varroa mites

Using oxalic acid vaporization when brood is present

A reader wrote that she was confused about scheduling oxalic acid vaporization treatments when brood is present. Some folks advised her to use three treatments five days apart, another advised three treatments seven days apart. She wants to know which is best.

I think the big question here is how long after a treatment does the oxalic acid continue to kill mites. As you know, oxalic acid vaporization sends a cloud of tiny crystals into the air which quickly attach to all the internal surfaces of the hive, including the combs, the woodenware, and the bees themselves. Although this substance is not particularly harmful to honey bees, it is deadly to varroa mites.

Some of the early reports claimed that the coated surfaces continued to kill mites for up to three weeks. But if this were true, mites would continue to be killed as they emerged from the brood cells. We know this isn’t the case. More recent reports say that levels of oxalic acid in the hive quickly return to normal after a vaporization treatment, which makes more sense.

In deciding how much time to leave between treatments, you have to decide how long the chemical stays active in the hive. My assumption here is that there is little or no residual effect.

A word about safety

Before I continue, I want to make it clear that I am not a fan of oxalic acid vaporization. As it happens, I have a love affair with my lungs, a co-dependency you might say. I need them and they need me. Stories and photos of haze-filled apiaries with coughing, hacking, half-blind beekeepers make me shudder. I know, I know. I’m silly and overcautious, right? Fine, I’ll cop to that. But just because “everybody” does something, doesn’t mean it’s a good idea.

Once you decide to vaporize, I strongly suggest you read the EPA label for using oxalic acid in a bee hive. Also read the Material Safety Data Sheet (MSDS) for oxalic acid, and read the instructions that came with your respirator which you undoubtedly purchased and plan to wear. Practice fitting the respirator to your face and check for leaks, and make sure you purchased the right cartridge. If you can smell the acid, the respirator isn’t working.

I think beekeepers are way too cavalier about breathing OA vapor, and while most will get away with it, some may not. In my most humble opinion, anyone who thinks that standing upwind of the vapor will protect him, doesn’t have enough IQ to be a beekeeper.

Timing the treatments

In the rest of this discussion, I’m assuming that there is little or no residual activity after an oxalic treatment. A publication by Dadant states “The hive returns to pre-treated levels [of oxalic acid] shortly after treatment. Within days of vaporization, the bees will remove the residual OA crystals from the hive.” Is there enough oxalic acid in the hive during the removal stage to kill emerging mites? I don’t know.

What we do know is the varroa mites under capped brood cells are protected from the oxalic acid crystals, but the phoretic mites—those that are moving freely through the hive or riding on adult bees—will be killed by it.

Varroa under the cappings

Now let’s say you treat on July 1. Theoretically, all the phoretic mites will die in the next day or two. But you have approximately twelve days of varroa mites that are preparing to emerge. Why twelve? Because the brood develops in three stages. Brood is in the egg stage for 3 days, the larval stage for 6 days, and the pupal (capped) stage for 12. The rule of thumb here is that each stage is double the length of the previous one: 3+6+12=21.

Because someone is sure to correct me on this, I will point out that the brood cycle is actually a bit shorter because the larval stage is closer to 5.5 days. In the end, that makes the entire brood cycle closer to 20.5 days instead of 21. But because most people use 21 days as the brood cycle, and because there is some variability among populations, I will stick with that.

The egg and larval stages don’t count for much here because the mated female mite doesn’t climb into a brood cell until just before it’s capped. Basically, honey bee eggs and early-stage larvae are ignored by the mites.

Tracking the emerging mites

To reiterate, after the first treatment you have a shed load of dead mites and 12 days-worth of safely capped mites. During each of the next 12 days, some of the mites will emerge along with the brood. These emerging mites are mated and ready to go. Presumably, they attach themselves to a honey bee and ride around until they are taken to a ready-to-cap brood cell. They detect this, probably by scent, and scurry inside and bury themselves under the brood food.

You want to treat often enough to kill the emerging mites, but not often enough to injure the bees. Some reports say that the drones and workers are not damaged by repeated exposure to oxalic acid because they don’t live much longer than four to six weeks. However, the queen—a bee that can live a long time—is more apt to be damaged by repeated exposures. How many treatments she can withstand, I don’t know. But in any case, it makes sense to not treat more than necessary.

The standard choices

Let’s say you choose 5-day intervals. If you treat on day 1, and then five days later on day 6, and then five days later on day 11, you still have one more day of mites that will emerge on day 12 with no follow-up treatment. However, if you assume that the brood cycle is really a half-day shorter as I mentioned above, or if there is residual activity for a day or two after the treatment, then you should be okay.

On the other hand, if there is virtually no residual activity after treatment, or if your brood cycle is closer to 21 days, then maybe you are leaving a day’s worth of mites in the hive. It’s a crap shoot with many variables and several unknowns.

It seems like six-day intervals might be better than either five- or seven-day intervals because it evenly divides the capped period. But each day you wait gives the newly-emerged mites more time to find a soon-to-be capped brood cell to inhabit, which is why using oxalic acid vapor is never as effective when brood is present.

Alternative choices

If I were doing this, I might consider using a five-day interval and then a six-day interval since there is no reason the intervals need to be equal. That means I would treat on day 1, then five days later on day 6, and then six days later on day 12. Anyway, it’s just an idea that may or may not make any difference.

