Varroa Mites in Backyard Beehives: Counting, Thresholds, and Treating on Schedule

The Short Version

Varroa mites are the number one killer of backyard beehives in North America. They do not just eat your bees. They spread viruses, weaken the bees that have to survive winter, and quietly turn a strong colony into a weak one over a single season.

The good news: varroa management is not hard, and it does not require fancy equipment. It is one skill you learn once and repeat every year: count the mites, compare the count to a threshold, and treat when it is time.

• Count mites every 3 to 4 weeks during the season, using an alcohol wash on a cupful of bees.
• Treat at 2 percent in summer, 1 percent in fall, which is the threshold most extension services now recommend.
• Time matters more than product. The same treatment can be great in one season and useless in another.
• Keep a written log. Mite counts before and after treatment are how you know if your treatment actually worked.

Why Mites Matter More Than Anything Else You Are Frightened Of

Beginners usually worry about bears, wasps, or queenlessness. Mites are the realistic threat. A colony with an uncontrolled mite load typically shows signs in late summer:

• Bees with deformed wings and crumpled bodies, which are infected with viruses the mite passes along
• Spotty brood patterns, with gaps where larvae died
• A colony that looks fine in May and is a dud by October

Mites reproduce inside the capped brood cells, so the worst of the damage happens where you cannot see it. That is why you cannot judge a hive by how busy it looks. You have to count.

Also know this: mites do not respect fence lines. Foragers from a collapsing hive bring mites to healthy neighbors. Your hive and your neighbor's hive are part of the same problem, and treating your own well is the main thing you can do about it.

The Alcohol Wash: The Core Skill

The alcohol wash is a five minute test that tells you the mite level on adult bees. Do it on a warm, sunny day when the bees are flying.

You will need:

• A cup or small jar with a lid, like a mayonnaise jar
• 30 percent isopropyl alcohol (rubbing alcohol mixed with water works fine)
• A marker for the lid, if you are testing several hives

The procedure:

1. Take a frame of open brood from the middle of the hive, preferably one with older larvae.
2. Brush a cupful of bees, about 200, into the jar from that frame.
3. Add about half a cup of 30 percent alcohol.
4. Shake the jar hard for about 30 seconds, then let it sit for 2 minutes so the mites fall to the bottom.
5. Count the mites in the alcohol.

The math: you have about 200 bees, so divide the mite count by 2. That gives you mites per 100 bees, which is how thresholds are quoted. If you shake out 8 mites, your colony is at 4 percent.

Do the wash the same way every time. Consistency is what makes the numbers comparable from month to month. Write the date and the count in a notebook. That log is the most valuable tool in your apiary.

Two honest limits of the wash: it measures mites riding on adult bees, not the ones hidden in capped brood, so the number is a proxy. And on a cool day, fewer bees are out, so the sample is thinner. A second method, the sugar shake, works the same way with powdered sugar instead of alcohol, and the drone brood check (uncovering a handful of capped drone cells and counting mites) gives you a rough read of the brood itself. Beginners who master the alcohol wash and the drone check have more than enough.

The Numbers That Matter: Thresholds

You do not want to treat a clean hive, and you do not want to ignore a loaded one. The thresholds widely recommended by US extension services and the Honey Bee Health Coalition are:

• Summer (active brood season): treat at 2 mites per 100 bees (2 percent).
• Fall, before winter bees are raised: treat at 1 mite per 100 bees (1 percent).
• Winter, during a broodless period: oxalic acid is worth using even at lower counts, because it is cheap, low residue, and it resets the colony for spring.

The fall threshold is stricter on purpose. Mites that ride into the last brood batch are the ones that live inside the winter bees, and winter bees are the ones that have to carry the colony through. A colony at 2 percent in September is a colony at risk in February, even if it looks strong.

Choosing a Treatment: The Practical Landscape

Treatments split into two groups, and the difference decides when each one works.

Treatments that work while brood is being raised

• Formic acid (Formic Pro, MAQS): evaporates as a mist and can reach mites inside capped brood cells. This is the main summer option. It is sensitive to temperature. Follow the label's temperature range carefully, because cold weather and very hot weather both hurt its performance.
• Amitraz (Apivar): a strip that has worked well for years, but resistance is now common in parts of the country. It also works poorly above about 90 degrees Fahrenheit, which rules it out for late summer in warm climates.
• Thymol (Apiguard): generally gentle on the colony, but thymol resistance is showing up too, and it only works when the hive is warm enough for the oil to evaporate.

Oxalic acid: the winter workhorse

Oxalic acid cannot get through a capped brood cell. When brood is present, it only touches the mites on the surface, so it looks disappointing. But when the colony is broodless, nearly every mite is on an adult bee, and a single oxalic dose in winter can achieve very high kill rates.

• Vaporization (a small electric device) takes a couple of minutes per hive and is the cleanest method.
• Trickling a sugar and oxalic solution is cheaper and works, but it is more work and more slop.
• In the north, the natural broodless window is roughly late fall through mid winter. In milder climates, the queen keeps laying all winter, so oxalic is less effective there, and a brood break becomes the tool.

The brood break is a classic late summer move: trap the queen or box the colony for about 3 to 4 days, then treat with oxalic. It is very effective, but it must come after the main honey flow is done, and you must watch what your treatment label says about honey harvest restrictions. If your harvest season is still running, a brood break is probably not your summer move.

Honey Safety: The Part People Skip

Whatever you treat with, the hive was making honey while you did it. The rules that keep you out of trouble:

• Use only products registered for use on honey bees, and follow the label. The label is not a suggestion. It sets temperature limits and, for some products, a withdrawal period before honey can be taken.
• Never treat in the middle of the harvest season unless the label explicitly allows it and you can keep that honey out of your jars.
• When in doubt, pull and set aside honey supers during a treatment, and do not sell or give away honey made during a treatment you are unsure about.
• If you treat in winter, there is no honey conflict at all, which is another quiet argument for the winter oxalic dose.

These products are designed to be safe for honey when used as labeled. The failures come from off label use, wrong temperatures, and treating during harvest and keeping the honey anyway.

A Simple First Year Plan

If you keep one or two hives, this sequence keeps you honest without turning beekeeping into a chemistry degree:

1. Spring: do an alcohol wash in the first warm week the bees are flying. Log the number.
2. Summer: wash every 3 to 4 weeks. If you hit 2 percent, run a formic acid treatment within the label's temperature window, after harvest is done.
3. Fall: wash again when the colony is preparing for winter. If you are at or above 1 percent, treat. If you used formic in summer, you can often wait for winter and let oxalic finish the job.
4. Winter, broodless period: a single vaporized oxalic dose. Then wash again in spring to see where the colony actually came out.

The before and after washes matter. If your spring count is barely lower than last fall's, your treatment did not do its job, and that tells you to pick a different product or different timing next year, not to treat the same way again hoping.

What Not to Do

• Do not use home remedies as treatment. Cinnamon, oregano, and garlic have no reliable effect on mites, and a season spent testing them is a season the colony may not survive.
• Do not treat a clean hive. Repeated treatment of a colony at 0.5 percent buys nothing and adds residue and cost.
• Do not treat in bad weather. Every product has temperature limits for a reason. A formic treatment on a cold morning is close to worthless, and a hot day can turn it into a hazard to the colony.
• Do not skip the log. Without numbers, you cannot tell a successful treatment from a lucky one.

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— C. Steward 🐐