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Where this comes from. Every dose, limit and safety instruction below is taken from the authorised product literature for Api-Bioxal, held on the Veterinary Medicines Directorate’s product information database, not from a supplier page. The monitoring and treatment-choice advice is the National Bee Unit’s. Sources are listed at the foot of the page - and the literature that came with your own product always outranks anything written here.
The brood decides everything
Oxalic acid works on the mites riding on adult bees. The Api-Bioxal literature puts it plainly: oxalic acid does not penetrate wax, so it will not kill mites within capped brood, and the presence of brood may noticeably reduce how well the product works. 1
In a colony with a good patch of sealed brood, most of the mites are exactly where the treatment cannot reach them. That is why the same product looks excellent one month and pointless the next. Nothing changed about the acid. What changed was how much of the mite population was out in the open.
So the literature does not tell you to treat on a date. It says to use the product only when the quantity of brood in the colony is non-existent or at its lowest levels. 1 That is the test.
Note that it is brood, not sealed brood. Oxalic acid is lethal to open brood. 3 A colony with eggs and young larvae and nothing capped passes a “no sealed brood” test and still loses that brood to the treatment.
Why September is usually too early
In most of the UK a colony still has brood in September. The queen is slowing down, not stopped. Treat then and you spend the dose on the minority of mites you can see.
The National Bee Unit asks for monitoring at least four times a year, and treats the October check as the one that tells you whether a winter oxalic acid treatment is needed at all. 2 That is the order to work in: the main treatment goes on once the honey is off in August, then you monitor, then you decide about winter. The August treatment is the one that protects the winter bees. Oxalic acid in the broodless window is the tidy-up, not the rescue.
Finding the window is a matter of looking rather than of picking a date. Take the roof off on a mild day around the turn of the year and look down the seams. Some years that is early December. After a warm autumn it is later, and in a mild winter in the south some colonies never go properly broodless at all.
Trickling: 5 ml a seam, and what the syrup has to be
The trickle solution is 4.4% w/v oxalic acid in 60% w/v sucrose syrup, made up as one 31 g sachet in 500 ml of syrup. 1
The syrup matters and it is easy to get wrong. The literature is specific that the 500 ml is made from 308 ml of water and 308 g of sucrose, which is a 1:1 syrup. 1 That is not the thick autumn feed in the sugar syrup guide, which is a heavier mix. Use the thick syrup by mistake and the solution is not the concentration the label describes.
The dose is 5 ml per seam of bees, meaning per gap between the top bars where bees are sitting, with a maximum of 50 ml per hive. 1 A colony covering six seams gets 30 ml. Count the seams rather than guessing at the colony: the dose follows the bees, so a small colony gets a small dose.
Once it is made up it keeps for 24 hours. 1 A bottle left over from last winter is not the product the literature describes. Warm it to roughly hive temperature first, work one seam at a time, and get the roof back on. The job is under a minute a hive.
Vaporising: 2 g, once a year, and fifteen minutes shut
The sublimation route is 2 g per hive as a single administration, using an electric vaporiser, with the hive kept shut for fifteen minutes afterwards to stop the bees and the smoke escaping. 1
Two limits on this route get missed, and both are in the same paragraph of the literature. The dose is also the maximum dose. And it is one treatment per year. 1 Repeating it at weekly intervals is a routine you will see described online, and it is not what the authorised literature allows.
Sublimation is also not recommended in summer. 1 So the brood-free slot that opens after a swarm or an artificial swarm is a trickling opportunity, not a vaporising one.
There is a temperature floor as well. The National Bee Unit’s own leaflet asks for an outside temperature above 3 °C for sublimation. 3 Worth knowing that the same leaflet prints 2.3 g where the authorised literature says 2 g. 31 Where the two disagree, the authorised literature is the legal document and it wins.
How many treatments, and it depends on the route
This is the part most often stated wrongly, because the answer is not one number.
- Trickling: up to two treatments per year, in winter and in the brood-free part of the spring and summer season. 1
- Vaporising: one treatment per year. 1
So a spring trickle does not use up your winter one, but a spring vaporisation uses up the only vaporisation you get.