This post is just a roundabout way of saying I don’t have a good answer to the question. But it shows how I would evaluate it if I was using oxalic acid vapor. I’m not a believer in one-size-fits-all answers, so you may have to experiment with different protocols to see which one works best for you.

Honey Bee Suite


  • Thank you! This is exactly the “answer” I was looking for. Thanks for taking the time to write this out.

  • I believe you use the drip or syringe method to apply oxalic acid according to previous posts. What protective equipment do you personally use to do that? What about just treating twice?

    • Kelsey,

      Twice is fine when brood is not present. In fact, once is fine when brood is not present. But with brood, the mites just keep hatching out and they will quickly reproduce.

  • I’ve used OAV successfully for four years. I use only when broodless. I use an approved mask with appropriate filter. I wear gloves and eye protection. And, finally, the way I would put it is: “In my most humble opinion, despite all the protection, anyone who does not stand upwind of the vapor doesn’t have enough IQ to be a beekeeper.”

      • Bloodless? Never. Broodless? Well, it’s possible in fall, winter until the first of the year, and in summer during severe hot weather and/or nectar dearth.

        • And when the colony swarms – the old queen leaves with the swarm and the remaining colony rears a new queen. Assuming the colony swarms on the day the developing queen cells are sealed there will almost always be a few days when the colony is broodless (and bloodless!).

          For similar reasons, during swarm control, it’s usually possible (with care) to create brief broodless periods in both the queenright and queenless ‘halves’ allowing mite management if needed.

          • Hey Dave,

            Excellent points. Sorry if I stole your question…once I get started answering, I sometimes forgot to see where the question was directed.

  • Rusty,

    If a person has two hives, then brood can be separated by capped and uncapped stage between two hives. The hive with uncapped brood treated after the transfer, with the new brood frames from the other one donated throughout the next 10 days. Then when the capped brood emerges in the other hive, it can also be treated. Overall two oxalic treatment during 11 day period with uncapped brood transfer in between.

  • Nice article! Quick question – what time of the day should OA be applied? Early before the girls go foraging, during the day, or in the evening when they’ve stopped flying? Or does it matter? Thanks.

  • I don’t think I would get involved with this method as it requires timing that may be difficult to adhere to depending on the number of hives you have and weather conditions.

    What do you think about the mite-away strips?

  • Very helpful article. Thanks for taking the time to share your thought process. I am a chemist and appreciate the fact that you cautioned people to read the Material Safety Data Sheet before treatment. Checking the fit of the respirator is also a must.

  • Hi Rusty,

    I wonder why you have to remove the honey supers if the bees remove the residual OA crystals from the hive soon after a treatment?

    • Linda,

      Maybe the tiny crystals would dissolve in the uncapped honey? If so, the bees couldn’t remove it.

  • The math also only works for an assumed 100% kill rate per application and does not allow for a non complete kill rate, forager bees returning with fresh mites, nor contaminated drifting bees entering the hive between treatments.

  • In addition to Rusty if you are interested in learning alternative methods check out Randy Oliver’s web site, scientificbeekeeping. It offers a wealth of information on various treatments. Randy has spent countless hours on treatments against the varroa. There have been concerns that varroa are adapting to some treatments.

  • Katherine – Rusty outlined this nicely. A useful thing to do is monitor your mite “drops” daily. Use something like an Excel spreadsheet and record your mite numbers dropped per day. This will give you a picture of just how OA is working. In 2017 I lost two hives to mites. Just too new to all this and had some bad advice – I did opt for OA when some other treatment would have possibly helped to save one or more of the hives. I used OA vapour – it is a great treatment option, easy, fast and effective but as Rusty explained does not kill mites emerging from capped cells. Unknown to me I was just too far into the mite epidemic to prevent colony collapse using OA. Once DWV started to emerge both hives nose-dived quickly. With OA – depending on the number of mites, my records gave me a steady drop of mites over each day of the treatment interval. I started with 7-days, but dropped that to 5-days. Largest drops did not always occur on the first day after treatment but most often. I had continual drops each day. This could simply be dead mites that fell several days after treatments. Would be logical given once OA crystallizes it is no longer effective. For all the reasons discussed by Rusty the best time to use OA is when hives have no capped brood in them. OA was very effective for me with this year’s package bees. They came with an Apivar strip but I soon realized the package was heavily infested with mites. Despite confidence by the supplier that the strip would control them I treated the package twice with OA. They only needed the first treatment but that treatment did see a mite fall of 440 mites within 3-days. Before that I had 17-87 mite falls per day on the mite board when Apivar was the single treatment. My post treatment numbers after 6-days were 0-3 and have remained through to the present (end of July). A recent alcohol wash (220 bees) produced no phoretic mites and none seen on drop boards in 3-queen rearing NUC’s made from that colony. The trick is early intervention. By the way OA vapour treatments for me have been painless – they take just minutes. Place the heating unit in the hive base, ensure the hive is sealed reasonably well and turn on the switch. Move well away from the hive – 6-10 m and let it do its thing. Within minutes OA is vapourized and vented from the area around the hive. I remove the heating unit and let the hive sit, just don’t breathe while this is done and move well away. If lucky, we’ll make it through to fall but mite monitoring is on-going and I will not hesitate to use something like formic acid after honey supers are removed it mites appear. Later in season all the hives will be treated with OA regardless of what I see on the mite board.

