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American Bee Journal

The Classroom – March 2022

By March 21, 2022No Comments
Mite bomb answer missed the mark
[This was mailed to me in response to my answer to a question about mite bombs in the December 2021 issue.] This problem needs some field experience and some science as the answers given are misleading and totally inappropriate. “Mite bomb” refers to a colony of bees whose mite population is a little ahead of the other colonies in the apiary. To field experience, this says one thing: If you are going to treat it for mites, you therefore must treat the entire apiary.

Secondly, if there is extensive brood present when the bees are first tested, then the mite count of 3/300 bees is false, a very serious mistake. Good science has demonstrated that 70 to 80% of the mites are sealed inside the unhatched brood, busily reproducing. When that brood hatches, the mite count will seem to skyrocket, showing the diagnostic error.

Thirdly, there is only one miticide that will even begin to kill mites within the brood: FORMIC ACID. The other miticides will kill only the phoretic mites. Therefore Apiguard would be totally inappropriate at this time.

The suppliers may not give you this information — they are interested mainly in selling goods for profit. And “educated” scientists may not give you this information either — too many of them lack good solid commercial experience in the field.

For your readers, the first question, appropriately asked, should be, “How much brood is in the colony and therefore how many mites do the bees really have?” And your supplier, if at all interested in your continued business, and especially if claiming to “know bees,” ought to ask you the same thing.

Hope this helps to set the record straight on “mite bombs” and what to do about it. The rest of the answer is spurious.

 Don Jackson

Minnesota, December



Thanks for the response to my answer. I went back and read the reader’s question and my response.

Though I appreciate your feedback (I honestly do!), I stand by my answer. Let me elaborate a bit.

First, you and I do not have the same definition of “mite bomb.” I have only ever heard it described as a colony that is dying, or significantly compromised, to mites and whose mites will be distributed over the other colonies as it is robbed or its heavily infested bees drift to neighboring colonies. I have never heard the definition you provided. Just to confirm, I looked up how other folks use the term and it seems consistent with my use. Thus, it appears we are both starting from a different premise.

Second, let me discuss the 3% infestation rate. I completely agree with you on some of the points you made regarding this. Probably 50% or more of the mites are reproducing in brood, when it is available, at any given time. Most folks I know cite the 70-80% number you used. I do as well, quite a bit in fact. The catch is that I cannot find the paper where this number originated. We all seem to say it, but I cannot find it (the “good science” you quote). That said, I have no reason to doubt that most of the adult Varroa are in brood cells reproducing when brood is available. Even still, there is a reasonable global consensus (as much as there can be consensus about anything) around the 2-5% (most citing ~3%) infestation rate number, which, by the way, holds true even when brood is in the colony. Sure, there are some who say that beekeepers should treat at 1% (because any mites at all = problems for colonies) while others say one should treat at or over 5% infestation. Regardless of the amount of brood, research has shown that the ~3% infestation rate means mite populations will quickly get out of hand if not addressed.

I agree completely with you that to know the actual infestation rate, you have to know the mites you have on adult bees, know the mites you have in brood, know the amount of adult bees, and know the amount of brood. We determine all of these in our research projects. However, this is not practical for a quick field diagnosis and not useful for beekeepers. While the 3% infestation recommendation is certainly not perfect, it is the best we have, and it is used by scientists and commercial beekeepers alike (though, of course, not by all of them). Nevertheless, you are correct. There is error associated with that number.

Third, as you suggest, there is evidence that formic acid can kill mites in capped brood cells. The reader mentioned using Apiguard, which as noted, clearly did not work. Yet, I have seen Apiguard work during the time of year it was used (October) so his use of it was very reasonable. [Look at the next question below and the questioner uses Apiguard with success.] He then defaulted to using formic, which I supported in this case. That said, formic cannot be used in all cases at all times. In Florida where I live, formic cannot be used some of the year (often, during the time we most need to kill mites) given we are outside the temperature window that it can be used safely for bees. Furthermore, many of the other Varroa treatments do not need to kill Varroa in cells as the treatments are designed to remain in colonies 6-8 weeks. That way, they kill mites emerging from the cells with adult bees. Sure, they cannot be used as a “flash kill” like what you might expect with formic, but they are no less useful if you follow the label directions.

