Winter Oxalic Acid: Treating in the Broodless Window

The Short Answer

In late fall and early winter, most colonies in cold temperate climates go through a spell with little or no capped brood before the queen starts laying again around midwinter. During that spell every varroa mite in the colony is riding on an adult bee, where oxalic acid can reach it. One well-timed oxalic acid treatment in that window can do more than repeated treatments while the colony is still rearing brood.

Three things decide whether it works: confirming you are really in the window, using the dose on the label in your hand (the US vapor rate changed in 2025), and choosing a day mild enough that the cluster is loose.

We're software engineers, not bee inspectors. This post explains why the window exists, how to tell you're in it, and what the labels and extension services say. Your product label, your state apiarist and your local association have the final word on what you apply.

Why the Winter Window Exists

Varroa mites breed inside capped brood cells. While a colony is rearing brood, a large share of the mites are sealed in with the developing bees, out of reach of anything that only works on contact. Oxalic acid is a contact treatment. The US label is blunt about it: use it "when little or no brood is present", because it "will not control Varroa mites in capped brood and may damage bee brood."

Brood rearing shrinks through the fall. Research on colonies through the winter found little brood before the winter solstice and a rapid increase after it, and honey bees are known for restarting brood rearing in midwinter. So in the north, the gap usually falls somewhere between late fall and the solstice.

When there's no capped brood, there's nowhere for a mite to hide. The Honey Bee Health Coalition lists brand-reported efficacy for oxalic acid of 82 to 99% when brood is not present, and describes it as less effective with capped brood. This is the same principle as the summer brood break, at the opposite end of the year. Our summer broodless window post covers the August version.

When the Window Opens Where You Are

The window moves with climate, season and bee stock, so a national calendar is only a starting point:

A mild autumn can keep a queen laying well past the date you expected. If you keep bees somewhere with short, mild winters, your colonies may never go fully broodless, and the winter oxalic treatment loses most of its advantage. Your local association will know whether that's true where you are.

Is This Colony Actually Broodless?

There's no single test that settles it without opening the hive. There are signals, and they carry different weight:

If you're going to open the hive to look, you may as well be ready to dribble while it's open, so the colony pays the heat cost once.

What the US Label Says

One of the main oxalic acid products sold in the US is Api-Bioxal (EPA Reg. No. 73291-2). Its label was amended in 2025, and the vapor rate went up. These are the label's own directions:

A few other label lines matter in winter:

• "DO NOT use in enclosed overwintering areas." Colonies wintered indoors are off-label.
• Oxalic acid can be used with honey supers on the hive in the US. That isn't true in Canada or the UK.
• Protective equipment is on the label, not optional. Chemical-resistant gloves and eye protection for both methods. A particulate respirator for dribbling. For vapor, an elastomeric half-mask respirator with acid-gas cartridges.

The 1 g vs 4 g question. Many guides still quote 1 g per brood box for vaporizing. That was the rate on earlier Api-Bioxal labels, and it's still the rate on the generic oxalic acid dihydrate label (EPA Reg. No. 91266-1). The current Api-Bioxal label says 4.0 g per brood chamber, and the Honey Bee Health Coalition's 2026 guide and WSU Extension both list 4 g. EPA allowed old-label stock to keep selling for a period after the change, so this winter you may find either label on the shelf. Read the one on your package. Other products have their own rates. EZ-OX, for example, is labelled at 2 to 4 g per deep.

Why the dribble strength looks different everywhere. The same recipe, 35 g of oxalic acid dihydrate in 1 L of 1:1 syrup, is described as 2.8% on the EPA label, about 3% by NC State, and 3.5% by other sources, depending on how the percentage is calculated. Go by the grams, not the percentage.

Outside the US. Canada's registered oxalic acid dihydrate uses the same 35 g per litre, 5 mL per seam and 50 mL maximum for the dribble, once per season, with a vapor rate of 1 g for a single brood chamber and 2 g for a double (per Manitoba's 2026 treatment guide). The UK's Api-Bioxal is a different formulation, with a vapor dose of 2 g per hive once a year and no use with supers on. Use the label for your country.

One Dribble, Not Several

For the trickle method, once is the plan. A major review of oxalic acid research concluded that a single treatment during the broodless period works, while repeated treatments were poorly tolerated by the bees, with high bee mortality and low efficacy. NC State Extension says that, done correctly during broodlessness, one application is all that's needed. Canada's label allows the dribble once per season, and the UK's allows each treatment only as a single administration.

The US label doesn't set a number of applications. That doesn't make repeat dribbles a good idea for a wintering colony. A second dose hits the same bees that have to carry the colony to spring.

Repeated vaporizing isn't a way around poor timing either. WSU Extension cites research that found seven vaporizations at 1 g every five days ineffective in colonies still rearing brood. Timing does the work, not the number of treatments.

Temperature and the Cluster

The US label doesn't set a temperature limit. The guidance around it does:

• The Honey Bee Health Coalition notes risks when opening a hive below 40°F (4°C), and that a tight cluster can make treatment less effective.
• The University of Minnesota Bee Lab recommends late fall, when bees are loosely clustered, at about 40 to 50°F.

