What to do if Oxalic Acid seems to fail?

Hi all, I have been having a discussion with @Arvada, who still has brood in his hive, but had mite levels which needed treating. I hope he will forgive me for sharing his experience here, but I think we could all learn something.

He is doing the recommended Oxalic Acid vapor - 3 treatments 5 days apart and has just done the second treatment. To his consternation, even more mites dropped after the second treatment. He asked me what happens if there are as many or more after the 3rd treatment. My response was that I would do a sugar roll (or alcohol wash) count about 5 days after the 3rd treatment. If the numbers were at or above treatment threshold, I would probably choose a different treatment. However, I know that the Brits and Canadians have extensive experience with Oxalic Acid, and perhaps somebody with more experience (@Dee and any others) could make a suggestion? Thank you! :blush:


I would first like to know what Oxalic Acid he is using, how is he sealing the hive if he has a flow hive with screened bottom board, what tool is he using to vaporize and an actual video of his process would be helpful.

His hives could also be actively robbing out a dead/weak mite bomb hive somewhere in close proximity.


Dawn, thanks for getting this out there for me. I appreciate it. It is possible this is not a fail at all. The jump on mite count after the second treatment was the surprise.

First, I had started using ApiGuard recommend to me by a local resource and was using it per label. I was seeing minor benefit and the bees nearly sealed the foil with propolis as they apparently didn’t like it much either. So based on recommendations I switched to OA which really seems to work well but it also is showing me I seem to have a lot of varroa.

I am using a fresh packet of Oxalic Acid from Florida Laboratories. Both flow hives sit on concrete blocks and are not suspended so the surface area of the bases are covered. For treatment, I remove the white boards and pack cloth rags in both front and back. I am using the Varrox 12V 150W brand heating element with a fresh battery. I have found the Varrox plate slides in the rear far better than stuffing it in the front entrance.

I use 1 gram per label per brood box and heat until completely vaporized. I did a trial run so I knew how much time that would take.

After vaporization, the Varrox is removed but the opening and the back remain sealed for 10-12 minutes while I move on to the next hive.

After treatment number one, there were several hundred mites on my white boards which encouraged me. This was FAR superior to the Apiguard I had tried. I waited 5 days and performed treatment number 2 yesterday. Today there were probably close to 1000 on the white board. Yes! Progress…but it left me wondering what happens if mite count remains high after three treatments? Even this three treatment plan is not the official recommended path and the recommended path is to wait until the hive is broodless. I do think however there is merit to treating earlier to allow the winter colony to be as strong and disease free as possible.

So I posted my what if question to Dawn. I do have a question though because I can’t find it easily…after the third treatment, what should my mite count look like? What is a problem threshold?

Red Hot…Your comment about robbing may actually have some merit. The colony with the very high jump after treatment 2 is my very productive hive with incredibly large colony numbers to boast. They aren’t mean but man they make my other colony look like whimps and that colony is actually doing quite well. This one is almost like a super colony. This hive had so many bees I actually added a third brood box (completely empty) three weeks ago to it just to give them some space. They have completely drawn comb and nearly filled the third brood box in about 2.5 weeks. That hive will winter with three brood boxes.

Other facts that might help. Yes I did of course remove the flow supers. In Nebraska they stop collecting mid August due to nectar flow patterns here locally. I have been allowing the bees to simply prepare for winter.

I have performed complete hive break down checks every two weeks monitoring for disease. I am not good enough to find my queen but I see full stages of life cycle inside. Everything seems to be where it is supposed to be.

And since my large colony has three brood boxes…they were treated with three grams of OA. I was more curious if anyone knew why “three treatments?” I have found that there are posts of Brits using OA routinely through the spring. the manufacturer of Varrox recommends treating while broodless. If the molecular density of OA gas is heavy, should I vaporize top down? Anyone know that little factoid?

Video, I can certainly make one. My next treatment is Friday. I keep two cellular cameras on the hives with microphones so recording this would be a simple matter.




Here’s what may be happening:
The biggest mite kill will happen 3 days after treatment: Could it be possible you are seeing those mites plus the ones you vaporized during the second treatment or are you checking the sticky board each day?


Could it also be a sort of geometric cumulative effect of Oxalic Acid accumulation in the hive? From what I have read, after a vaporization, the levels remain therapeutic for about 4 days, which is why treatment at 5 day intervals is recommended when brood is present. If Don is using 3 grams of OA, could there be a longer persistence, and so the second dose really makes the mites’ eyes water?? :blush: Just a thought. I am really curious to read what happens after the 3rd dose.

By the way, I think that Don is incredibly wise to treat now, rather than wait until the hive is broodless. With a heavy mite infestation, that may be way too late for recovery.


I’m going to watch this thread with anticipation.

I don’t have the answer to Don’s (potential) problem, but I exclusively used OA on four of my hives during autumn. Through checking drone cells I still saw plenty of varroa, but the hives were all strong and healthy going in to winter. I was concerned that I hadn’t got rid of them (did two sessions of vapes through late summer and autumn, every five days for five visits). So, I also did a month in winter when I hoped they were broodless.

Long story short… all hives came through winter very well and are building up strongly - it’s only 13 days into spring here…

But, it’s very time intensive, so only if you have the hives close at hand… and an understanding wife (not always…)!

In saying all of this Don, I didn’t have as many varroa to begin with as you, so best practice at this stage might be to put strips in.

Good luck, Paul

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I don’t know the real facts, but I can give you anecdotes. :blush:

The OA vapor is hot, so it rises. Several British beekeepers have told me that they have hives over open mesh bottom boards (no slider). When they “vape” with OA, they don’t try to put a slider in, they block the hive entrance and just hold the wand under the mesh. They still get good therapeutic results. Of course the vapor condenses as soon as it touches something cool (comb, bees etc), so it doesn’t all go straight to the top of the hive.

