What can be achieved by the application of Oxalic acid against Varroa?

Charriere Jean-Daniel, Imdorf Anton and Fluri Peter
Research Institute for Milk Products
Beekeeping Department
Liebefeld
3003 Bern
SWITZERLAND

Today the concept of recommended alternative Varroa control is seen to
complement the summer treatments with Formic acid or Thymol as an autumn treatment during the colonies' brood free state. (Imdorf and Charri‚re, 1998). This action at the end of the season reduced the Varroa population so much so that the mite numbers remained below the critical level up to the next treatment period in August-September of the following year.

(Qualification: No reinfestation from outside)

In practice there are several preparations and methods of treatment for the Autumn:

Additionally the systemic acaricide Perizin and Apitol, which are simple to apply but have a risk of residues and of resistance (Lodesani, 1996) Oxalic acid is however frequently recommended by itself.

At the moment Beekeepers may choose to spray or trickle as an application method.

In this article we compare both these procedures regarding the efficacy on the mite and the additional effects on the bees.

Application method

You should use an oxalic acid solution of 30 grams of hydrated oxalic acid dissolved in one litre of water.

Spray 3- 4 ml of this solution from a hand sprayer onto each side of the combs covered with bees.

The method is especially suitable for colonies in single chambered bee
hives, e.g. Dadant hives.

Many apiarists also treat colonies in Swiss hives in this manner, which is satisfactory.

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

The bee department implemented trials with spray application in brood free colonies in 1994 and 1995 during the months November and December at several apiaries in central Switzerland.

The control treatment with 50 ml Perizin solution followed on, at the
earliest, two weeks after the oxalic acid treatment.

With the help of a varroa screen floor the mite drop should be checked and recorded weekly throughout the entire experiment period.

The mite drop from Oxalic acid treatment and the control should be
calculated as 100%

Spraying the Oxalic acid solution.

3- 4 ml per occupied comb side

Effectiveness of the spray treatment

The average result of the Oxalic acid treatment in 1994 was 98.3% and 1995 was 97.4%. Of the 112 trial colonies treated 101 resulted in mite kills of over 95% on (chart 1). 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.

An experiment in 1995 authenticated this:

At the beginning of September we treated 10 colonies, which averaged 12 dm square of sealed brood (the min. 8 dm 2; the max 15 dm 2). The efficacy reached only the average of 61% (min 42%, max.

The spray treatment with Oxalic acid in brood free colonies

Results of the mite fatalities after the oxalic acid and Perizin treatment.

Additional (Side) effects on the bees

No noticeable side effects could be established by normal dosage of the
bees.

They kept very quiet during and after treatment.

Subsequent observations also showed no increase in bee mortality.

Opening the colonies in November had no negative effect or consequence on the colony development.

Application method

Use a solution of 1 part by weight of Oxalic acid dihydrate and 10 parts water likewise 10 parts sugar.

Trickle 5 ml with help of a Perizin applicator or a syringe into each space between the occupied combs.

A weak colony requires altogether 30ml, an average colony about 40ml and a strong colony 50ml of the solution.

The treatment should be undertaken at above 5 degrees C.

Before application the solution is warmed to room temperature.

The labour requirement in comparison to the spray method is less because the colonies do not need dismantling.

We have tested the trickling method in the autumn & winter of 1997-98 on 95 colonies on 11 sites distributed over the whole of Switzerland.

The treatment occurred in the November or December in brood free colonies.

Two weeks later, at the earliest a control treatment should be undertaken using 50 ml Perizin solution or oxalic acid sprayed throughout.(Imdorf and associates, 1997). The mite mortality was recorded weekly with the help on a varroa screen during the whole test period.

Trickling the oxalic acid.

5 ml per occupied comb alleyway.

Efficacy of the trickling treatment.

The average treatment result for the 95 colonies was 98.5%. Between the
colonies and sites there were relatively modest variations (chart 2). The treatment result is with the one spray treatment (chart 1) absolutely comparable.

The evaluation of the efficacy might not alone support the treatment results in percentages itself.

Crucial for each individual colony is the number of mites not killed by the treatment.

They were included through the control treatment with Perizin or with
sprayed oxalic acid.

The average mite mortality of the control treatment was between 1 & 23 mites per colony, per site.

The values for individual colonies are with only a single exception (few exceptions)less than 40 mites.

In practice these overwintering mites will form the starter population for the following beekeeping season.

From such low survival rates the mite population cannot develop much danger for the colony itself in a beekeeping season.

This is based on the condition, that no large reinvasions occur from outside untreated colonies.

Efficacy of the trickling treatment method with oxalic acid in brood free colonies.

control treatment with Perizin, all the others with sprayed oxalic acid.
Summation (Results oder Review)of mite mortality following oxalic acid and the control treatments.

Side effects on the bees

You only reckon with single dead bees during the two weeks after treatment.

We found on several trial sites, generally feeble colonies in the Spring of 1998.

This was also confirmed elsewhere in Switzerland and abroad in many apiaries where oxalic acid was not used to treat bees.

Because conclusive comparative trials still miss the effect on bees, the oxalic acid trickling method cannot be definitively assessed at the present time.

For the Autumn 1998 until Spring 99 trials are planned to clarify this
specific question in order to optimize the formulation, the concentration and dosage.

Temporarily, a repeated treatment of colonies in the Summer and Autumn with the oxalic acid trickled treatment must be avoided.

The rationale for this is the insufficient impact in colonies with brood and perhaps the poor tolerance by the bees.

Protective measures

During the application of oxalic acid, acid resistant gloves and goggles must be worn.

Whilst spraying oxalic acid, an organic acid resistant face mask must
additionally be worn. Type FFP2SL EN 149 refers.

The complete safety equipment which is required for the alternative Varroa control treatment is available from the bee trade suppliers or specialist firms.

Producing the oxalic acid solution should be undertaken only by trained personnel.

There were no differences between the treatment results of either spraying or trickling.

The average was over 95%. Such good efficacy against Varroa can only be
achieved in brood free colonies.

The tolerance of the bees to the trickle method is less than that to the spray method of application.

Therefore the application by spraying is recommended.

Multiple treatment with oxalic acid by trickling is NOT recommended.

Imdorf A., Charriere J D., Bachofen B. (1997) The control of the efficacy of control methods against Varroa jacobsoni with help of the oxalic acid.

Apiacta

Imdorf A., Charriere J.D

(1998) How can the resistant Varroa mites be kept below the harmful
threshold level?

Switzerland.

$Bienen-Zeitung 121 (5) 287-291. Lodesani M. (1996) Variabilità
dell'efficacia terapeutica ottenuta con trattamento di Perizin.

$L'Ape Nostra Amica (5) 4-9.

[L'Ape Nostra Amica] (5) 4-9.