – no wires, batteries or treatment administration delays. I will apply OA in October-November when all winter brood have hatched.

          2. David Post author

            Sounds like you’re on top of things … I’m presuming temperatures aren’t too high if the bees are tolerating the formic acid? I think MAQS has an upper limit of 30C, but someone commented to me recently that they considered the upper limit to be well below that, based upon their experience of the stuff. I’ve not used MAQS/formic and so am little more than an interested observer. Because it’s usable with supers on I always think of it as a mid-season treatment, though I do know some who use it after the summer honey harvest. Fortunately, my mite levels are manageable (and in places low) and I get from the midwinter treatment to late summer without additional intervention.


  9. vince poulin

    So far no adverse effects with respect to temperature as far as I can tell. Proof in the pudding will be once treatments are concluded I’ll search for queens. All the hives have good numbers of capped brood and I have two spare queens should an issue arise. Now, last week it was hot for Vancouver with temperatures going to 28ish and even one day at 30. You’d think I was in Thailand. One has to think dosage may well be in play with respect to temperature as well. With Formic – data sheet says 15 – 20 ml per deep Lang. Perhaps if one went with the higher end dosage risk is increased by temperature. All said, at 7.5 ml per Warre Brood box bees look to be tolerating it. I hear you on the value of a mid-summer treatment and why working out the kinks with Formic Acid is important.
    I’m 4-days after the above post. T2 on Yellow is showing a drop range of 35-41/day. There has been no real slowing down. I started treatment on another hive – “Cedar”. Phoretic mites got hit hard over the first two-days (123, 117) but numbers remain strong at 50 to 37 today. The second hive I treated, “Green” is still dropping mites with its initial treatment (T1) with no slowing down in drops. It started with a hard hit on phoretic mites (186, 143) then dropped to 54, but has gone up 63, 112, 100, 90 (today). One live mite today. This is Day-9 on T1. Formic Pro recommends a second treatment on Day-10. I plan to hit the hive with T2 tomorrow – Day 10.
    I have a strong 2020 colony that I also treated 5-days ago (4-days into treatment). No immediate phoretic mite drop (12, 22) but over the last 2-days some large numbers – 78 and 103 respectively. This did surprise me a bit but mite board counts were high so should be expected. Total kill for this small hive now exceeds the estimated infestation number. I’m tracking total mite removal loads with the infestation estimates that come from your UK calculator. Let’s see how this goes. A Mid-Season treatment in my situation is essential. Had I not proceeded with the treatments all our hives would have been heavily infested by the end of the month and without question by the second week of September and likely doing great harm to winter bees. I do need to see these numbers come off significantly over the next few weeks. Let’s hope Formic does its thing.

    1. David Post author

      Hi Vince

      I’ve not given it a lot of thought but would assume mite drops would reflect the rate at which new brood was emerging. If the queen is laying steadily and with a certain level of mite infestation, you can expect a proportion of cells to be occupied each day. ~12-13 days later, on emergence, those mites will meet the formic and (hopefully) meet their end as well. That means you might expect about 12-13 days of (roughly) similar mite drops, before newly capped cells start to get infested at a lower rate.

      If I had more energy this could probably be modelled with BEEHAVE, based upon known egg laying rates, capping dates and the range of time that mites remain phoretic.

      Something for the winter I think … I have a huge pile of honey supers to attend to 🙂


  10. vince poulin

    Let’s look back on this come winter David. Lots to talk about. Take care and enjoy the year’s efforts! Watch the back those supers are heavy.


  11. vince poulin

    David – wow, a serious turn of events. A bit hard to take actually but worthy of discussion. Summary: I originally was going to treat our hives using OA (vapour). I have used OA for the past 4-years with good success. Never a queen loss nor any indication hive numbers declined (visual inspections). After some thought and the notion that IPM strategies are best (reduce resistance, etc.) I diversified. I went with Formic Acid as the treatment option (first time). I started treatments on August 7 – all went well. 7.5 ml formic dose per single Warre brood box (1/2 recommended rate for Lang). Mite counts variable but sufficiently high to think the treatment protocol was OK and needed continued. All hives went to T3 with one T4. I left for a camping trip on Sept 11 and returned Sept 25. On returning 2-hives were queen-less and one seriously diminished. It subsequently collapsed fully. These were especially strong hives before treatments. When examined I found up to 44% mite infestation of uncapped (dead) worker cells. A few drone cells remained amoung the hives. One drone cell contained 35 mites. A worker cell on that same frame 14 mites. These are astonishing numbers. The implications huge and help to explain those exponential mite population curves. 4-hives have been spared but three have high mite loads. Immediately on return I resorted to OA – my former treatment method and now into OA3 with still high loads (40-500) with totals from the OA treatments about 600 – 1600 depending on the hive. These are being delivered in 5-day intervals. I will continue them through to sub double digit mite counts.