  • As the use of acids has increased so have queen events. Queen events are a major issue now, especially among the acid vaporizing crowd. There is a big reason most commercial operations requeen 60% or more of their apiary in any year…their queens quickly peter out by late summer or early fall, and the following spring. Acid vaporization has severe problems associated with its use and performing chemical experiments on complex insect nests is quite an absurd activity in my opinion. My treatment-free apiary has better survival rates than most other heavily treated apiaries in my area and yet I’m told every year that my bees don’t stand a chance and my apiary will collapse…10 years later our colony counts increase by 20% every year and our numbers are way over 120 now just from brood breaks, wintering colonies on real nectar based honey, keeping colonies small, and respecting the natural world not trying to micromanage things we don’t understand or have very little understanding about what we are truly doing to our beloved bees. I imagine that in another 5 years the beekeeping “experts” will come out with yet another “breakthrough” treatment to “save” the bees…what we should be wondering is that if the multitude of treatments are saving honeybees from varroa and 100s of thousands of queens, packages, and nucs are being sold every single year and beekeepers of every size are splitting constantly, why are numbers of colonies in the USA barely remain level at 2.5-3 million? American beekeepers should be keeping over 10-20 million or more by now…there is something fishy going on with all these statistics and yet few if any are talking about!!!!

    Forage remains the #1 health issue honeybees and all pollinators face!!!! Animal and human health studies all show that improper diets exacerbate immune system problems and lead to an organism becoming sick, dysfunctional, and eventually death.

    • Bill, I’d say you are very lucky (as well as knowledgeable and the other things that keep your apiaries alive and well, of course).

      Here in France, the most popular – and readily available – hives are Dadant which are huge and simply exacerbate mite and other health challenges. We have started to shift our bees into Warré hives and will observe the results over the next few years but for now, all I can say is that varroa are a reality. They are at tolerable levels in most of our hives but others do require treating.

      The one year we decided not to treat because we believed that the agro-pharma industry was cynically leading us down the path you allude to, the colonies in question died and showed huge mite counts towards the end. Constant treatment is not the long-term answer, I totally agree, but in commercially-oriented hives, designed for beekeepers and not bees, what are the REAL choices do you think?

      Genuinely interested in your views.

  • Thank you, Rusty – again a very useful article. Just a little comment from me. Here is France where I live, and keep bees, we are always advised to treat with oxalic acid (whatever method) only ONCE and only in a warmish day in winter, when the chances are that there are (almost) no brood in the hive. I treat my bees after the summer harvest with a variety of other methods and then in about December or January I use oxalic acid dissolved in syrup (then apply by syringe) or now I use Apibioxal. So far I have been content and my bees have come through winter happily and strong.

  • I use the dribble method exclusively and only in winter when the hives are broodless (in Oregon we have some winter days near 50 degrees F when we can lift off the cover and apply the OA). I don’t like to apply OA in late summer or fall because the bees are raising “fat” bees for the winter. Every “fat” bee counts!

  • Thanks for the post Rusty, but the concern over physical safety with regard to oxalic acid vaporization seems overblown and makes me wonder if you have any personal experience with trying it? I have noted the same thing with my local bee club. I’ve used it for about three years now with anywhere from one to five hives and believe it to be extremely safe, efficient, effective, and cheap. I don’t even understand the comment, “Stories and photos of haze-filled apiaries with coughing, hacking, half-blind beekeepers make me shudder.” When I smoke my bees you likely couldn’t even tell when I started or stopped if you weren’t informed. Nary a puff gets out. In any case, I think you do a small injustice to the beekeeping community by coming down harshly on vaporization (far outweighed by the good you do. .. keep it up!). – Tim

  • Shouldn’t the treatment regimen for OAV when brood is present be based on the 24-day cycle for drone brood instead of the 21-day cycle for worker brood? In other words, treat once a week for 4 weeks in order to fully cover the drone brood cycle.

  • Thanks Rusty for a good discursive article.

    We have found eggs on the varroa monitoring boards when after vaping during the “broodless” period in December. We are convinced the two events were related. Why? Not entirely sure. And why didn’t the bees eat the (damaged?) eggs for protein?

    The hive is a complex environment dominated by pheromones- what affect do the regular fumes have on the chemical messages?

    And finally, the queen’s spermatheca holds the only supply of sperm for fertilising eggs that she will have in her lifetime. The sperm is kept in good condition, in part, by a supply of oxygen from a trachea net around the spermatheca. Same question for the scientists…. what damage?

    Sorry for more questions than answers Rusty!


  • With this summertime OAV approach, you might consider the need for a fourth treatment. Drones are disproportionately affected by varroa (which is why some folks use the green drone frames as one arrow in their IPM quiver), but the drone pupal stage is 15 days. And with the worker larval stage being ~5.5 days, you need to treat every 5 days. So, to get all of the varroa using OAV in the summer you really need to do 4 treatments, not 3 (Day 1, Day 6, Day 11, and Day 16). Otherwise, you miss 3-4 days of varroa in the capped drone cells, not 1 day.

