1 December 2023

Something I notice quite often: beekeepers, vets and authorities don’t really know about varroa treatments and their characteristics. This leads to bad decisions and wrong assumptions. Which, in the end, leads to problems with the treatments. The issues are different for beekeepers or authorities, but wrong assumptions are always the foundation of it. So, let’s break down the main things, shall we?

First steps to keep varroa mites under control

When varroa arrived in Europe and it became clear how devastating the effects can be, a rapid reaction was necessary. As far as I know, nobody tried the eradication approach like in Australia until September 2023. It was spreading way to fast, it was clear very soon that this wasn’t an option. So, treatments were needed to deal with this novel parasite.

The first step was to try out acaricides – so substances that were already used in veterinary medicine and agriculture to kill mites. Several substances did the job to kill varroa, but not the bees. These were all synthetic substances: Some of them are still on the market, others aren’t used anymore.

Refining solutions

Though this was a pragmatic decision, it didn’t go well for ever. First of all, these were substances that partly also harmed the bee colonies, so it wasn’t really helping. Then, these substances left residues in the wax and, even more importantly, in the honey. Finally, these first treatments weren’t a solution for organic beekeepers.

So, the European Group for integrated Varroa Control took up its work. One of the examples of very successful and fruitful collaboration. This group developed and tested the treatment methods with substances from natural origin we have until today. And most products on the market are based on this work.

Synthetic compounds vs. substances of natural origin

A very important thing to know about varroa treatments: They’re veterinary medicine. Both synthetic or “natural” substances. There is no good or bad in this. We just have to use all of these products according to the label, as we would use any medicine for ourselves. “Natural” doesn’t mean that it can’t do harm. Many of the most potent poisons are “natural”. Every substance that works will also have side effects. Two things matter in the end:

  1. That the benefits (killing the mites) are larger than the risks (harming the bees).
  2. The substance can be applied to the colonies in a practicable manner.

Though I have my personal preference, I honestly don’t care if a beekeeper uses products with synthetic compounds or substances of natural origin. As long as they use it correctly. And doesn’t fall into the “I can do this myself” – trap. In this video, I explain what’s behind a varroa treatment.

However, there are a few more things to consider for taking the right decision of which varroa treatment to use. Every treatment has advantages and disadvantages. So, let’s get into the characteristics of the five main substances used today.

Easy to use, but risk of residues and resistance

Synthetic varroa treatments come with a big advantage: They’re easy to use, as they usually comes as strips. You put them between the frames, close the hive and forget about it for six weeks. Great. These treatments are also independent of external temperature, i.e. you can use them during the season as well as in the dormant phase. What I call “summer” and “winter” treatment from habit.

However, this comes with a cost: These compounds dissolve in fatty substances and accumulate in the wax (which is a complex fat in the end). This leads to residues, mostly in the wax, but also in the honey. We’ll get into more detail with this in two weeks in a separate post. But for now: If these residues are too high, you may not be allowed to sell your honey. Another argument to use these products only according to the label.

Amitraz

This is a very “popular” substance, there are several products out there which use amitraz as an active substance. It acts by contact with the strips: the bees “rub” on it, examine it, and may also try to get rid of it. Such a strip is a foreign element in the colony after all. By this, they get some amitraz from the strips on their body. They distribute it within the colony by all the social interactions worker bees have.

By this, it kills the mites on the adult bees. It doesn’t reach the reproducing mites in the brood though. This is the reason why the strips have to stay in the colony for at least six weeks. These are two brood cycles of worker bees. Workers that hatch in that period with varroa mites on them will get the treatment.

However, exactly this detail is a caveat with amitraz: With larger amount of brood, its action is quite slow. Too slow to reach the efficacy that’s really necessary. That’s why in some products the treatment is extended to up to ten weeks. Too long if you want to put supers on again. More on why that’s an issue next week. Newer formulations try to increase the efficacy in shorter time but aren’t as comfortable as strips. Formulations are the form a medicine comes in, if you were wondering.

The pyrethroids fluvalinate and flumethrin

These are two substances, but I treat them as one. Because they’re very similar to each other, they belong to the same substance group, pyrethroids. These substances are used in agriculture, too, as acaricides (kills mites) and insecticides (kills insects). The latter may make clear why varroa treatments have to be handled with care.

Fortunately, these substances kill varroa mites at lower concentrations than they kill bees. This difference is called the “therapeutic window”. But its potential harm for bees is also the reason why you definitely have to stick to the dose and duration on the label.

Anyway, fluvalinate and flumethrin are applied as convenient strips, too. Like with amitraz, the treatment has to remain in the hive for six weeks and then – important! – has to be removed. Dose and time are important to prevent resistance, too. Especially, a these substances are so similar that in case of resistance against one, the other one doesn’t work anymore, too. But as I said, more on resistance and residues in two weeks.

For now only one thing: Resistance problems are the reason why you should rotate treatments. This means that if you treat with fluvalinate during the season, you shouldn’t use flumethrin in the dormant phase. If you want to stick to strips, amitraz would be an option.

