Oxalic Acid Treatment

Oxalic acid is an organic acid that kills varroa mites on the bees by contact; it does not reach mites in capped brood cells. It therefore works best when the colony has no capped brood: in the broodless period at the start of winter, after a brood break, and in swarms and package bees. It should be applied with an authorised product, in the amount stated on the label and with protective equipment.

How does it work?

Oxalic acid occurs naturally in plants such as spinach, chard and rhubarb, and is also present in small amounts in honey. In the hive, however, it is a veterinary medicine that acts on the mites by contact; being "natural" does not mean it is harmless.

• It kills only the mites on the bees. Mites reproducing in capped cells are not affected.
• It is very effective in a broodless colony; when brood is present, a single application clears only part of the mites.
• There is no known resistance problem. This makes it a valuable tool in programmes that rotate with synthetic medicines.
• When used correctly, the risk of persistent residues in honey is low; even so, it is not applied while supers are on.

When is it applied?

• Early winter, the natural broodless period: About three weeks after the cold has set in and the queen has stopped laying, when the capped brood has emerged. In coastal regions with mild winters brood may never stop; before treating, check a frame to see whether there is capped brood.
• After an artificial brood break: Once all the capped brood has emerged after the queen has been caged or the colony split.
• In swarms, shook swarms and package bees: In colonies placed in a new hive that do not yet have capped brood.

In summer, when the hive is full of brood, a single oxalic acid treatment is not enough on its own; at this time the programme needs to be supported with other tools.

Application methods

1. Trickling (with syrup)

The authorised oxalic acid product is prepared with syrup according to the label instructions and trickled from above with a syringe or a special applicator into the spaces between the frames that are full of bees (the seams of bees). The mixture is not a feed for the bees to eat; the bees spread the acid by contact with one another.

1. Open the roof to see where the cluster is; apply only to seams full of bees.
2. Give the amount stated on the label per seam of bees; do not trickle into empty seams or onto the hive floor.
3. Use the mixture slightly warm; cold liquid chills the cluster.
4. Close the roof quickly; the whole operation should be brief.

Do not store the prepared mixture; use it the same day. As the sugary acid mixture stands, substances harmful to bees form and its effectiveness changes.

Warning about repeat treatments: If trickling is repeated at short intervals in the same colony, it harms the bees and the open brood and can shorten the life of the wintering bees. One treatment per broodless period is the rule; do not repeat it unless the label says otherwise.

2. Sublimation (vaporisation)

Crystalline oxalic acid is vaporised inside the hive with a special heating device; as the vapour cools it settles on the bees and combs as fine crystals. Again, it acts by contact.

• Because it can be applied without opening the hive, the cluster is disturbed less; bees generally tolerate it well.
• During and for a short time after the treatment, the entrance and other openings are closed; the duration is set by the product and device instructions.
• The acid vapour is very dangerous to humans: it severely irritates the respiratory tract and the eyes. Keep the wind at your back and wear a mask with a suitable filter and protective goggles.
• The effectiveness of repeated sublimation treatments at short intervals while brood is present is limited and disputed.

Check whether an authorised product and method for sublimation are permitted in your country.

3. Spraying

Some products are applied as a fine spray onto the bees on the frames. Because each frame has to be taken out one by one, this method is used mainly for swarms, package bees and small colonies.

Safety

• Oxalic acid is corrosive and toxic if swallowed. Wear acid-resistant gloves and protective goggles; for sublimation a mask with a suitable filter is essential.
• Store the product in its labelled container, out of the reach of children and away from food.
• If it gets on the skin or in the eyes, rinse with plenty of water; in case of eye contact or swallowing, see a doctor.

Measure the result

Put the mite drop tray in place before treating; the number of mites falling over the following days gives an idea of the load in the colony. If a count some time later still shows a high mite level, check whether there was capped brood, whether the treatment was carried out correctly, and whether reinfestation from neighbouring hives is likely.

Common mistakes

• Expecting a single treatment to clear the mites when the hive is full of capped brood.
• Repeating trickling several times in the same winter.
• Preparing the mixture days in advance and letting it stand.
• Pouring acid into empty seams and onto the hive floor; this is medicine that never reaches the bees and is wasted.
• Making mixtures by eye with unauthorised acid of unknown purity.
• Sublimating without protection and breathing in the vapour.