    What happened? 1. Formic as we heard – is in fact – hard on queens and bees. My first time and 3-hive losses. 2. Fomic (liquid acid) looks to me to have been ineffective in arresting mites early in August. That failure allowed mite populations to continue exploding while not being readily apparent from the daily mite drop counts. Mite mortality seemed to level off while remaining consistently high. The occasional first-2 day big drops but then still high until next treatment. 3. I left for 2-weeks and unable to respond to worsening mite numbers. I left thinking Formic was doing its job and getting under those wax cappings. I did some uncapping on 30-August to assess % live to dead mites in cells. The infestation rate at that time was very high despite all the three-four Formic treatments. 21-47%. Of those mites 17-29% were live. Scary – this meant about 30% of winter bees were being subjected to mites and their viruses. No wonder DWV was present. 4. With these failures I spoke to our local supply people. It is looking like 2020 has become a high mite infestation year with many hives being lost to mites. Oddly a contributing factor may well be lack of control on part of many people. That supplier said treatment sales were down in August but they ran out of supplies in September. A friend lost his only two hives in August (2020 packages from mite-free Tazmania). Notably – he did not treat despite me trying to convince him to get moving on treatments. Another lost his queen and hive is in serious decline (another mite-free Tazmanian package). He did sugar dusting. An additional friend, a science teacher reported his class hive also collapsed. He mused about Formic acid resistance.

    We worry about OA honey contamination but frankly – OA has been a valued treatment option for me since year 1. Given this experience I’m certain to go back to it. I could easily have applied OA this year as all supers were off the hives in early August when I began treatments. An incredible year for bees here with lots of honey, strong bee colonies and fine weather. Possibly too fine and why mites took off before most people knew it. Certainly not impressed with Formic – 3 queen losses (out of 7) and clearly ineffective as seen in the % live to dead mite counts from uncapped cells.

    1. David Post author

      Hi Vince

      Interesting. I have zero experience with formic, either DIY or as MAQS (which is the name it’s sold under here). When introduced the dosing was wrong as it was for Langstroth boxes. Our boxes are smaller, so everyone was overdosing and there were significant queen problems.

      I’ve been giving some evening talks to beekeeping associations on rational mite management. In discussions with beekeepers who do sometimes use formic there have been several comments about how poorly tolerated it is when the temperature is high. Did your final treatment – when you were away – coincide with a very warm spell? The instructions on MAQS says don’t use over 30 centigrade (~86F) but several commented that the upper temperature limit should be lower than this.

      I think formic acid resistance is unlikely – it’s supposed to affect the respiration of the mite by inhibiting mitichondrial function. It probably has multiple targets and this makes resistance more difficult to acquire.

      Finally, the mite levels you’re reporting are very high. I’m not sure we’ve ever seen 14 mites in a worker cell even in hives deliberately maintained with high mite levels for research. Nasty! We have had a lot of success with cleaning hives up using a combination of miticides and shook swarms. We’re in the process of preparing this for publication. It’s an intervention for midseason, but it’s fantastically effective.

      I hope they respond to OA. My hives have been ignored most of this year and the mite levels are higher than I’d like – or were, as treatment is nearly finished. I’m going to take care to treat them during a broodless period in the winter to maximise the impact of using OA.


  12. vince poulin

    Frustratingly high David. This after being truly proactive with control – early season OA, and drone uncapping throughout June-July. I don’t think we approached that 30 degree threshold but I do recall one day in particular being very warm and may well have got close. While away – I don’t know. We did have great weather our entire trip. That cell with 14 mites was the highest for a worker cell but I had many with 3-5 mites and some higher (worker cells). 35 in the one drone cell was shocking. I knew drone cells are favoured by mites and support larger numbers but this was a bit much. My guess – over half the drone cells contained mites in excess of 1-2. Mite counting is no small job. Adults readily seen but juveniles another matter especially when extremely small and nearly transparent. I’m still seeing them! One hive is promising – OA3 – Day 1 only 32. However, other 3-hives are on Day 2 with their OA3 yielding 112, 208, and 244. The battle goes on! Suspense full – what day will it be when those numbers crash? May not be until OA4 but eventually I will get them. Question is – has too much damage already been done????? Last winter I followed your advice and completed OA treatments on all the hives that first week of December. Busy in November.