There you are, that is Level IV evidence for you. :smile: OK, I know, Level IV doesn’t exist… :stuck_out_tongue_winking_eye:

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I should have treated myself; not for mites but for chiggers before I worked in my forest last week: Now I’m full of itchy bites :open_mouth:

Maybe since I didn’t treat, my kids and future grandkids will be better able to defend against chiggers without spraying their clothes lol.

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I have one colony on its fifth vape
I’m sure it’s robbing another colony somewhere, one that is collapsing. The bees have more stores than the others so I have assumed that as well as stealing stores they are stealing mites too. We have no EFB or AFB in the area so I have fingers crossed that they are not bringing that back as well. I have now reduced the vape interval to four days in case the phoretic phase is shorter than the supposed 5-11 days.


Cumulative Effect

It would be great if that is the case. Treatment three comes tomorrow but I have to leave town so I will have to do an early check Saturday.

It was good to see @Sting 's and @Dee 's comments that continued treatment without harm has been helpful. Glad that Sting has a strong spring colony. This is what I think I needed to know. If rates aren’t low enough, I will continue and winter.

What type of treatment thresholds’ should I hope for?

Depends on who you ask. I quite like the Canadian levels of 3 mites per 100 bees for an alcohol wash count in August. If you do sugar roll instead, I would lower that to 2 mites per 100 bees. Any more than that, and I would consider further treatment.

oh great…now what. So in checking the dead mite count on the board today…this is what I was presented with. I have not had this happen before. Can the Varrox melt the wire in the screen bottom?

Very much doubt it. The Varrox gets to about 300C and that is at least 1000C below the melting point of steel. However, it can melt wax in the lower parts of the frames (or burr comb). If there was honey in that, you may get some robbing from the drips that fall through the screen. If you do more treatments, I would probably put the slider in the upper slot for a few days afterwards.

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ok…after treatment tomorrow, will put the slider in the upper slot. I cross searched and see you have a bit of verbiage on the topic of screen bottoms anyway. Can you tell me what that is you are using and how big are the holes supposed to be?

I have both the Flow screened bottom board and two from Bee Thinking. I hate the Bee Thinking ones, because there is enough space above the slider for ants and wax moths to build small cities. I have actually switched them out for solid bottoms. However, to answer your question, Bee Thinking uses #8 hardware cloth, which has 8 squares per linear inch, or about 3mm per square.

Flow decided to go with #6 hardware cloth, because they have humongous SHB on that continent, and they wanted the mesh to be big enough for the bees to chase the beetles down through it. Unfortunately, for about the first year of production, they used a mesh with moveable wires. Each square is meant to be about 4mm per side, but because the wires can slide, the holes can get up to 7mm, which is plenty big enough for a bee to pass through. I adjusted all of my wires, but I didn’t like the ant and other pest access when the slider is in the lower slot, so I just keep mine in the upper slot all of the time. I would switch to a solid bottom, but the tilt on the Flow SBB is useful, so I have kept it for now.


Update 9/27

All, I was out of state for well over 10 days so I finally got back and have an update. I am happy to report that the mite count dropped dramatically. After third treatment I had to leave so I could not do a 24 or 48 hour check. What I am reporting is a 10 day check. Rather than seeing over 1000 dead on the bottom board, the entire board count is about 120-150. This would technically still be above threshold but is now about 1/10th what it was.

Based on comments, I think I may slip in a fourth treatment. #Dawn_SD was correct, the heat of the varrox caused some honey to drop down causing the bees to guard and collect what was lost. So this third time, during the treatment, I moved the varrox during heating like a pendulum so that its total time in one spot didn’t cause excessive heat. This solved the problem of losing honey through the bottom screen. I also moved the plastic bottom board up to the top slot.


Once the daily drop is less than 10 I’m happy



I know this comes much later but time and patience can be a virtue. I started taking a bee keeping class. The instructor with a masters in entomology talked to me about my varroa situation. First comment he made was identical to yours.

He felt my bees found a mite bomb hive. I learned (sadly) that Nebraska has no monitoring program or educational program for bee keepers to encourage management. Because of this, our state has almost the highest hive loss rate in the country.

Well over half of our bee keepers when surveyed on Bee Informed neither check for pests nor treat for them. Even though I am in my first year my colonies are strong so they are pillaging.

Being aware of this, I actually treated two more times in November. I had a re-infestation. Now that I understand what my “neighbors” are doing or not doing, I am simply going to have to stay on it like a hawk late summer through almost beginning of winter.

I also think I am going to play the numbers game too. I have a third brood box on one hive, I will add a third brood box to the other hive next spring. Most literature favors better survival in my cold climate with three brood boxes. If I have to fight varroa, then I can also use a numbers game and just try to go into winter with a larger colony.

We haven’t hit our hardest part of winter yet (January, February) but so far so good on both of my hives.

Thanks for the help @Dee.



It’s so frustrating when your neighbours are doing nothing about pests. I have nothing but admiration for beekeepers who have gone down the no treatment route properly. By that I mean selecting for varroa resistant/tolerant stock and being ruthless in culling those bees that do not cope. It’s laborious and time consuming and needs dedication. I suspect that TF beeks are more on the ball with monitoring than the rest of us. As you say, you can’t just buy a box of bees and walk away going back only to rob their stores. What’s the answer? VSH bees are just a tool not a cure for varroa. I wish you well, stick with it you are doing a grand job.