  • My experience with OA is limited to only two seasons but have to echo comments made by Tim Tovar. The method is anything but complex or dangerous to any reasonably informed beekeeper. I suspect in clustered apiaries fumes could hang-around a treated hive for a short period of time preventing moving on to an adjacent hive but my experience is this is measured in minutes. After vaporizing OA it makes sense to leave the hive contained for 10 minutes in any event to allow the vapour to reach throughout the hive. I have never experienced clouds of vapour – just the opposite. I have to use a timer to tell when the treatment is completed. I more often apt to wonder if the heating element is working or not. The treatment is best delivered when the hive is reasonably sealed to prevent the vapour from vacating. If a beekeeper smells vaporized OA just move away and next time seal the hive better. It is just another tool, but an effective one when used at the right time.

  • Rusty,
    First, thank you for nice coverage of a controversial topic.
    I’ve used OA vapor intermittently for years now and have been pleased with the results-at least as far as mite drop following treatment is concerned. Granted, that’s a surrogate marker; my very non-scientific study has not produced a dramatic difference in treated vs. untreated colony survival. There are so many factors that go into that that it’s hard to make any real judgment one way or the other.
    I don’t use OA by any method when there is appreciable brood present. Part of the controversy about this method revolves around the question of whether repeated, closely-spaced treatments have unacceptable toxicity to the bees-particularly the queen as somebody mentioned above.
    I haven’t tried it myself, but inducing a ‘brood-less’ state by isolating the queen for an interval of about 10-14 days has been widely published. A brood-less period is beneficial in itself by interrupting the mite life cycle (e.g. a swarm). During the brood-free time, all or most newly laid eggs develop and are capped, including the drones. Treatment is then done soon after day 24 when the drones have emerged and nearly all the mites in the colony are phoretic.
    There is also a slow release version of OA that has not, to my knowledge, been approved by FDA. I’ve never used that either, but mention it just for completeness.
    Finally, I couldn’t agree more about the MSDS, but you may have overstated your case about the hazards of vaporized OA. Certainly, a respirator mask, approved for organic gases, is required, but they are available for a modest price. Also, if you are seeing apiaries in a cloud of vapor, that beekeeper is using the product incorrectly. If used correctly, very little vapor should escape. I use a respirator without fail, but rarely see more than a few wisps of free vapor. It is nasty stuff if inhaled, though, so why take the risk.
    Of course, if one uses the dribble method, skin, and especially eye, protection are mandatory. OA is a strong organic acid and will ruin your day if it splashes in your eyes while you’re mixing.

    • A very good article with a very sensible contribution regarding fumes of any toxic ‘gas’.

      A great many workers who had to handle asbestos back in my younger days were not told of the product’s dangers. When the under-the-carpet knowledge surfaced, we then got the very best quality masks. Some missed the boat and died! It is always prudent to protect the body but if the fumes are not inhaled whatsoever they will not hurt.

      The authorities are remarkably stupid when they say a mask does not protect the wearer. Yes, they are talking about Covid but they might as well include oxalic acid by saying if you wear a mask that is designated to protect against acid ‘fumes’ it will not protect the wearer. Try telling that tripe to the Institute of Heating and Ventilating Engineers or Honeywell.

      It seems surprising to me to say that getting bees to eat the oxalic acid by spraying it in liquid form is less dangerous to bees than the fumigation of them? Please elaborate and explain please. I see the liquid form of treatment is frowned upon by many oxalic acid authorities as of 2021.

      It would be optimum to not have to administer any treatment of any description that might hurt the bees but when the wing virus is seen on 6 or so bees and after fumigation with oxalic acid in winter 60 mites are killed is that deemed to be bad? Was the correct approach chosen? Will the 60 bees that had those mites on them be better off without those mites now dead. I for one, sincerely hope so. The old saying, being cruel to be kind, comes to mind but some would say you’ve done more harm than good? It is with a great shame that no one has ever spoken to a bee to see how it feels! There are some that will say they know how bees feel!

  • Hi Rusty

    I’m in agreement with you about the cavalier attitude of some (but certainly not all) beekeepers with regard to use of various chemical substances. I worked all my life with numerous hazardous chemicals and just because 40 years ago we were happy to slosh things around without protection doesn’t mean we should do so now…we’ve learned a lot over the decades!

    Organic acid vapours (oxalic, formic, acetic) are extremely hazardous to the lungs. If you inhaled any you’d soon know about it! Just because you’ve managed never to inhale any so far doesn’t mean you won’t get caught out one day so why take the risk? Splashes on the skin and esp in the eyes are also dangerous. I wear gloves, mask and goggles regardless.

    When I began beekeeping I was quite shocked at the casual use of many chemicals including vaporizing acetic acid to sterilise brood comb and use of boiling alkali solutions etc!!! Often done in people’s homes and gardens with other family members running around!

    So please people as Rusty says, read the safety instructions which should come with all products these days and protect yourself accordingly…it’s for a reason.

    As Philip also says, if you are using the products correctly according to instructions then you shouldn’t be at any appreciable risk from ‘clouds of fumes’ etc…but you never know.

    Happy beekeeping all!!

    • Thank you, Dorothie. People frequently write to me offline and tell stories of the “clouds of chemical fumes” they have encountered in bee yards. Many have felt the choking effects of chemical vapors and some have passed out. The ones who write are often too embarrassed to leave a comment, but for sure they are out there. Since the products can be used safely and effectively with minimal effort, that is what we should be doing.

  • It’s 90 plus degrees here in central Pa. I want to treat my hives by vapor but I’m not sure if I should. Or wait till the temperature drops to more reasonable outdoor levels say in low 80s or high 70s.