    1. David Post author

      Hi Vince

      One of the interesting things is that mite numbers in similarly treated colonies can vary from season to season. This year looks to be quite a ‘high mite’ year for us here. Last year mite levels were low. The colonies were treated broadly the same. I suspect it reflects differences in weather/forage and the availability of brood of a suitable age for infestation. I do a bit of drone brood removal, but since many frames are foundationless my colonies tend to have quite high drone levels anyway.

      If I was a little better at mathematics all this could be modelled and neatly explained … but I’m not 🙁

      Keep slaughtering those mites

  13. vince poulin

    David – grim here. Hard to watch but now down to a single hive (from 7) with another one a set of brood boxes where I’m combining remnant bees from 3 others. All were 7 very strong, healthy, robust hives first week of August. The last ditch effort to combine the three collapsed hives is in the interest of learning. I see no chance they will make it but until the last bee goes I’ll keep them. Our one remaining hive was strong through to this week but frankly it looks to me it will share the same fate as the others. 58 dead bees on the landing board today with 4 DWV visibly infected bees. There remains a lot of phoretic mites. Completed OA5 last night with 112 hitting the mite board today previous day count was 33. It has been an epidemic of serious proportions. Many area hives being lost. What brought this on ??? Possibly: It was a hugely successful nectar season for bee keepers. Lots of honey, crazy amount of swarms, people reporting productive hives due to good weather and perfect flowering conditions. Mite populations look to not have wasted the opportunity. By early August when all our productive hives began down-sizing – as your graphs show – mite densities in every hive grew exponentially. As I mentioned 35 mature, nice brown mites in one drone cell. 14-adult mites counted in a worker cell. Some untold number of juvenile mites present I would not have seen. Formic did not work for me. Rather the opposite – 2-queens lost and a 3rd queen that did not survive. I administered OA after the FA treatments because mite drops did not abate. It is obvious from the numbers of mites killed all capped brood in the hives were infested and without question the viruses they carry. It will be interesting to see our regional statistics on hives lost this fall. One thing for certain 2021 will have to begin with packages.

    1. David Post author

      Sorry to hear that Vince

      It’s always grim watching a colony perish. These mite explosions at the end of the season are not unusual – the brood levels drop and suddenly the phoretic mite numbers go through the roof. With high levels of phoretic mites the likelihood of new pupae becoming mite infested is high. Brood levels continue to drop as the queen shuts down egg production. Suddenly you’re a mitekeeper rather than a beekeeper.

      How can this be avoided? Monitoring through the season and intervening earlier is likely to be beneficial. Do I remember you used a DIY formic treatment? I suppose one possibility is that it was ineffective because the concentrations or delivery were somehow incorrect. Of course, not so incorrect that the queens survived 🙁 When I’m lecturing on Varroa management I always make the point that the commercial approved treatments are approved because, under the conditions tested and used appropriately, you should achieve a 90%+ kill of mites.

      My mite levels have been higher than usual here. Not catastrophically so, but more than I’m comfortable with. I put it down to my “hands off” beekeeping this season, though it’s notable that some colonies are appreciably better than others within the same apiary. I’ll add the overall mite drop numbers to my notes and refer back when I start queen rearing next season.


  14. vince poulin

    Earlier intervention is a very good point. I had been drone uncapping and removing drone cells throughout summer to assess just when I needed begin those interventions. I did a lot of full stop scrapping out of drone cells from frames. Some removed and froze entirely but must say did not find that process fun. I would freeze then replace in the hives but upon opening two nice hives and getting hit with the smelt of rotting bees I stopped. I then resorted to using a hive tool and scrape out cells down to foundation. On less dense frames I tediously removed individual larva-pupa with twizzers. I then found pressure washing the areas worked with a hose would flush them well no bits left to rot. In my early work I did see mites but numbers small – the odd mite. I did not think enough to raise concern. During these days mites rarely dropped to the varroa boards. Reading one of your posts I learned about the UK mite calculator and saw where I could use mite#s/cells uncapped to assess mite populations. When I ran those numbers it was already first week of August. At that time some came back “treat in 1-month” most others were “ASAP”! Those results surprised me given the drone work I had been doing and visually seeing few mites. However, data does not lie. I wasted no time after that but of course went with Formic – and as you say the DIY method may well have been the problem but it is a method most commercial guys here use – easy, fast and cost effective (so it seemed). Formic Pro is quite expensive and comes with the warning that it too can be hard on queens. I have a friend who applied Pro and lost the queen. As you well said – early intervention is essential. Had I had those drone-uncapping recommendations earlier I would have treated sooner. I would have removed all honey supers earlier – its never been about honey for me anyway. I may well try Randy Oliver’s OA with glycerin in saturated paper towels. I can see that working but do like OA vapourized. It fills the hive with miticide. I will for certain continue with uncapping but will take every opportunity to run the numbers. I like drone uncapping – I used it initially to help reduce congestion in hives. My frames like yours are heavy to drone cells. It did help mitigate swarm impulse in a few of the hives and provides provide good data. I should add alcohol washes to the monitoring. The loss of 300 bees looks trivial after loosing 6-strong hives. Thanks greatly David – all you do is much appreciated.

