Oxalic Acid Trickling Method
By David Ashton
 
Late season varroa control is important. The treatment should take place when autumn feeding has finished and the colony is free of brood.

Oxalic Acid Dripping Method
Oxalic acid is an organic acid, which is found naturally in honey. The method is extremely effective, very quick and inexpensive.

Safety
For varroa control the Oxalic acid is used in a very weak solution (3.2%). This solution is a very weak corrosive. But you must be very careful to avoid contact with the skin as oxalic acid can be absorbed through the skin. During treatment it is important to use acid resistant rubber gloves. Whilst mixing the solution you should use gloves, safety goggles, and respirator (P2 Dust Mask).

Treatment in Brood Free Period
The ‘Oxalic Acid Dripping Method’ does not work behind the brood cells’ sealed cap. This is the reason why treatment should take place in periods when no or very little brood is present. We would normally recommend a treatment in late October - November (but account should be taken of climate for example in western seaboard of British Isles the gulf stream keeps the temperature up so brood rearing goes on into December) With climate warming, it is important to check if brood rearing has stopped, so it could well be that treatment should take place in November or December. The important thing however is that the beekeeper must not fall in to the trap of forgetting to use an oxalic acid treatment. Bees, which for various different reasons go into winter and hibernation with too many mites, will suffer a lot of damage and harm.
It has been said that the colonies should be treated so the bees will have a couple of flying days after treatment. There is, however, no research that shows this to be necessary.

Mixing the Solution
For the treatment use oxalic acid bihydrate, mixed in the proportions 1 litre distilled water: 1 kg sugar: 75 g oxalic acid bihydrate. This is enough for 55 colonies. Be aware that mixing very small amounts, for example for just two colonies, can create problems, as ordinary kitchen scales can give the wrong mixture. Consider weighing a minimum of 0.5 litre distilled water: 0.5 kg sugar: 32.5 grams oxalic acid bihydrate.

Durability (Shelf Life)
We only recommend autumn treatments, so there is no reason to keep your surplus mixture; it is best put down the drain. If however you wish to keep your mixture, it will keep up to six months in the dark and cold of a cellar. The mixture should also be kept in child safe and secure bottle.

How to use it?
The Oxalic Acid mixture should be lukewarm. Using a 50 ml syringe, trickle 3- 3.5 ml per frame space with bees. That is to say 35 ml per colony. This is a very small quantity so it is a good idea first of all to practise with ordinary water. It is important that the trickling takes place direct onto the bees and not just on the frame tops where the bees would leave it.

Temperature
The outdoor temperature must be above 0oC.

Damage to bees
If the Oxalic Acid is used correctly and in the correct dosage, there is little or no risk to the bees. If you overdose, you can lose a lot of bees and weaken the colony.

How often should you treat?
Danish and international research has shown that if bees are treated more than once per generation, it can result in damage to the bees and a reduction in the strength of the colony. We recommend therefore that you should only treat the bees once each season. The best time to treat is in the autumn/early winter when the colony is brood free. However, with large-scale beekeepers, we have successfully treated both in the autumn and in the spring with the Oxalic Acid Trickling Method.

 

(Reference: Danish BKA website www.biavl.dk/varroa)