Varroa treatment: oxalic acid, formic acid, thymol, and the timing that decides it

Varroa is the reason most colonies die, and the reason is rarely the mite itself. It is the viruses the mite carries, the winter bees raised while the load was high, and the treatment that was done to a date rather than to a count. This guide is the version we would want on the wall of the honey house: how to count, what to use, when, and what to write down.

Count first

A mite count is the only reading that tells you whether to treat. Sightings on bees mean the load is already high; a clean-looking colony can be carrying plenty.

• Alcohol wash. Half a cup of bees, about three hundred, shaken from a brood frame, having first made sure the queen is not on it. Cover with alcohol or windscreen washer fluid, shake for a minute, count the mites in the liquid, divide by three. The result is mites per hundred bees. It kills the sample; it is also the most accurate method.
• Sugar roll. The same three hundred bees, a tablespoon of icing sugar, a jar with a mesh lid, a minute of rolling and a minute of resting, then shake the sugar out onto a white surface and count. Less accurate, and the bees go back.
• Sticky board. A tray under a mesh floor for three days, counting natural drop per day. Useful as a trend, poor as a threshold, because drop depends on brood and weather as much as on mites.

Thresholds vary by region and season. Much current guidance treats at two mites per hundred bees in spring and early summer and at three in late summer; some programmes use one in spring. What matters more than the exact figure is that you have one, and that you count the same way each time so the numbers compare. In the app the count is the varroa reading on the inspection row, so it sits next to the population and brood figures it should be read against.

The two treatments that matter, and the rest

If you only do two things a year, do these two.

1. Late summer, straight after the main harvest. The bees that will live through winter are being raised now, from August in the north and from February in the south, and they need to be raised in a colony with few mites. This is the treatment that decides whether the colony is alive in spring. Supers off, a full course of whatever your count and your weather allow, and a count three weeks later.
2. Midwinter, when the colony is broodless. A single oxalic acid dribble or vaporisation reaches nearly every mite, because none are hiding under cappings. December or January in the north, June or July in the south, on a day the cluster is tight.

Everything else is a response to a count: a spring treatment if the winter one did not hold, a mid-season one if the load climbs with supers on, and a second autumn course if the first did not move the number.

The families of treatment

Labels differ between countries and change over time, and the label on the product in your hand is the authority. What follows is the shape of each family, so that the label makes sense.

Oxalic acid

The workhorse of the broodless treatment. It kills mites on adult bees and does not penetrate cappings, so its timing is everything. As a dribble, a solution of oxalic acid in thin sugar syrup, commonly about three and a half per cent, is trickled along each seam of bees, about five millilitres per seam and around fifty per colony, once, when the colony is broodless. As vapour, a gram or two per brood box is sublimated into a closed hive; with brood present the labels repeat it three or more times at five to seven day intervals so each cohort of emerging mites meets a dose. Slow-release oxalic strips are registered in some countries and cover several weeks in one application. Cheap, effective, and no known resistance. The vapour is hazardous to you: respirator with acid-gas cartridges, eye protection, stand upwind.

Formic acid

The one that reaches mites under the cappings, which is why a course is short: pads or strips in the hive for one to three weeks depending on the product, typically at daytime temperatures between about ten and thirty degrees Celsius. Formic Pro and its relatives are labelled for use with honey supers on in several countries, which makes formic the usual answer to a high count in the middle of a flow. The risks are to the queen and to open brood in hot weather, and to the beekeeper's skin and eyes. Ventilate the hive as the label says and do not apply it in a heatwave.

Thymol

Gels and wafers that evaporate over two to four weeks, usually two applications a fortnight apart, and only in warm weather: too cold and nothing evaporates, too hot and the bees may beard out or abandon the brood box. It taints honey, so supers come off. Gentle on the bees within its window and with no resistance of note; the late-summer treatment for many beekeepers where the weather suits.

Amitraz and the synthetic pyrethroids

Plastic strips hung between brood frames for six to eight weeks, working continuously as bees brush past. They cover several brood cycles in one application, which is their advantage, and they leave residues in wax, which is one of their problems. The other is resistance: amitraz resistance is now documented in many regions, and the older pyrethroids, fluvalinate and flumethrin, are often ineffective where they have been used for years. Supers off, strips removed on the day the label says, and a count afterwards, because a strip that did nothing looks exactly like a strip that worked.

Without chemicals

Drone brood removal takes mites out with the drone comb they prefer; a caged queen or a made-up split creates a brood break that exposes every mite to a following oxalic dose; screened floors drop a small share. None of them replaces the two treatments above on their own, and all of them are worth recording, because a brood break shows up in the brood readings for a month afterwards and the trend engine should know why.

By country

• United States. Products are registered with the EPA and labels are federal law. Formic Pro and, since the 2021 label amendment, Api-Bioxal oxalic acid can be used with supers on; amitraz and thymol products cannot.
• United Kingdom and Ireland. Only VMD- and HPRA-authorised products are legal, treatments must be recorded as veterinary medicines, and the oxalic labels do not permit use with supers on. Formic products do.
• Australia. Varroa was found in New South Wales in 2022 and the national response moved from eradication to management in September 2023. Treatments are available under emergency permits from the APVMA, including amitraz, pyrethroid, formic and oxalic products; alcohol-wash monitoring and reporting requirements vary by state and change often, so the state department's current page is the source.
• New Zealand. Varroa has been established since 2000 and resistance to the older synthetics is widespread; registered products include amitraz, formic and thymol lines, and rotation between families is the standard advice.

A season plan

Northern-hemisphere months first, southern in brackets.

1. Early spring (early autumn): count at the first inspection. Treat only if the count says so; an early spring formic or oxalic course is possible if there is little brood.
2. Late spring and summer (late autumn and winter in reverse): count monthly. Consider drone brood removal or a brood break with splits. If the count crosses the threshold with supers on, formic where the label allows.
3. Late summer, after the harvest (late February to March): the main treatment. Supers off, full course, count three weeks later, second course if needed.
4. Autumn (autumn): count again before the cluster forms. A high count now means winter bees were compromised; a second course is still worth it.
5. Midwinter, broodless (June to July): one oxalic dose. Count in spring to confirm.

Every dose on that plan is a dated item. The app's treatment scheduler generates the course from the treatment type, puts each dose on the to-do list for the apiary, and stores the date you actually did it next to the date it was due, which is why the third dose stops being the one that slips.

What to write down

For every course: the product and dose, the date of each application, the hives treated, the temperature that week, the count before and the count three to four weeks after. In the UK and EU that record is a legal requirement for bees as food-producing animals; everywhere it is the only evidence you have that the treatment worked, and the only way to spot the year the strips stopped working before the colonies tell you in March.