    1. David Post author

      Hi Vince

      Drone brood removal is probably most effective early in the season. There are studies of using it later in the season and it’s of marginal benefit at best. I run a lot of foundationless frames and find the easiest way to remove drone comb is to simply cut out an entire panel of comb:

      Drone comb panel removed from foundationless frame

      They rebuild the comb quickly if there’s a good nectar flow. If you’ve got chickens they love to remove the pupae. Alternatively you can freeze it and then run it through the wax extractor. It’s a messy and unrewarding process.

      Monitoring drone brood for mites is informative, though probably not very quantitative. Alcohol washes are quantitative but be aware of the limitations … they tell you only about the phoretic mite load which is dependent upon mite numbers and availability of brood to infest. Someone should do a proper analysis of phoretic mite numbers and changes in the laying rate of the queen … I think it would be interesting.

      With mites, forewarned definitely is forearmed.


  15. Eduardo Gomes

    Hi David,
    Last year in the treatment I usually do in early August with apivar I did not get good results in about 20% of the colonies. This year I will keep the calendar: 1st treatment with apivar started in late January and last treatment in August and again with apivar. However, this year I will introduce an intermediate treatment in late May / early June with cardboard strips with oxalic acid and glycerin sold in Europe by Lyson. What I intend with this intermediate treatment is to keep the infestation rate below 3% and eliminate varroas that are eventually resistant to amitraz. Another aspect I would like to comment and ask is about residues of amitraz in honey. The maximum level of amitraz residues in honey is 200 μg / kg in Europe. Do you know of any study in which the apivar was used experimentally during the flow to determine if this limit has been exceeded?
    Best regards,

    1. David Post author

      Hi Eduardo

      I don’t … but in the UK using Apivar with supers on is ‘against the rules’ and I’m aware that the bee inspectors are intending to start testing honey for Apivar residues. Do you really need to use Apivar in the winter? I presume you don’t have a broodless period in Portugal? Could you temporarily cage the queen and treat with oxalic acid?

      Your experience supports my view that Varroa is easier to manage when there’s a ‘proper’ cold winter period during which brood rearing usually stops.

      It’s worth noting for UK-based readers that the OA/glycerin strips are not a VMD approved treatment here.


  16. Eduardo Gomes

    In Portugal, and I believe that in the EU, it is also not allowed to use the apivar with the super on. At the end of winter, in January / February, colonies in general already have 2 or 3 frames with brood. Oxalic acid in these conditions, I believe, is not as effective as apivar. I already thought about treating in late November / early December, when there is no brood, but I’m afraid it is too early, because afterwards I will only be able to treat again when I take the super ones, in early August with approved treatment. The exception would be to use the maqs in May / June, but I’m afraid the temperatures are too high – usually above 30ºC. Caging the queen, at this time in the middle of the season, so after 14 days using a dose of sublimated oxalic acid is a good option, however very laborious. I have 160 hives and work alone. From all the options that I have analyzed it seems to me that cardboard strips with oxalic acid and glycerin will be the balanced option in the middle of the season in terms of effectiveness, increasing oxalic acid in honey I think is not a concern, and it is a little laborious treatment. In Portugal, these strips with oxalic acid and glycerin are also not approved. I believe the reason is because no company has yet submitted an approval process. If any company does so, I am very confident that it will be authorized by our veterinary entities. I continue to read very carefully what you write, and much of what you refer to in terms of the population dynamics of bee colonies and the population dynamics of mites is very close to reality in the territory where I have my hives, which is the coldest territory in Portugal. Best regards,

    1. David Post author

      Hi Eduardo

      Apologies for the delay in getting to your response … if your OA treatment in late November/early December is effective (because colonies are broodless) then your colonies must build up much faster than ours to allow the mite levels to reach dangerously high levels by May/June. We very rarely have significant mite levels that early in the season, which I suspect is because our season starts later. My colonies are all rearing brood now, though I doubt they’re up to 3 frames yet. They are certainly using more stores now than a month ago.

      160 colonies single-handed is a lot of work. I struggle with 20 (though in fairness, that’s on top of my normal job).

      Take care of your back!

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