Dr. Cameron Jack (University of Florida) and I just published a review on Varroa Integrated Pest Management. You can find the document here: In it, we review the research behind the 2-5% treatment threshold (see “Determining Thresholds”), identifying and monitoring the mite, the efficacies of the available products, the efficacies of other treatments for the mite (breeding, drone brood trapping, etc.), etc. Essentially, we reviewed the world literature on how to control this pest, both now and in the future. We go into considerable detail in this review and address many of the points you raised in your letter.

We also included a Varroa treatment decision tree that we hoped beekeepers would find useful. Please have a look at the manuscript as I think you might find it useful. Thanks again for your comments. I appreciate your feedback. …

Use of OA to supplement Apiguard

For some years, we have applied Apiguard to our bees after taking off supers to control Varroa. This treatment protocol has been reasonably successful and we are continuing it. Other beekeepers in our club are now using vaporized oxalic acid (OA) in the winter for mite control. How beneficial is this application as a supplement to Apiguard and should we be doing both? Mite counts after Apiguard are favorable, but that is at the end of the summer.

 John Moore



This is a very interesting question, mainly because many beekeepers have been discussing how treatments can be integrated to maximize their efficacy against Varroa. Thus, your question about following one treatment with a second treatment gets to the heart of what I hear many folks chatting about these days.

My answer with Varroa treatments remains fairly constant. First, I would sample the hives to determine what the Varroa loads are. I typically recommend treating at 3% infestation rates, but as discussed in the previous question, recognize that this rate varies a little, depending on the presence of brood, time of year, condition of the colony, etc. Nevertheless, I feel that this is a good target rate.

In your case, you take supers off and treat with Apiguard. You stated that you have favorable mite counts after treatment. I will take that to mean your use of Apiguard knocked your infestation rates below 3% (or somewhere near that rate). If this is the case, then I suggest you do not need to treat with OA, given your mite counts seem low. However, you are in that very tricky time of the year, summer, when Varroa numbers can increase rapidly. Considering this, I would recommend sampling your colonies monthly through October just to confirm that your mite counts do not exceed the target infestation rate. If they do, or are inching close, then I would recommend following with another treatment.

You specifically ask about OA vaporization. This could be a good option, but only if you have very little brood in the nest at the time. Ample research has shown that OA is not a good treatment option when you have a significant amount of brood in the hive. It is more of a flash kill, thus only killing mites on adult bees and not mites in the brood. If you have a lot of brood in the nest, you are going to have to use something that stays in the hive for a while. In that case, I strongly recommend having a look at the Honey Bee Health Coalition’s “Tools for Varroa Management” document. It shows product efficacy and helps you develop a treatment strategy based on the time of year/condition of the colony.

I do want to speak a little more to your point about immediately treating with a new active ingredient (OA in this case) after using another active ingredient (your use of thymol in Apiguard). Many farmers of various agricultural commodities will treat for something, and then follow up with another treatment using a different active ingredient. Sometimes, the applicators will apply two or more active ingredients simultaneously to control the same pest. This has led some beekeepers to ask me about treating with two active ingredients at the same time, or in succession.

Honestly, this idea is a little intriguing to me as I think it may make controlling Varroa easier. The only catch is that more research needs to be done on the effects of treating honey bees with multiple active ingredients at the same time, or active ingredients in succession, to make sure this strategy is safe for bees. Furthermore, one would have to look at the label language to confirm that products can be used simultaneously. I do not currently recommend treating with two active ingredients simultaneously. Yet, I think there may be a day, with more research and possibly more active ingredient options, that this will be an option for beekeepers. Until then, my recommendation would be to sample the Varroa methodically (monthly when colonies are not in winter), and treat when you are at 3% infestation rate. To answer your question specifically, I would only follow the Apiguard treatment with an OA treatment if my mite numbers were high.

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