  • I actually do what Philip Hopkins suggested. Egg laying naturally slows in early summer. I go out and cage my queens. Call this day one. Day 13-14 I go in and make sure there are no queen cells and remove any I find. Day 16 is easy, pop the top where I have cages sitting and let queens out. Now there are 9 days for last drone brood to emerge and 9 days before the eggs she starts laying will be capped. Which brings us to day 24. I think people are using too much OA when they vaporize. I use the required mask but nothing comes out of the hive unless I didn’t shove the wet towel in enough, which I correct when I see it escaping. No toxic clouds here. Piles of dead varroa first day. A little the next, a few the day after that. Done. Start last week of June. Remember when you cage queens, plenty of worker brood is still emerging for 21 days. You’ve only lost 16 days of egg laying and that’s not a problem for August foraging.

  • I have not seen how quickly the adult female once it emerges from the capped cells enters the next cell to reproduce. If it is less then the time interval between treatments you will be missing some mites allowing them to increase in numbers. For that reason I look at other methods of treatment.

  • New beekeeper, opened hive to do a mite check. Found many brood capped cells. Queen was gone. I had not checked hive for 3 weeks. I have a new queen. I have been instructed to move the hive about a football field away, remove all of the bees and return hive to same spot. Then install queen. Is this the way?

    • Fred,

      What is the reason for this? It sounds like someone thought you had laying workers, but you don’t say. Plus, if you do have laying workers, this probably won’t work. A bit confused here because if you still have a lot of capped brood, I doubt you have a laying worker problem.

  • In response to Daniel on 8/23/2018:

    In the February, 2018 American Bee Journal article “The Varroa Problem: Part 15” Randy Oliver mentions that “some experienced European beekeepers recommend repeating the vaporizations at shorter intervals– every 4 days–in order to prevent any mites from exiting and reentering the brood between treatments”. Randy tested his spreadsheet model and saw a rapid drop in the number of mites using treatments every 4 days, but he tested with only with 4 treatments over 16 days, so by the end of the time period of his model – 28 days the mite count came back up without further treatments. Randy’s best results in that article came with 4 treatments spaced 7 days apart – 28 days of evenly spaced treatments. The adult mites leaving the cells on the young bees and then reentering brood cells to attack the next batch of brood is the main problem with ending the Oxalic Acid Vapor treatment after just 16 days.

    So, what I would suggest would be to start a Varroa treatment with 6 or 7 or even more Oxalic Acid Vapor treatments 4 days apart to get the mite count down quickly and to catch any mites that exit and re-enter the brood cells, followed by continued treatments every 4 to 7 days to continue to kill the phoretic mites that enter the hive from other colonies and thus keep the mite count low forever.

    Since mite infested colonies are dying around you all the time, and your bees may be robbing these dying colonies or the last remaining bees of a dying colony may be joining your colony at any time, I believe you should be treating with Oxalic Acid Vapor any time it is warm enough for your bees to be active, plus at least a few times when there is no capped brood, such as in the winter when the queen is not laying, in order to kill all of the phoretic mites in your hives at those times also.

  • With a very high mite count and the hive close to collapse, K wing, white mummies, black mummies and any other thing that puts it’s head up. First thing make sure your hive is well venerated with ventilating top box (anything will do to get the water out) then start the oxalic treatment.

    This will work, one teaspoon of oxalic acid in the vaporizer every third day, do this nine times covering a 25 day period, this should bring your hive back into order.

    PS At this point of collapse, your hive is all put lost, it worked for me in 2016.

  • Hi Rusty – I am so grateful for your site. Although I took a beekeeping course with the local beekeepers association and have The Beekeeper’s Handbook (recommended by you) – I get the best information from your site. On that note – I am in a quandary and could really use some of your sage advice. I apologize in advance for the length of my “comment”…

    I’m a newbee – I lost one of my two hives earlier this summer – lost my queen but didn’t discover in time and then the hive was robbed (probably by my other colony!) and then finished off by yellow jackets. My current hive seemed to be doing pretty well, but then the yellow jackets (they knew about this hive from wiping out its neighbor I’m sure) nearly wiped them out. I was so freaked out about my other hive that I was determined to keep this hive alive no matter what and spent nearly three weeks personally guarding their entrance. I found three yj nests nearby and burned them out, plus I’ve gone through a ton of soapy water bottles killing yjs around my hive.

    The colony is pretty small – I only have about 2 frames with bees, brood and honey, maybe 2-3 frames of honey and beebread, and a couple of frames with just a honey arc. I saw some larvae the other day trying to do a quick inspection when the hive was so quiet I was convinced they were dead – but it started to rain so I needed to close up the hive and couldn’t see if there were any eggs.

    I’ve checked periodically for signs of varroa mites all summer and never spotted any falling through the screened bottom board until yesterday. Another beekeeper suggested that since my queen took a break from laying a while back, that would have set the mites back and so maybe that is why I hadn’t seen any.

