bee-keeperIf there's one reason a modern beekeeper can't afford to relax, it's varroa. The mite Varroa destructor feeds on the bee's fat body and spreads viruses that deform wings and kill brood. A colony that looks strong in August can collapse by November if varroa isn't kept in check. The good news: varroa is manageable — but only if you monitor it, treat at the right moment, and then check that the treatment actually worked. This guide covers the whole loop: measure, decide, treat, verify.
Why it's so dangerous
Varroa breeds in capped brood, right where you can't see it. While brood is present, the mite population roughly doubles every month — a colony that carries a barely noticeable load in April can be overwhelmed by August. The problem isn't just the mite itself, but the viruses it spreads (deformed wing virus above all) — which is why even a modest number of mites can do great damage once viruses take hold. And mites travel: bees drifting and robbing carry them between colonies, so your neighbour's untreated hive is your problem too.
How to measure infestation
The biggest mistake is treating “blindly” or “when the neighbour does”. First measure how many mites you have:
- Alcohol wash — take about 300 bees (roughly half a cup) from a brood frame, making sure the queen isn't among them, shake them in alcohol and count the mites that fall; the most accurate method
- Powdered sugar roll — the same idea, but the bees survive; slightly less accurate but kinder
- A screened bottom board — count the mites that drop naturally over 24–72 hours; good for trends, rough as an absolute number
The goal is a number of mites per 100 bees, because only the number tells you whether and when to treat. Count monthly through the active season (April–October) — a single measurement in spring says nothing about August.
When it's too much
A rough rule used by many advisory services and in the bee-keeper app: 3 or more mites per 100 bees (3%) means treat now, at any point in the season. From late summer the bar drops: with winter bees about to be reared, even around 1 mite per 100 bees in August–September is a call to action, because the bees that must live until spring are the ones the mites will damage. Below that, keep watching — levels can climb fast.
Treatments and when to use them
- Oxalic acid — most effective when there is no brood (winter, at a cold 5–12 °C), because then no mite is hidden under cappings; applied by trickling or vaporization
- Formic acid — penetrates even capped brood, so it works with brood present (best at roughly 15–25 °C; follow the product label exactly)
- Thymol and other organic treatments — per the manufacturer's instructions, usually in late summer
- Synthetic strips (e.g. amitraz, flumethrin) — effective where mites are not resistant; never during a honey flow
Two rules apply to every product. First, follow the label — dose, duration and temperature range decide whether a treatment works. Second, rotate active ingredients between years: using the same chemistry season after season is how resistant mites are bred. The most important treatment of the year is the one right after the last harvest, in late summer — that's when you save the winter bees. An additional broodless-period oxalic treatment in late autumn or winter mops up whatever remains.
Verify it worked
A treatment is not a result — a low mite count after the treatment is. Count again 7–21 days after the treatment ends and compare with the pre-treatment number: the knockdown (the share of mites removed) should be high, and if the follow-up count shows the load barely moved, repeat with a different active ingredient rather than more of the same. This monitor → treat → verify loop is exactly what a varroa tracking tool automates: bee-keeper stores each count (you can photograph the wash and tap the mites to count them), flags when a check or recheck is due, computes the knockdown, and warns when the same active is used twice in a row.
Biotechnical methods
Chemistry isn't the only lever. Mites prefer drone brood, so a drone comb added in spring and cut out once capped removes mites with it — repeated regularly it takes real pressure off the colony. A brood break (caging the queen, or the natural break after a swarm or split) leaves all mites exposed on adult bees, where treatments reach them. These methods don't replace measuring — they buy you lower counts and fewer treatments.
The harvesting rule (withdrawal period)
After treating with a medicine, respect the prescribed withdrawal period — the time during which you must not harvest — so the honey stays clean and safe. Never treat with honey supers on unless the product's label explicitly allows it. In the app you log the treatment and it calculates and shows you a “do not harvest until” warning for that date, so you never mix medicine and honey.
Frequently asked questions
How often should I count mites?
Monthly through the active season, April to October, and always twice around a treatment: once before to justify it, once 7–21 days after to verify it worked.
Can I treat while the honey supers are on?
Only with products whose label explicitly allows use during a flow. For everything else: harvest first, then treat, and respect the withdrawal period before the next harvest.
My colony looked strong in September and was dead by December. Was it varroa?
Very often, yes. Mites and the viruses they spread damage the winter-bee generation in late summer; the colony looks populous but the bees are short-lived, and it dwindles or collapses in late autumn. The clue is a post-harvest mite count that was never done — or done too late.
What knockdown should a treatment achieve?
A well-applied treatment should remove the large majority of mites — as an internal alarm bee-keeper flags anything under about 70% knockdown as a failure. If your follow-up count is still high, re-treat with a different active ingredient.