When to treat bees with oxalic acid in fall
Oxalic acid timing follows the brood. This page collects what two sources say about it, and walks one mite count through the varroa calculator to see whether there is anything to treat. It gives no dose or method. Those come from the product label.
Why the broodless window matters
NC State Extension's publication Oxalic Acid for the Control of Varroa Mites (Tarpy and Keller, December 2025) says oxalic acid is not effective at killing mites underneath cappings. It says the best time to apply it is a time of broodlessness, and that going into winter is an ideal time. For fall, it says nighttime temperatures begin to drop in North Carolina in late October or early November, an ideal time of year to treat because capped brood becomes less common. Your dates will differ with your climate.
The same publication says to follow the label, and to read the label first for vaporization because the recommendations have changed over time. For the dribble method it says one application is usually enough, if it is done correctly in a broodless period. It also says oxalic acid should be one of several treatments in rotation.
Honey Bee Suite makes the same point about capped cells in its article on vaporizing with brood present: mites under capped brood are protected from the crystals, and mites that emerge with new bees survive. In a reply to a reader (November 2019), Rusty Burlew writes: "I believe treatment needs to be based on testing, not on the calendar."
What a count tells you before you treat
An alcohol wash is the testing step. Say you wash a half cup of bees (about 300 bees) and count 6 mites. That is 6 ÷ 300 × 100 = 2.0 mites per 100 bees. The Honey Bee Health Coalition's Tools for Varroa Management (9th edition, Table 1) uses 1 per 100 bees for the Dormant phase, which is when the colony is clustering and has little or no brood. So this count is at or above it. A count taken earlier in fall, before the cluster forms, may fall under Population Decrease instead, where the same table uses 2.
My suggestion: a count well under the threshold in November is a reason to test again later. A count over it, with the brood already gone from the combs you can see, is the situation these sources describe. A colony that is still raising brood is a harder call, and your state apiarist or local club is the better place to ask.
Enter your own wash in the varroa mite calculator, set to the dormant phase. Sources count 300 or 400 bees per half cup, and the calculator shows both.
Questions to bring to your club
- Is there still capped brood? Look at a couple of frames before deciding the window has started.
- What is the latest wash count, and what threshold does your source use for this phase?
- Does the label of your product cover the method and the temperature you plan to work at?
- When will you test again to see whether the count dropped?
Feeding has its own fall deadline. See when to stop feeding syrup and how much fondant to give in winter.