Oxalic Acid Vaporization Treatment Schedule: A Month-by-Month Calendar

Oxalic Acid Vaporization Treatment Schedule: A Month-by-Month Calendar

If you take one thing from this page: most of your year’s varroa control happens on two days. The last warm day of autumn and the first workable day of late winter, when the colony is broodless and every mite in the hive is exposed at once.

Everything else on this calendar is monitoring, and reacting to what the monitoring tells you.

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The calendar

Written for a temperate US climate. Shift everything earlier as you go north and later as you go south. If brood never stops where you are, read the warm-climate section below — this calendar will not work for you as written.

Why August is the month that decides your winter

Beekeepers lose colonies in January and blame the cold. The cold is rarely the cause.

Winter bees are physiologically different from summer bees — fat, long-lived, built to last months rather than weeks. They are raised in late summer and early autumn. A varroa mite feeding on a pupa is eating the fat body, the exact organ that makes a winter bee a winter bee, as Ramsey and colleagues established in 2019.

So a colony with a high mite load in August raises a generation of winter bees that look normal and are not. They die early, the cluster shrinks below the size it needs to hold heat, and in January you find a small dead cluster with honey still in the box. The damage was done five months earlier. We follow that mechanism through in winter bees and why colonies die in January.

Treating in November does not undo it. August is when the decision matters.

The two treatments that do most of the work

A broodless colony is the only situation where a contact treatment reaches the entire mite population at once. Al Toufailia and colleagues measured 97% kill from a single 2.25 g sublimation on broodless winter colonies — an efficacy number that is simply unavailable at any other point in the year.

The mechanics are easy. Wand through the entrance, entrance blocked, lid stays on, cluster undisturbed. You are not opening the hive in December. Finding the window is covered in when is my hive broodless, dose per box in how much oxalic acid per hive.

If you keep bees where brood never stops

Gulf Coast, Florida, South Texas, coastal California — this calendar does not apply to you cleanly, and you should know why before you follow it.

With brood in the hive year-round, most of your mites are sealed under wax cappings at any given moment, and oxalic acid vapor does not penetrate cappings. A single vaporization into a brood-right colony reaches only the mites riding on adult bees. That is not a failure of oxalic acid — it is a contact treatment doing exactly what a contact treatment does — but it means one shot will never be enough in a warm climate.

Your options are a proper series spaced across a full brood cycle, a treatment that penetrates capped brood for the autumn work, or making your own broodless window by caging the queen. All three are legitimate. Treating once and feeling finished is not.

Rules that apply in every month

• Count before, count after. A treatment you did not measure is a treatment you are guessing about. Five minutes with an alcohol wash settles it.
• Two grams per deep brood box. Not per hive, per box. Do not answer a timing problem with a bigger dose — no amount of oxalic acid opens a capping.
• Read your label about supers. The residue science is reassuring, but the label is the law. Details in treating during the honey flow.
• Respirator, every time. The EPA label requires one and the requirement is warranted. Hot oxalic acid vapor is corrosive to eyes and airways.
• Treat the whole yard together. Mites move between colonies. Treating one hive while its neighbours seethe just restocks it.
• Write it down. Date, dose, count before, count after. Two seasons of that and you will know which of your colonies and which of your queens actually hold their numbers.

Related reading

• How much oxalic acid per hive: dose by box
• When is my hive broodless?
• Varroa mite life cycle: why timing beats dosage
• How to use an oxalic acid vaporizer safely

Sources

• Al Toufailia, H., Scandian, L. & Ratnieks, F.L.W., “Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid”, Journal of Apicultural Research 54(2):108–120, 2015
• Ramsey, S.D. et al., “Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph”, PNAS 116(5):1792–1801, 2019
• EZ-OX Tablets label, EPA Reg. No. 101743-2, accepted 27 January 2026, US EPA

Happy Beekeeping from the Lorob Bees Team

General beekeeping information, not a substitute for the product label. Always read and follow the label of any product you use in a hive.