Broodless Window Finder: When Is My Hive Broodless?
The broodless window is the best oxalic acid opportunity of the year: with no capped brood in the hive, every varroa mite is riding on an adult bee where a single vaporization can reach it. The problem is knowing when it happens where you live. This tool pulls the last three winters of daily temperatures for your ZIP code, works out when cold weather shut down brood rearing each year, and gives you the dates to check your hives and treat.
Tool Broodless Window Finder
What we saw in recent winters at
| Winter | Brood rearing shut down | Window opened | Window closed | Length |
|---|
Temperatures from the ERA5 reanalysis via Open-Meteo. Dates are estimates for planning; a frame check is the only way to know a hive is broodless.
How the finder works
A colony goes broodless when the queen stops laying and the last capped brood emerges. Queens stop for two reasons: forage ends and sustained cold keeps the cluster tight. The tool looks for that point in each of the last three winters, then adds the time the last brood takes to emerge, and closes the window when lengthening days restart laying in early January or a warm spell arrives first.
- Shutdown. The first day the 7-day average temperature falls below 45°F and stays cold for the following two weeks, or the first hard freeze (28°F), whichever comes first.
- Window opens. About two weeks after shutdown. Queens taper off before they stop completely, and worker brood is capped for 12 days, so the last sealed cells are usually empty within two to three weeks.
- Window closes. January 10, when day length has been increasing for three weeks and most queens resume laying, or earlier if a week averaging highs of 60°F or more arrives in December.
- Typical vs. range. The headline dates are the median of the three winters; the range shows how much they moved. Plan for the typical, watch the weather for the range.
If you have no window. Gulf Coast, Florida, coastal California and much of the Deep South often have brood every month. Two workable answers: make your own window by caging the queen for about 24 days and treating once when the last brood has emerged, or run a proper series of treatments spaced across a full brood cycle. What does not work is treating once into a brood-right colony and calling it done.
What to do in the window
- Confirm it. On a mild day, lift the inner cover and pull one frame from the center of the cluster. No eggs, no larvae, no capped worker brood: you're in.
- Treat once. 2 g of EPA-registered oxalic acid per brood box, up to the label maximum, on a dry day above freezing. Our dosage guide has the numbers; the safety guide has the PPE.
- Count. A sticky board for three days after treatment shows the drop. It will be the largest drop you see all year, because every mite in the hive was exposed.
- Don't over-do it. One treatment in a truly broodless hive reaches roughly 97% of the mites. A second one a week later adds little and stresses a clustered colony.
Frequently asked questions
Why does the broodless window matter for oxalic acid?
Oxalic acid does not penetrate wax cappings, so it only reaches the mites riding on adult bees. In a broodless colony that is every mite in the hive, which is why a single vaporization in the window reaches around 97% of the mites, while the same treatment with brood present only reaches the minority that are not sealed in cells.
How does the tool estimate my broodless window?
It pulls the last three winters of daily temperatures for your location, finds the point each year where sustained cold or a hard freeze shuts down foraging and brood rearing, adds the two weeks or so it takes the last capped brood to emerge, and closes the window when day length restarts laying in early January or a warm spell arrives first. The result is a typical window with the range seen in recent winters.
What if I live where it never gets cold?
Along the Gulf Coast, in Florida, coastal California and much of the Deep South the queen may never stop laying. The tool will tell you when a natural window is unlikely and offer the two workable answers: make your own window by caging the queen for about 24 days, or run a series of treatments spaced across a full brood cycle.
How do I confirm I'm actually broodless?
Pull a frame from the center of the cluster on a mild day. No eggs, no larvae and no capped worker brood means you're in the window. A few cells of capped brood is close enough; a full frame is not. The estimate tells you when to look, the frame tells you the truth.
What temperature can I vaporize at in winter?
Oxalic acid vaporization works on a clustered colony. Pick a dry day above freezing, ideally 40°F or warmer, with little wind, and treat in the middle of the day. Our guide to vaporizer temperature covers the unit; this is about the weather.
More free beekeeping tools
See all Lorob Bees beekeeping tools →
Related reading
- When is my hive broodless? The full guide to finding the window
- The varroa mite life cycle: why timing beats dosage
- The oxalic acid treatment schedule, month by month
- Why colonies die in January
- Does oxalic acid vaporization work? What the studies show
One treatment in the window beats five at the wrong time
The InstantVap oxalic acid vaporizer runs on the 18V battery you already own and turns a 2 g dose in as little as 10 seconds. Pair it with EPA-registered EZ-OX and a full-face respirator. Not sure which model? Try the InstantVap Sizer.
General beekeeping information from Lorob Bees, not a substitute for a product label or your state apiary inspector. Always read and follow the label of any product you use in a hive.