Oxalic Acid, Spray Treatment of Varroa Infestation Of Honey Bees

Organic acids like oxalic can be used to treat honey bees for varroa infestations, spraying the diluted acid is one such method of delivering the treatment.

Information Mainly from Swiss Dairy Institute Liebefeld, originators... Anton Imdorf, Jean-Daniel Charriere and Peter Fluri.

Strength

The oxalic acid solution used should be compounded of 30 grams of hydrated oxalic acid dissolved per litre of water.

Spray 3 or 4 ml of this solution from a pressurised hand sprayer over each side of the combs that are covered by bees.

The method is especially suitable for colonies that are housed in a single brood chamber.

Safety Precaution

Before and during application, a breathing mask suitable for organic acids, eye protection and acid resistant gloves must be worn, so that there is no danger to the operative.

1994 trials

The Liebefeld bee department implemented trials with spray application of oxalic acid against varroa mite, on brood free colonies in 1994 and 1995 during the months November and December, at several apiaries in central Switzerland.

The control treatment, which was 50 ml Perizin solution, was performed at least two weeks after the oxalic acid treatment.

Record varroa mite drop weekly throughout the entire experiment period, using a varroa screen floor.

The varroa mite drop from both the Oxalic acid treatment and the control should be summated and considered as the infestation level, with the varroa mite drop that is due to the oxalic acid being expressed as a percentage of this figure.

Results

The average result of spraying Oxalic acid at a rate of 3 to 4 ml per occupied comb side, during 1994 was 98.3% and 1995 was 97.4%. Of the 112 trial colonies treated 101 resulted in varroa mite kills of over 95%. Between the years, the locations and the hive type there were no significant differences.

A further quality assurance of the treatment was the minimal differences in the results between individual colonies.

At the beginning of September we treated 10 colonies, which averaged 12 dm² of sealed brood (the min. 8 dm² the max 15 dm². The efficacy reached only the average of 61% (min 42%, max.

For best effect the spray treatment with oxalic acid should be used in colonies with no brood.

Standardisation

In all cases the oxalic acid solution was made of 30 g oxalic acid crystals (oxalic acid dihydrate) dissolved in 1 litre of de mineralised water (Radetzki 1994). 3-4 ml per comb side were sprayed (Imdorf et al. 1996) with a hand operated atomiser (Ginge®, 0.5 l, no. 20-05-00, adjusted to the finest atomisation).

Colonies sprayed with oxalic acid do not build up as quickly, during the following spring, when compared with colonies that have been treated using the trickle (dribble) method.

Beekeeper Protection

It cannot be stressed too strongly that oxalic acid is an aggressive substance and needs to be treated with respect. Acid resistant gloves and goggles should be worn and an apron of the type used by mortuary attendants, along with wellington boots that have the tops covered by gaiters so that any falling liquid cannot fall into the boot. A respirator that has specialised organic acid filtering will be required in cases where the acid is sprayed or vapourised. Oxalic acid is also poisenous to humans by ingestion.