Treating varroa mites: all methods compared for Dutch beekeepers

If there is one topic that keeps Dutch beekeepers awake at night, it is varroa. Before 1980 nobody knew what a Varroa destructor was. Today it is by far the biggest reason colonies die, harvests disappoint, and beginners often quit again in their second year. It is also the topic where you ask ten beekeepers and get eleven answers.

In this article I lay out what you really need to know about varroa treatment in the Netherlands. Honestly, with the products you can buy here, and with my own observations from practice. No "the truth", because there is no such thing with varroa. But a workable overview from which you can make your own choices.

What is varroa and why is it such a problem?

Briefly: the varroa mite is a parasite from Asia that feeds on the fat body of bees and larvae. One mite alone is not a problem, but mites multiply rapidly in capped brood (one mother can produce 50 to 100 offspring within the brood season).

The real problem is not the mite itself, but the viruses it transmits. Especially the Deformed Wing Virus (DWV) is deadly. An untreated colony in the Netherlands has a 90 to 95 percent chance of dying within 1-2 years, usually in winter, often with an empty or near-empty hive.

Step 1: measure first, then treat

This is the most important part of this whole article.

The biggest mistake I see beginners (and myself, years ago) make: blindly following a treatment protocol without knowing how infested your colony actually is. That costs money, time, and gives you no idea whether it is working.

There are three methods to measure your mite pressure:

1. Natural drop (varroa floor under mesh). Simplest method. Count the fallen mites on your inspection tray for a few days in a row. Cheap, non-invasive, but less accurate. Gives you a trend, not an exact number.

2. Powdered sugar method. Collect 200-300 bees in a jar, add a spoonful of powdered sugar, shake, and tip through a fine sieve. Count the mites that come along. Bees survive, you get a fairly accurate snapshot.

Honest aside: I still remember the first time we did this during the course. It looks a bit pitiful, that shaking with the jar of bees. Yet to my mind this is the best practical method for the average beekeeper. I have applied it a few times since, and the numbers it produces are far more useful than a gut feeling at the inspection tray.

3. Ether/alcohol method (alcohol wash). The same 200-300 bees, but in alcohol or beeswax oil. The bees do not survive this. But: this is by far the most accurate method (also catches mites buried deep in the body hair).

When should you treat?

• Spring (April-May): more than 1 mite per day on natural drop. Your colony is already heavily infested.
• Summer (June-July): more than 5-10 mites per day.
• Autumn (August): more than 5 mites per day means: treat now, otherwise you will not produce healthy winter bees.

Step 2: choose your method

There are four main routes in the Netherlands. They do not exclude each other, you typically use them in combination across the year.

1. Thymol (Apiguard, Thymovar)

What: gel strips or trays that release thymol vapour (essential oil from thyme). When: after the summer harvest, July to September.

Pros:

• Easy to use (strip on the frames, done)
• Proven effective, often 80-90 percent mite kill
• Allowed in organic beekeeping

Cons:

• Only works at temperatures between 15 and 30C
• Treatment lasts 4-6 weeks
• Strong smell, so only after the honey harvest (otherwise it gets into your honey)

2. Formic acid (FAM, MAQS)

What: formic acid 60 or 85 percent, in liquid form or as strip. When: after the summer harvest, or emergency treatment during the season.

Pros:

• The only product that also works in capped brood. Mites are 80 percent of the time hidden in brood, so this is unique.
• Works fast (within a few days)
• Allowed in organic beekeeping

Cons:

• Highly weather-dependent (15-25C ideal)
• Wrong use can kill the queen
• Unpleasant for the beekeeper. Good gloves and goggles are not a luxury.

3. Oxalic acid

What: acid, in powder form dissolved in sugar water or vaporised as fume. When: November to January, in the broodless period (winter).

Pros:

• Highly effective: 90-95 percent mite kill in one treatment
• Cheap (a few euros per hive)
• No residue in honey
• Allowed in organic beekeeping

Cons:

• Only works broodless. So only in winter, or after artificial brood interruption. In summer it is pointless.
• Vaporising requires equipment (200-400 euros). Trickling is cheaper, but less friendly to the bees.

4. Biotechnical (without chemistry)

What: beekeeping techniques that reduce mite population without products.

The three most-used methods:

• Drone brood removal: varroa has a strong preference for drone brood. Place a drone frame, let it fully grow, cut it out before it emerges. Many mites die with it.
• Brood interruption: cage the queen for 21 days. No brood, no hiding place for mites, then you can effectively apply oxalic acid.
• Splitting colonies: deliberately splitting colonies disrupts the mite cycle.

Pros:

• No chemistry
• Makes you a better beekeeper (you learn your colony better)

Cons:

• Time-intensive
• Almost never enough as the only method. Usually combined with chemical treatment.

The Dutch beekeeping year for varroa

A workable schedule, based on what most associations advise:

• April-May: first measurement (natural drop). No action unless pressure is high.
• June: possibly drone brood removal in strong colonies.
• July (immediately after honey harvest): summer treatment with thymol or formic acid.
• August-September: check measurement. Second treatment if pressure is still high.
• December (colony is broodless): oxalic acid trickling or vaporising.
• January-February: check measurement. Evaluate result of winter treatment.

The three biggest misconceptions about varroa

Misconception 1: "One treatment per year is enough." Often not. Most beekeepers treat 2-3 times per year. One summer treatment plus one winter treatment is what most associations advise as a minimum.

Misconception 2: "Organic beekeepers do not treat." This is a stubborn misconception. Organic beekeepers do treat, just with different products (formic acid, oxalic acid, thymol). All organically allowed. What they do not use are synthetic products like amitraz or fluvalinate.

Misconception 3: "My colony looks healthy, so no treatment needed." By the time you see symptoms (deformed wings, slow flying, shrinking colony), your treatment window is usually past. Only measuring gives you reliable information.

Honest note from my own practice

In an earlier article I wrote it already: I have once let a colony go an entire season without treatment. Deliberately, to see what would happen. That colony just survived, and at the spring count it had no more or less varroa than my treated colonies. Sometimes I do not understand it myself anymore.

What that taught me: be humble about what we know for certain. The science around varroa is still evolving. Some bee strains develop resistance. Some mite populations become resistant to products. What was the gold standard ten years ago may be outdated today.

My personal approach: measuring is my best friend, blindly following a protocol is my enemy. In practice it often comes down to the following: once a year formic acid in July, after the harvest. That seems to work well for my colonies every year. But note: I do not claim this is the best route. I am open to other ideas, and if you use a combination that works for you, please react on the forum. This article gets better with every experienced beekeeper who thinks along.

What if you have doubts?

Doubt is a healthy signal with varroa. What I recommend if you are just starting:

• Join your association. Many associations organise joint treatment days, especially for the oxalic acid vaporising method (handy, because one beekeeper can vaporise for ten others).
• Start with the standard NBV route (summer thymol or formic acid, winter oxalic acid). That works for most beekeepers in most situations.
• Keep a logbook. Counts, treatment dates, weather conditions. After three years you have a goldmine of personal data.
• Ask your questions. On the Imkerplein forum, in the Bee Health category. No question is too silly. Especially not about varroa.

Finally

Varroa is to Dutch beekeeping what weather is to cycling: you cannot avoid it, it is always unpredictable, and the best beekeepers are not those who know everything for certain, but those who keep their eyes open and keep learning.

Got additions or corrections? Post them in the forum. This article is a living document, and your experience makes it better.

Good luck with your counts.