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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.

Honey bee coated with oxalic acid crystals.
Honey bee coated with tiny oxalic acid crystals. By Chamblis – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44483567

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.

Rusty
Honey Bee Suite

Comments

Katherine Sherrod
Reply

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

Kelsey larsen
Reply

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?

Rusty
Reply

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.

Dave Maloney
Reply

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.”

AramF
Reply

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.

John Gobel
Reply

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.

Rusty
Reply

John,

To get most coverage, I would do it when the bees are home—morning or evening.

Robyn Gray
Reply

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?

Rusty
Reply

Robyn,

I have never used them.

Kathy Ackley
Reply

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.

Linda Mehlenbacher
Reply

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?

Rusty
Reply

Linda,

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

Jeff, bottom of the South Island New Zealand.
Reply

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.

Jerry Truett
Reply

In your opinion, what is the best and safest way to treat mites?

Billy
Reply

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.
http://www.scientificbeekeeping.com

Vince Poulin
Reply

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.

CHET CALHOUN
Reply

What do you use if not oa? have a link to your preferred method for dealing with mites?

Rusty
Reply

Chet,

I use OA, I just don’t vaporize it.

Bill Castro
Reply

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.

Kevin
Reply

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.

Kourosh Bassiti
Reply

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.

Rusty
Reply

Kourosh,

That’s basically the way I do it, by dribbling OA once in the winter.

Judy Scher
Reply

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!

Tim Tovar
Reply

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

Mark
Reply

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.

Rusty
Reply

Mark,

The question was asked recently, and for most of us, the drones are basically gone for the year.

Steve
Reply

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!

Steve

John
Reply

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.

Rusty
Reply

John,

At this time of year, drones aren’t much of an issue.

Vince Poulin
Reply

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.

Philip Hopkins
Reply

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.

Dorothie Jones
Reply

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!!

Rusty
Reply

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.

Brady
Reply

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.

Rusty
Reply

Personally, I would wait.

Daniel
Reply

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.

Fred Levy
Reply

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?

Rusty
Reply

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.

Fred Levy
Reply

Is there a way to send you a picture?

Steve Winchell
Reply

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.

Graeme Hunter (New Zealand)
Reply

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.

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