In practice: pick a calm, dry day in the window when the cluster is loose, not the coldest week of the season. The dribble means lifting the lid and working over the cluster, so be quick. Vapor goes in through the entrance with the hive closed and sealed, which is one reason some beekeepers prefer it for winter.

Count Before, Check After

A winter treatment is a reset, not a guess. The Honey Bee Health Coalition's thresholds put prompt control at 1% or more (1 mite per 100 bees) in the dormant phase, and recommend sampling just before the bees cluster to see how healthy they are going into winter. Our varroa treatment guide covers the alcohol wash and the full treatment calendar, and the varroa calculator turns a count into a percentage.

The same guide says sampling during the dormant phase matters less, because mite reproduction is low and it can be too cold to sample safely, and that you should always repeat sampling after a treatment. So the useful follow-up is a wash once milder conditions allow, usually at the first proper spring inspection. If that count is already climbing, either the treatment missed the window or mites came in from somewhere else. Either way, the date you wrote down in December is what tells you which.

When to Leave It

• Brood is clearly present. You'd be treating the mites you can reach and leaving the rest, and the label warns about brood damage. Wait, or treat in the window you have.
• The colony is small or weak. The label itself warns that the dribble can cause bee losses in weak colonies. A small cluster may not survive the disturbance and the dose.
• The bees are wintered indoors. Off-label.
• It's bitterly cold or windy. Opening the hive costs more than it gains. Wait for a mild day.
• You treated in the fall and your count is low. A winter treatment you don't need is still a dose the colony has to tolerate. For why winter losses really happen, see do beehives die in winter.

The HiveSense Angle

Almost every judgement in this post compares something to last time: when the window opened, what the count was going in, whether the spring wash says it worked. That comparison only exists if you wrote it down. In HiveSense you log the treatment, the product and the date against each hive, record mite counts at inspections, and set a task to remind you to check, all offline at the yard, with voice notes if your hands are cold and full. Free for up to 15 hives. If you run BroodMinder or BEEP sensors on Pro, the brood-temperature trace sits on the same timeline, so next year's broodless window is a line you can look back at. The varroa tracker page has the details.

Frequently Asked Questions

When is the winter broodless window?

In cold temperate climates it usually falls in late fall and early winter, after the queen stops laying and before brood rearing restarts, which in many colonies begins around the winter solstice. Extension guidance for North Carolina puts the ideal treatment time in late October or early November; the University of Minnesota Bee Lab says late fall, when bees are loosely clustered. Mild autumns and warm regions shift it, and some southern colonies are never fully broodless.

How much oxalic acid do I use for vaporization?

Use the rate on the label of the product in your hand. In the US, the current Api-Bioxal label (EPA Reg. No. 73291-2, dated 2025) says 4.0 g per brood chamber; earlier Api-Bioxal labels and the generic oxalic acid dihydrate label say 1.0 g. EZ-OX tablets are labelled at 2 to 4 g per deep. Canada and the UK have their own labels with different rates.

Can I dribble oxalic acid more than once in winter?

Plan on one. Research on the trickle method found that a single treatment during the broodless period is effective, while repeated treatments were poorly tolerated and caused high bee mortality. NC State Extension says one application is enough if it is done correctly during broodlessness. The Canadian label limits the dribble to once per season.

What temperature is too cold to treat with oxalic acid?

The US label sets no temperature limit, but the Honey Bee Health Coalition notes risks when opening a hive below 40°F (4°C), and that a tight cluster can make treatment less effective. The University of Minnesota Bee Lab recommends treating in late fall when bees are loosely clustered, at about 40 to 50°F. Pick a calm, mild day rather than the coldest week.

Does oxalic acid work if there is brood in the hive?

Only partly. Oxalic acid kills mites on adult bees and does not reach mites sealed in capped brood cells, and the label warns it may damage brood. The Honey Bee Health Coalition lists brand-reported efficacy of 82 to 99% when brood is not present, and less with capped brood. That gap is the whole reason to time it to the broodless window.

Sources

• Api-Bioxal US label, EPA Reg. No. 73291-2 (2025) and the earlier 2024 label
• Oxalic Acid Dihydrate US label, EPA Reg. No. 91266-1
• EZ-OX US label, EPA Reg. No. 101743-2
• Honey Bee Health Coalition, Tools for Varroa Management, 9th edition (2026)
• NC State Extension: Oxalic Acid for the Control of Varroa Mites
• University of Minnesota Bee Lab: Varroa mite testing
• Penn State López-Uribe Lab: Oxalic acid treatment protocols
• WSU Extension FS403E: Oxalic Acid Options for Controlling Varroa
• Manitoba Honey Bee Health Treatment Guide (2026)
• UK VMD: Api-Bioxal summary of product characteristics
• Rademacher and Harz (2006), Oxalic acid for the control of varroosis in honey bee colonies, Apidologie
• Avitabile (1978), Brood rearing in honeybee colonies from late autumn to early spring, Journal of Apicultural Research