    Well – that is no longer true.Yesterday is the first day I’ve seen mites – there were 5 – and today I counted 13. I don’t have that many bees so I really didn’t want to scoop a bunch into a jar to do a shake test if the mite board count is high enough to warrant treatment. I assume the brood cells are my winter bees and they are probably afflicted with mites. I realize this small of a colony doesn’t have a great chance of surviving this winter (or maybe even fall 🙁 ), but I can’t give up on them, so I read everything I could trying to determine the best mite treatment given the time of year and the circumstances of my colony.
    I decided that if this colony has any chance at all I should treat with something that will also penetrate the brood cells and do it right away, so I picked up some MAQS this morning and just finished applying it.

    So here is my quandary – the directions say you need at least 6 frames of brood. I read one of the links you referenced on about applying the MAQS – he had used a half strip in one of his experiments, so I thought that since I only have a couple of frames of brood I applied about a third of a strip. Since you aren’t supposed to disturb the colony for 7 days, I loaded up their feeder to which I added some homemade bee cheer. Now that the hive is all buttoned up, I just noticed in the instructions that you shouldn’t feed during treatment, but they don’t say why. Do you know? Do you think I should pull the feeder out? I did it because they have been struggling – I started feeding them a few weeks ago. I thought not feeding them for a week might be a bad thing. Do you have any other thoughts/advice on my strategy? Do you think a colony this small has any chance at all? Thank you!!!

  • Hey Rusty, I’m treating my two deep hive with OAV. I wanted to ask about whether or not I need to remove the inner cover when I treat. I’d rather not so I don’t have to open anything up but didn’t know if the vapor would reach the bees between the inner cover and outer cover.

    My plan was just to slide back the outer cover to stop the vapor from seeping out of the ventilation hole, but I wasn’t sure if enough vapor would make it inbetween the two covers. I know you don’t treat with vapor but wanted to see what you thought. Thanks.

    • Matthew,

      I don’t know why you would have bees between the two covers. In any case, I don’t think you need to remove the inner cover.

  • Thank you Rusty and friends for all the information on this site. New to bees and reading everything I can about this new hobby! I’ll be getting my bees (2 nucs) end of April. I live in central PA.

    I am wondering if anyone is familiar with the heat method of ridding hives of Varroa? Basically, it’s a unit that is inserted into the hive entrance, it heats hive to an internal temp of 106*. Hot enough to kill Varroa, but not the bees. They say it also kills the bugger inside capped cells. I believe I will try this method. It’s not cheap, but neither are most of the chemical treatments.

    Regarding your OA dribble treatments, is that all you use?

    Thanks for all you do.


    • Diana,

      I’ve heard many rumors about the heat method, but I personally don’t know anyone who uses it. I use a variety of treatments, and always rotate them to prevent resistance. I use OA dribble, HopGuard, thymol, and formic.

      • There is some Czech research on using specially constructed hives to heat the brood and kill the Varroa in the brood cells: they found the bees could easily defeat the heater by fanning, and it was extremely hard to get the brood to the “Varroa kill” temp and keep it there long enough to do the job. In addition, an unintended consequence of heating brood is rendering drone brood infertile (and the queen if she is present). That could have disastrous consequences, with local Drone Congregation areas seeded with infertile drones who mate the queens but give her no sperm. She only tracks the number of matings, not the quality, so could come home to a very truncated career.

        • Janet,

          That is interesting. I was given a hive top “heater” that is red translucent plastic that is used in place of a lid and is supposed to let the hive heat up inside without the bees seeing the light. Anyway, I tried it a couple different ways, but the mites didn’t seem to mind. No data, just an observation. I imagine additional fanning played a part.

  • Hi Rusty, I will be getting my first package of bees in May for my two top bar hives which I have recently made. I’m concerned that I really don’t have a plan for mite control due to the limitations of the hive design. From my research I effectively seem to only have OAD or OAV available outside of splits and brood beaks as my options. I’ve just read about too many issues with MAQS and thymol treatments to feel that they are safe alternatives.

    Currently, my plan is to do an OAD on the package about 7 days after queen release and then do monthly dribbles recognizing that this does nothing for mites under capped brood. I may have a neighbor beekeeper do an OAV (his lungs, not mine) in mid-August for the anticipated run up in mite population before doing another OAD late fall/early winter.

    Short of having bee resistant stock, what treatment schedule would you suggest for a first year top bar and would you suggest an alternative schedule in the second year?

    Love the site, and thank you!


    • Kevin,

      I can’t recommend a treatment schedule because I don’t know how many mites you will have. I believe treatment needs to be based on testing, not on the calendar. You test first and then, depending on results, you treat or not. Personally, I think proactive, once-a-month dribbles may be unreasonably hard on your queens. If you have a significant mite load, the risk is probably worth it. If you have low counts, I’d say it’s probably not worth it.

      By the way, have you considered HopGuard II? It’s been my primary treatment for the last three or four years with no negative outcomes.

  • Just a couple of comments based on this interesting discussion:

    -We have in our club found that most new beekeepers are not comfortable doing regular mite counts. Very often they reach out for help as winter approaches and share that they did not really know how to do one so they just put it off. I think that first year or few you are just getting used to opening that bustling, growing colony of stinging insects and the idea of doing a capture and count is overwhelming. And most refuse to do alcohol washes as they are understandably reluctant to kill any bees at all. Given that fact, and most particularly for those who live in areas of high bee density (where drift shares the local mite load around all hives within a shared flight range), we give them the option of treating by the calendar. Which in our area sort of translates into “when you get them (you do not know what the mite management strategy was in their home apiary), right after honey harvest, right before winter wrap-up, and midwinter”. Most choose to use Apivar, formic acid and OAV in rotation. If you are doing OAV, use a full face respirator.

