Why varroa sets your calendar
Varroa destructor feeds on developing and adult bees and, worse, spreads viruses — deformed wing virus above all. Colonies that die of varroa usually don't die in summer, when you're watching. They die in late autumn or winter, because the bees raised in August and September were parasitised as pupae and never became the robust, long-lived winter bees the colony needed.
That single fact organises the whole treatment year: the most important intervention happens right after the summer harvest, so that the winter bees are raised under low mite pressure. Everything else — spring monitoring, drone brood removal, the midwinter oxalic round — supports that main event.
Measure before you treat
Treating blind means you never learn whether a treatment worked, and you can't compare colonies. Three monitoring methods are standard practice:
Alcohol wash
The most reliable count. Scoop roughly 300 adult bees — about half a cup — from a brood frame, after checking the queen is not on it. Swirl them in alcohol or winter windscreen-washer fluid, shake for a minute, and count the mites that come loose. The sample is sacrificed, which feels harsh, but 300 bees is a small price for a number you can act on.
Sugar roll
The same 300-bee sample, rolled in powdered sugar in a jar with a mesh lid, then shaken out over a white surface. The bees survive and can be returned to the hive. It tends to under-count slightly compared to a wash, but it's accurate enough for routine monitoring if you work methodically.
Sticky board
A tray or sheet under an open mesh floor catches the natural mite fall over several days. It costs the colony nothing and needs no handling, but the signal is noisy: mite fall varies with brood cycle, colony size and weather. Use it to watch trends and to check that a treatment is actually dropping mites — not as your only decision tool. Compare counts against your own colony's history rather than against absolute rules of thumb.
The 2% and 3% thresholds
Infestation is expressed as mites per 100 adult bees. With a 300-bee wash or roll, the arithmetic is simple:
- 6 mites in 300 bees = 2% — the alert level. During the season, plan a treatment soon.
- 9 mites in 300 bees = 3% — the action level. Treat without delay.
These are the widely used practical action thresholds (the Honey Bee Health Coalition's guidance follows the same lines, and HiveTrack's varroa charts draw them in for you). Two caveats: in spring, a lower count already deserves attention, because mite populations grow with the brood nest and a 1–2% load in April can become a crisis by August. And a low count in a single wash is not a guarantee — mites live mostly in capped brood, so repeat the measurement every few weeks through the season.
Timing for Belgium and the Netherlands
A workable varroa year in the Low Countries looks like this:
- Spring (April–June): monitor every few weeks. Removing capped drone brood is a useful biotechnical brake — mites prefer drone cells, so cutting out a filled drone frame removes a disproportionate share of them.
- July–August: the main treatment, immediately after the supers come off. Don't let this drift into September — the winter bees are being raised now, and they are the whole point.
- September: check that the treatment worked with a follow-up wash or roll. If counts are still high, apply a follow-up treatment with a different active ingredient.
- November–December: a single oxalic acid treatment in the broodless period, usually after the first sustained cold spell. With no capped brood to hide in, the remaining mites are exposed and one well-timed round is highly effective.
One autumn trap deserves its own warning: reinvasion. Robbing bees plundering a collapsing, mite-ridden colony nearby will carry mites home. A colony that measured clean in September is not guaranteed clean in November — keep a monitoring board in and glance at it through autumn.
Treatment options
| Active ingredient | Typical products | When it fits | Points to watch |
|---|---|---|---|
| Formic acid | MAQS, Formic Pro | Summer, right after harvest | The common option that also reaches mites under the brood cappings. Strongly temperature-dependent — follow the label's temperature window closely; heat spikes during treatment can stress queens and brood. |
| Thymol | Apiguard, Thymovar | Late summer, after harvest | Works by evaporation over several weeks, so it needs mild weather — check the label's temperature range. Never with supers on: thymol taints honey. |
| Oxalic acid | Trickle (dribble) or vaporisation | Broodless period, midwinter | Only kills mites on adult bees, not under cappings — hence the broodless timing. Trickle at most once per winter; repeated trickling is hard on the bees. Vaporisation requires proper respiratory protection. |
Whatever you use:
- Use only products authorised for beekeeping in your country, and follow the label — dose, duration, temperature.
- Rotate active ingredients between years to slow resistance.
- Never treat with honey supers on unless the label explicitly allows it.
Write the numbers down
A single mite count means little; the trend is everything. Six mites in July reads very differently if the June count was one than if it was five. Note the date, the method, the count and any treatment applied, per colony — it takes thirty seconds and it turns next year's decisions from guesswork into pattern-reading. Our inspection checklist covers what else is worth recording while the hive is open.
Track your mite counts in HiveTrack
HiveTrack logs alcohol wash, sugar roll and sticky board counts per hive and charts the trend with the 2% and 3% action thresholds drawn in — plus reminders when a treatment or follow-up check is due.
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