How I Time My Varroa Treatment Around A Cold Snap To Kill More Mites
- Laurence Edwards
- Dec 15, 2025
- 6 min read
Varroa is still the number one health threat to honey bees in the UK.
You feel it in spring when colonies that looked fine in autumn come out weak, slow and virus ridden. One of the biggest improvements I have made in my own beekeeping is learning to time winter oxalic acid treatments around a natural brood break, instead of just picking a random date on the calendar.
In this guide I will show you how I use the first proper cold snap as a trigger, apply a simple three week rule and then treat during a short window where most of the mites are on adult bees. The method is aimed at UK beekeepers and is based on what I actually do in my own apiaries.
Why Timing Matters For Oxalic Acid To Kill Varroa Mites
Oxalic acid only kills varroa mites that are on adult bees. It does not touch mites that are hidden under capped brood. That is why so much guidance talks about using oxalic acid during a broodless or very low brood period in winter. If you treat when there is still a lot of sealed brood, a large proportion of the mites survive and your oxalic treatment looks weak even though the product itself works very well.
Worker brood takes about twenty one days from egg to emergence. If the queen slows or stops laying during a cold snap, and you count forward three weeks from that point, you land in a natural low brood window. Most of the brood that was sealed before the cold snap has emerged, and a high percentage of the mites are forced out onto adult bees where oxalic acid can reach them.
How To Kill Varroa Mites Using Oxalic Acid Treatments
Step 1: Choose Your Winter Oxalic Method
Before you think about timing, you need to be clear on how you are going to apply oxalic acid and what is actually legal where you live. In the UK that usually means using a licensed product such as Varroxal or Api Bioxal by sublimation or trickle, following the label exactly.
In my own operation I prefer sublimation because it is quick, repeatable and fits well with the way I manage my boxes. At the moment I use Varroxal in combination with an Instant Vap Turbo vaporiser. It gives me fast heat up times and consistent dosing, which is exactly what I want when I am working multiple hives in cold weather. The important point is not the brand. The important point is that you pick a legal (gunk free) product and a delivery method you understand and stick with it.
Step 2: Use The First Real Cold Snap As Your Trigger
Queens do not lay at a constant rate all winter. When a proper cold snap hits, flying stops, clusters tighten and queens usually cut back strongly on egg laying. That change in behaviour is exactly what we can use to our advantage.
In my North Wales apiaries the first real winter hit this season landed on Friday 21 November 2025. Temperatures dropped, colonies pulled into tight clusters and flight activity stopped. That is the kind of event I am looking for. I mark that date in my notes as the start point for my winter oxalic timing.
Your date will not be the same as mine. It depends on where you are in the country and what the weather does on your sites. The principle is the same wherever you are. Watch for the first proper cold snap that clearly changes how your bees behave and use that as day zero.
Step 3: Apply The Three Week Rule And Set A Treatment Window
Once I have my cold snap date, I apply a simple three week rule. Worker brood takes about twenty one days from egg to emergence. If a queen has slowed sharply or stopped around that cold snap, by three weeks later most of the sealed worker brood from that period will have emerged.
Using my own example, three weeks after Friday 21 November 2025 lands on Friday 12 December 2025. That is when I expect the proportion of mites on adult bees to be at its highest, because there is far less sealed brood available for them to hide in.
Rather than chase a single perfect day, I set a short treatment window. This year my window is from Friday 12 December to Friday 19 December 2025. That gives me flexibility to pick a calm, dry day while still staying inside the period where I expect brood levels to be at their lowest following that initial cold snap.
Step 4: Treat Blind Without Pulling Brood Frames
One of the most important parts of my approach is what I do not do. I do not go digging through brood boxes in December trying to prove the colony is brood free. In my view that level of disturbance in winter is a much bigger risk to colonies than treating on a well chosen temperature window.
Rolling or killing a queen in winter because you insisted on seeing a broodless frame is an expensive mistake. Instead, I trust the biology and the weather pattern. The cold snap tells me when laying will have dropped. The twenty one day development cycle tells me when most of the brood from that period will have emerged.
On the treatment day I seal obvious draughts and entrances with old towels, then sublimate through the dedicated hole in my Abelo boxes or from above through a simple eke. I do not split boxes apart and I do not pull a single frame. The cluster stays intact, the queen stays protected and the oxalic acid vapour still penetrates the cluster very effectively.
Step 5: Pick The Right Day Inside The Window
For sublimation I actually prefer a slightly milder day inside that December window rather than the absolute coldest day of the year. If it is too cold, the bees pull into an extremely tight cluster and vapour has to fight its way into a very dense ball of bees. On a day that is clearly winter, but a touch softer, the cluster relaxes just enough to allow oxalic acid vapour to flow through the bees and around the combs.
So the sequence is straightforward. Use the first proper cold snap as your trigger. Count forward three weeks. Set a one week window around that date. Then watch the forecast inside that window and pick a calm, dry day that is cold but not brutal. Work efficiently, apply the treatment once in line with the product label and then leave the bees alone. My motto for this is, you want the warmest day possible, without ANY bees flying. Around 9-10c is perfect.
Safety, Legal Use And Common Sense
Oxalic acid is a powerful tool and needs to be treated with respect. I always use a licensed product, follow the label and stick to whatever routes of administration are allowed where I live. I wear proper personal protective equipment, including a suitable respirator, eye protection and gloves, and I keep other people and animals away from the treatment area.
It is also worth saying that an oxalic treatment in winter is not a complete varroa plan on its own. I still use other controls at different points in the season. The point of this method is to make that single winter oxalic treatment count as much as possible by aligning it with a natural low brood period instead of guessing.
Watch Next: My Varroa And Oxalic Acid Videos
If you want to see real colonies being treated and more detail on the products and equipment, these two videos from my channel are a good next step.
How To Kill Varroa Mites With Oxalic Acid Treatment
The Secret Weapon Against Varroa Mites
Recommended Playlist: Varroa
If you want everything in one place, this playlist brings together my core videos on understanding varroa and controlling it more effectively throughout the season.
See This On Social
For a quick look at oxalic sublimation kit in action, here is a Facebook post where I review the Varrox Eddy sublimator and talk through how I use oxalic acid to kill varroa mites in winter.
Bringing It All Together
Timing your winter varroa treatment around a cold snap is a simple but very effective upgrade to your beekeeping. Instead of guessing when colonies might be brood free, you use the weather and the brood cycle to create a logical treatment window where the majority of mites are exposed on adult bees.
Pick a legal oxalic product and a delivery method you are comfortable with. Use the first proper cold snap as your trigger. Count three weeks forward. Set a short window and treat blind without tearing colonies apart in winter. Done this way, a single oxalic treatment can make a big difference to mite levels and viral pressure, and give your bees a much better chance of exploding into spring.
If you want more structured, step by step help planning your varroa strategy across the whole year, as well as guidance on inspections, feeding, swarming and queen management, you can learn more inside my 14 Day Beekeeper online training here:
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