    -many new beekeepers just don’t believe they will get mites, hope IPM will do the trick (locally it barely makes a dent), and many do not want to buy a vaporizer unit…the units are expensive and you need that safety gear to use it, adding to the cost.

    -Oxalic acid dribble does cause some brood kill when young larvae are fed OA laced brood food. So keep OA dribble treatments to a minimum, particularly if you are treating at a time when the hive really needs to grow (here we have little time to build the hives in spring before our June honey season, so OA dribble would not be used then). Monthly treatments would be too often.

    -time treatments with the mite life cycle in mind. There is a lot of discussion of the proper timing online, but I dimly remember it was three treatments done 5 days apart??

    -always remember the impact of drift bees…in studies in Europe, between 10-40% of the bees in your colony were not made there, they came from the colonies in your area. Those drift bees bring along the mite and disease levels from their home hives. This is one reason our club offers a complimentary OAV treatment to anyone in the area in midwinter who signs up for one, and encourage area beekeepers to ask for one any time they think they need it.

    -the advice from treatment free/Darwinian/natural beekeepers is to run resistant stock. So far no one in our club has ever found any bees better at mite control than any other bees, even when sold as “resistant”. In Canada there are no resistant stocks available for purchase. On the topic of resistance…”resistance is futile” unless it is so effective it allows you to extend your interval between treatments and/or skip treatments altogether (the latter is, alas, what many newbees assume is meant by “resistant”).

    Finally, read this blog post that arrived in my email inbox this morning…very germane to this discussion:

  • Thanks, Rusty and Janet, and hope you both had a nice Thanksgiving.

    Also, thanks, Rusty, for the reminder on Hopguard II as an alternative during bloodless periods to oxalic acid. I was drawn to OA over Hopguard due to the economics but recognize the harsher effects of OA on the bees so I need to take that into consideration.

    Perhaps I can refine my question a bit. A reminder that I will have top bars so formic acid and Apivar may not be good options in that set up accordig to many. I did not mean to imply that I would do treatments based off of a calendar, but that being said, I seem to interpret some of Randy Oliver’s presentations along with others as implying that it is critical to be ahead of the traditional explosions in mite numbers that often occur in mid-August and September. Therefore, I thought that many suggest to be aggressive in treating in advance of that timeframe to keep the mite population under control. This might be especially important in a top bar as I don’t know of any treatments short of oxalic acid sublimation which can get the mites back under control. And from what I understand the sublimation is not a great option when th mite population has peaked. Could be wrong on all fo this, of course.

    So I guess I’m wondering in a top bar, what would the suggested treatment be if the mite numbers are above threshold as it often times is in late summer/early fall, and, also, would it be best to plan on doing OAV in early August regardless of mite counts in an attempt to stay ahead of the typical mite explosion??

    Again, many thanks for helping this not even new beekeeper!


    • Kevin,

      I have mentored a number of people with various top-bar hive configurations. For mite treatments, we have used Formic Pro, Mite-a Way strips, Apiguard, Apivar, HopGuard, OA drip, and OA vapor, usually depending on what the beekeeper was comfortable with and how the hive was designed. I hope that anyone who said, “formic acid and Apivar may not be good options” took a good look at your set-up before they said that.

  • Thanks for the link 😉

    In the UK the 3 x 5 days was probably first published by a very thorough commercial beekeeper called Peter Little living in Somerset. He determined he interval empirically … having tried 4 – 7 or 8 day intervals if I remember.

    It’s what I use and it works well. However, it’s worth emphasising that multiple treatments should only be needed when there is sealed brood present. Most colonies will go through a broodless period (at least in much of the UK) at some point between mid/late autumn (fall!) and the start of the New Year. That’s when to treat. The point of my recent post ( was that beekeepers don’t have to wait for the end of the year, the vacation or when they have a bit of spare time … treatment is maximally effective when the colony is broodless.


  • I evacuated and removed honey supers before fumigating with OA in mid-summer. How long should I wait before replacing the honey supers? With the dribble method, 14 days are recommended but that is understandable given the different mode of action (liquid vs gas).

  • The Internet posts warned me of the same thing. Then the acknowledged expert in our local club told me he just kept treating (oxalic acid vapour, not drip) until mites were knocked back, irrespective of the presence of brood. I followed this advice last summer and not only did I not kill brood significantly, I got my hives to the point where overwinter survival was better than recent averages.

    • Vince,

      Maybe I’m not reading this right, but the reason OA vapor isn’t recommended during brood rearing is not fear of harming brood, but the problem of emerging brood spitting out new varroa that was protected from the OA.