Substances of natural origin

An important thing to know about varroa treatments: Beekeeping is in the exceptional situation of having highly efficient substances of natural origin against the parasite. In other areas of animal husbandry, this isn’t the case. “Natural” treatments are often worse than “synthetic” ones, but not in beekeeping. Here, not only organic beekeepers can fully rely on these substances.

There’s a common misconception though: Organic acids and essential oils used for varroacides (i.e. substances killing varroa) are referred to as “soft” treatment. Amitraz and the pyrethroids, on the other hand, are called “hard” treatments. This originates from a publication by Rosenkranz et al. in 2010. They called it this way, to avoid the “synthetic” and “natural” discussion. Organic acids and essential oils are “soft” in the sense that they don’t have the resistance and residue issues I talked about in the previous section. All the following substances are natural components of honey.

Oxalic acid

I already wrote a longer post on what to know about treating varroa with oxalic acid. It’s the most efficient way to treat varroa. Like the previous two, it acts by contact. It’s also completely independent from external conditions, it can be used at any temperature.

Something to consider when treating with oxalic acid: It acts only on the mites on the adult bees and it’s eliminated quickly out of the colony. This is an advantage when it comes to how well the bees tolerate this treatment and it also reduces the risk of residues.

As it doesn’t reach the varroa mites in the brood, the efficacy goes down in presence of brood, though. This is why in summer, you can cage the queen and then treat when the colony is brood free. I describe that in the post I mentioned above.

Treating with oxalic acid is quite convenient, too: There are ready to use solutions you can trickle directly into the hive. No preparation needed anymore as with former products.

Formic acid

The most prominent thing to know about this varroa treatment is that it’s the only substance that acts into the brood cells. This means that it kills also the reproducing mites in the sealed brood cells. Great advantage!

However, as I said, there are disadvantages to every treatment. In this case it’s that formic acid has to evaporate and reach a therapeutic concentration in the hive air. This isn’t bad by its own, but it makes this treatment very dependent on the temperature. Especially the balance of killing mites without harming the bees.

Formic acid evaporates already at relatively cool temperatures. The best results occur between 15-18°C. When it’s warmer than 25°C it evaporates too quickly and harms the bees. This results in losing brood (especially younger stages), sometimes also the queen, and big bee beards at the hive entrance. The latter because they try to get the stuff out of the hive by ventilating as much as possible. In warmer areas, it may be a good winter treatment if this meets the temperature range.

In addition, the dose has to adapted to the strength of the colony to avoid side effects. This is why formic acid strips don’t convince me: it’s a fixed dose and from my experience the evaporation isn’t as controlled as claimed by the manufacturers. You also need an evaporation chamber, i.e. an additional hive body, to place the evaporating device in. And you need such a device for each colony you have. Quite a bit of material.

Thymol

From all the tested essential oils, thymol showed the highest efficacy. Though, mixtures of thymol with other essential oils are more stable in different conditions. I wrote also about this before

Like formic acid, thymol has to evaporate. But it’s better at warmer temperatures – it does best between 20-25°C and can be used between 12-30°C without harming the bees. At this temperature range, the concentration in the hive air reaches levels that kill varroa mites without harming bees.

Also this treatment, however, acts only on the mites on the adult bees. Therefore, it has to stay for several weeks in the colony. The length depends on the product. But in general it comes in a carrier you put on top of the frames. But you don’t need a special evaporating device or an additional hive body.

So, what to use for varroa treatments?

Now that you learned quite a bit about different treatments – how does that help you to decide what to use?

Well, if you ask for my personal recommendation, I would always say oxalic acid. Highest efficacy of all, well-tolerated by the bees without any restrictions by the climate. Caging the queen in presence of brood may be a little bit too much work for professional beekeepers with thousands of colonies, though. I know professional beekeepers with more than 1,000 colonies who still use this method. It’s a matter of organisation. But I see that in North America or Australia, with operation of several thousands, this may not be practicable.

So, let me propose the following:

  1. Professional beekeepers with several thousands of colonies: Use oxalic acid when your colonies are naturally brood free. For the second treatment, to prepare the colonies to this period of low resources, use strips with synthetic acaricides. Rotate between the substances to avoid resistance. If you decide to go with synthetic only, this is even more important.
  2. Professional (or sideline) beekeepers with up to thousand colonies: You can try the oxalic only strategy if you’re well organized. You could also go for alternating strips and oxalic acid like larger operations or do the treatment in the active season with thymol or formic acid when you have the right conditions. This latter option is an advanced one, though.
  3. Hobbyist beekeepers or small operations: You have all options. I’d go with the organic ones, though. Preferably all oxalic acid. As you don’t have as many colonies, you can do the work to cage the queen during the active season.

Basic things to know about varroa treatments

Knowing about the characteristics, the pros and cons of each treatment is only the first step. This may seem a bit overwhelming, also considering the length of the post. But consider that you can come back to this and with time it will sink in.

This post is also the first in a series: Every week in December, I’ll be posting about what to know about varroa treatments. Next week, we’ll talk about some basic considerations when it actually comes to the treatment itself. In two weeks, as I already mentioned, we’ll talk about resistance and residues and how to avoid that. And in the final one, why I think that treatments are the way to go when managing varroa. Stay tuned!

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