Whichever route, it must not be used at the same time as another acaricide. 1 A thymol tray still on the hive is a treatment in progress. Finish it, record it, then decide about the next.
Treat the whole apiary together. The literature asks for all colonies in the same apiary to be treated simultaneously to avoid reinfestation, and the National Bee Unit says the same. 12 Treating the three that are broodless and leaving the two that are not means the mites simply move back. If some colonies are not ready, wait for them, or accept that the treated ones will be reinfested.
The product has to be an authorised medicine
Anything placed in a hive to control a pest is a veterinary medicine. Without a prescription from a vet, administering one that is not authorised for use in the UK is an offence. 4
This matters more for oxalic acid than for anything else a beekeeper uses, because the raw chemical is cheap and easy to buy. Oxalic acid crystals bought as a wood bleach or a cleaning product are not an authorised veterinary medicine, whatever they cost and however well they appear to work. The authorised oxalic products are listed in the National Bee Unit’s bee medicines fact sheet. 4
The doses in this guide are Api-Bioxal’s. Other authorised oxalic products have their own routes and their own numbers, and one is not a substitute for another. Read the literature that came with the product in your hand.
Supers, honey and the withdrawal period
The honey withdrawal period is zero days, and the product must not be used on colonies with supers on. 1 Those two statements are not in conflict. The zero days covers honey already in the jar. The instruction covers honey the bees are still working on.
For a winter treatment the supers are normally long gone. If you have left one on for stores, that colony cannot be trickled until it comes off, which is one more reason to settle the box arrangement in September rather than in December.
Whatever you use goes in the veterinary medicines record: product, batch, dose, date and which hives. That is a legal duty rather than good practice, and the medicines record covers what it has to contain.
What to wear, and what to expect afterwards
The literature specifies an FFP2 mask conforming to EN149, protective gloves and eye protection, and that applies to both routes. 1 Oxalic acid is not gentle because it is called organic. It is a strong acid and the vapour is worse than the liquid.
Mix outdoors or somewhere well ventilated, keep the powder away from your face when you open the sachet, and if you vaporise, stand upwind. A sealed hive leaking vapour is leaking it at you.
Expect some dead bees. An increased adult bee mortality rate after treatment, and slight agitation of the colony during it, are listed as very common at the correct dose. 1 That is worth knowing so nobody reads a drop of dead bees as evidence they got the dose wrong. Genuine overdosing has a measured cost: double the sublimation dose, or triple the trickled dose, and mortality rises significantly and overwintering suffers. 1
When the simple rule breaks down
- Mild southern winters. Some colonies keep a small patch of brood right through. Treat the best window you get and accept a less complete result.
- Nucleus colonies. A nuc covering three seams gets 15 ml, not a hive dose. The seam count is the whole method.
- Double brood. Count the seams where the bees actually are, which in winter is usually the upper box. Trickling into an empty lower box treats nothing.
- Very cold days. Do not open a hive to trickle when the cluster is tight and the air is near freezing. Wait for the mild day, and remember the 3 °C floor if you are vaporising. 3
- A colony treated in spring. If you trickled, you have one left. If you vaporised, you have none. 1
The short version
- Oxalic acid does not reach mites under cappings, so it only works when brood is absent or at its lowest. 1 It is lethal to open brood too. 3
- September is normally too early. Monitor in October and decide from what you find. 2
- Trickle: 5 ml a seam, 50 ml maximum, made up fresh at 4.4% in a 1:1 sucrose syrup, used within 24 hours. 1
- Vaporise: 2 g per hive, one administration, hive shut fifteen minutes, once a year, not in summer, not below 3 °C. 13
- Trickling allows two treatments a year. Vaporising allows one. Never alongside another acaricide. 1
- Treat every colony in the apiary at the same time. 12
- The product must be an authorised veterinary medicine. Wood-bleach crystals are an offence. 4
- FFP2 mask, gloves and eye protection, both routes. 1
This guide is general information, not legal advice. Rules change - always check the current guidance on GOV.UK before relying on it.