  • Rusty – I have used OA for close to 4-years and treated hives both at times with brood and without. I bought a vapourizer to administer the treatment. I monitor mite counts during every treatment and can easily conclude OA is not persistent. Peak mite drops happen within the first two-three days of treatment followed by significantly lower counts. Those days are the “wishful” period where you hope numbers come under control. Zero counts are the target for packages and the last week of November/first week of December. With brood present, OA is not effective for the reasons you talk about. What is? I detected high mite loads at end of July this summer. I began formic acid (65%) treatments the first week of August. My first time using FA. Results: 3-queens lost and ultimately 6 hives out of 7 by mid-October. Mite kills ranged from 1,534 – 5,508. Uncapping in one hive gave an infestation rate of 21.3% (n=183) 20-days into the FA treatments. Out of those 183 mites, 20.5% were alive – feet kicking! Clearly, for me, mites were getting past FA and not enough killed by FA from under caps to prevent re-infestation of the hive. I resorted to using OA on a remaining hive only out of frustration. That hive received two earlier FA treatments. OA was applied every 5-days. First on 29 Sept 2020. By OA2 numbers were down to 21 (Oct 7) – seemed in the right direction (not so many brood?). Wanting sub-10 counts OA3 was applied for a 32-drop. Five-days later (OA4) counts trebled to 90-54 on Day 1 and 2. Unquestionably high. Hence, OA5 and an even larger drop with 112-111 in the first two days. Clearly, mites were still coming out of brood cells or my vapour OA was not killing enough phoretic mites. OA cannot reach mites under caps but as I found out in my other 6 hives nor did 65% FA in pads. On November 5-6 after a persistent OA8 another 300 mites dropped – trebled again! By mid-November, numbers hit 5-13. Unfortunately, this last hive failed in mid-February 2021. Tests showed Nosema at 1.5-10 million spores/bee likely the cause but equally problematic all the other varroa transmitted viruses and horrific exposure to acid after acid treatments. Lessons: FA (liquid 65%) was no more effective than OA when brood were present. What’s worse the cure or the disease? I lost 3-queens. A friend using formic pro lost a queen as did our local school kids. I never lost a queen to OA. This last hive went to OA9 and she survived through to the last day in Feb. The number of mites in peak drops following each 5-day treatment period showed I was unable to kill enough mites to control the infestations. Starting early did not help (Aug 7, 2020). Even phoretic mites – while I’ve seen them well-controlled when cleaning packages and for post-December brood-less treatment neither FA or OA was effective enough to handle what happened this past year.

  • Yes they will. So you must treat repeatedly at 3 to 4 day intervals. A tedious and unpleasant exercise but unfortunately necessary when mite loads are high. The fact that the hive seems to tolerate this is the good news.

    • Vince2 – what happened here is a quite massive infestation of mites. So much so the best of beekeepers lost a significant number of hives. Losses are on the order of 70% according to a regional inspector. Over so many hives and operators, you can be sure every treatment approach was used with most not effectively controlling the level of infestation – kind of like Covid-19 variants. The key is don’t hang your hopes on chemicals when brood is present. David Evans (UK) -The Apiarist – read his work has good results with Amitraz – something I will try this year. If your hives are exploding with bees, honey supers over-filling, it means mites are taking advantage of the bountiful times as well. Do everything you can to control them (Rusty and David make this point continuously). Sugar dusting does not control mites but it will cause some mites to fall out and make you feel good. Drone uncapping is similar but a more effective and useful tool. It lets you see the infestation outright. Uncap 100 drones and find 1 mite = 1% infestation, get 25 mites, and you’re at 25% infestation). Neither gets rid of mites! Both highly variable but never-the-less you see what is happening and helps you plan. German researchers have experimented with brood-less hives. Their results are impressive. Such as <2% infestation without chemicals. I will be going into the 2021 winter bee production season with several "brood-less" hives. Those will have low mite numbers by queen exclusion and trapping and or exclusion followed by single to double OA treatments when all mites are phoretic. Another option for me will be to dispose of brood frames – and go after phoretic bees with OA. Then let the queen and colony replace those brood with fresh winter bees in a near-mite-free environment. This year I learned one-to-two mite-free hives in early August is worth 3-4X that number of hives having the potential to carry 1,500 – 6,000 mites into early winter. The latter are doomed, the former a good chance for successful wintering. Winter 2 successfully, split early, split again, and your off to the races. This of course is for we who raise bees not for honey but for the pleasure of having bees.

  • Here is my logic and why I don’t think 3 – 7 day treatments work long term.

    I’ve read many articles on vaporizing and the treatment differential was anywhere from every 4 days to 7 days. The most popular being every 5 days and each suggests 3 treatments.

    The pupate stage for Drones is 14.5 days. If the first treatment is done on day one of a cell being capped and two more treatments happen 5 days apart, the last 4.5 days of hatched drones won’t get treated.

    Therefore, I feel treatments should be 4 times every 5 days.

    Some say 3 times every 7 days to span the entire pupate stage.

    But, mites that emerge with a Drone can re-enter another cell within 5-11 days. So if treatments are 7 days apart, mites will reenter calls and remultiply. By treating sooner, you kill more mites before they reenter cells. That’s why I think 5 days is the correct interval time but it also needs to be 4 treatments to cover the entire drone life cycle.

    The best time to treat is when there is no brood, and should give near 100% results.

    Treating in August when mite counts are at their highest, is intended to reduce counts to a controllable number. I recommend a second single treatment in early December on a sunny day. Chances are the hive will be broodless then, depending on your local so you should get a very high kill.

  • Research by Jennifer Berry at UGA has shown the same thing that we found in real life, that in colonies with a lot of brood, repeat OA treatments on a 5 days interval as many as 7 times in a row does not bring mite loads down below the treatment threshold. They can keep mite loads from climbing out of control but don’t reduce them; if you start with a 10% mite load, that’s what you end up with, and “the colony is toast: in her words.

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