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The Varroa Mites Weakest Link is in their Feet

Experience with Oxalic Acid

By Gerhard Brüning, Buchenrein 5, 8704 Herrliberg.
Translated from the Schweizerische Bienen Zeitung 2/2005, pp 17 – 19.
by A.E.McArthur MIL


I have applied oxalic acid as a single treatment for the past four years and although I am perfectly content with it and experience colony losses which are well within the acceptable limits, I am still aware that there are no definitive data relative to the actual mechanism by which the substance acts during the brood rearing time of the year.

Late Summer and Winter Treatment

After the honey harvest is removed I carry out three treatment cycles using 1,5 – 2g oxalic acid at weekly intervals in parallel with my winter feeding regime. I am aware that at this time of year some 90% of the mite population exists in the brood, with this in mind I try to maximize the effect of the three week treatment by exposing the bees, hive internals and combs to a fine oxalic acid spray. By so doing the emerging mother and daughter mites come into contact with the acid at some point in their life cycle.

The oxalic acid ‘gassing’, winter treatment is carried out when the ambient temperature has remained around 5 C over a three week period. The optimum ambient temperature is 7 – 11 C, because at this temperature the bees are not too tightly clustered. This method however, can also be employed at lower temperatures.

Lethal Dose Window ‘Deviation’

When the first biological treatments appeared, I experimented with formic acid using a drip method and also the flat sponge method of application, actually achieving quite good results, but there were too many variables. The ‘window’ between mite kill and bee kill, I found was too narrow.

I initially used thymol and then Thymovar and for four years I had the greatest difficulty keeping the mite population under the lethal level.

Sublimation Devices

I used the sublimator device for the first time in 2000. It was a marvellous experience to see the mites dropping onto the hive insert in their hundreds after the winter treatment. The device is electrically operated using a 12 volt battery and designed with a small plate having two hollow depressions each capable of taking 1g of oxalic acid crystals. The design of this device had a weakness so far as I was concerned – in that during the sublimation process a portion of the crystals foamed and ‘spluttered’ and landed at the side of the plate where crystallisation took place. Using a fine meshed ‘hood’ fixed to the small plate I was able to overcome this spluttering tendency. The device was effective and the oxalic acid sublimate caused no harm to the bees. It was however very time consuming due to the time required to heat and the time to cool the device during use.


I have used a sublimation device in conjunction with a gas blowlamp for the last two years. The gas outlet tube is 43 cm long with an 11 mm length at the end flattened to 7mm. The outlet tube heats up during the treatment procedure just enough that the foam insert blocking the hive entrance does not deform or melt. This temperature is sufficient to totally sublimate the oxalic acid without any re-crystallisation.

In operation it takes around 5 minutes until the device is hot enough to sublimate the charge. Before I commence the treatment I close all the hive entrances with a foam strip. The device is charged with 2 – 3 g of crystals using an apple coring tool, depending on the capacity of the hive being treated. The sublimation process takes about 60 seconds so that with practice I can now reload the device and treat on a 90 second cycle.

During the summer treatment the entrances are closed to the diameter of the gas tube in the early morning. This is done because at this time of year the bees can form clusters in way of the entrance. These clusters must be dispersed before treating the colony. The exposed flame should be protected by a wire cover.

Powder or Tablets

Oxalic acid dihydrate purchased from a chemist contains 30% water, this acid form is the best suited to the sublimation procedure, because the water, as steam, acts as a carrier for the gas causing it to gush from the pipe at a low pressure. In addition to this aqueous oxalic acid dissipates with higher pressure. The tablet form is less effective due to the lower water content and during the sublimation process only produces a weak trickle of smoke.

No pressure build up occurs in the hive , because the steam pulse recondenses to water almost immediately and reduces to it original volume. If air were the pulse mechanism into the hive an equal amount of air would have to be released from the hive to maintain atmospheric pressure.

It is interesting to observe how the bees try to fan the sublimate out of the hive during the treatment, this is noticeable if the hive body is not airtight by white vapour swirling from any gaps. Thus the bees themselves assist in the even distribution of the substance throughout the hive. Air tight hives of course reduce the loss of gas to a minimum, but the operator must none the less wear suitable protection during treatment application.


To establish if oxalic acid acts as a systemic I set up a feeding experiment, using a dish filled with sugar syrup placed 20 metres from the hives. Within 60 minutes the dish was covered in bees with clouds of bees waiting their turn. When the dish was empty I replaced it with the same amount of sugar syrup laced with 3% oxalic acid. The bees lost interest on the instant with only a few landing on the dish, probably because they could not believe such a dirty trick had been perpetrated on them. Within 30 minutes the feeding area was clear of bees and not a drop of the tainted syrup had been touched. This proves that oxalic acid is not a systemic treatment. There must be another mechanism by which the poison enters the mite.

Vitality Test

To test the mite ability to ‘grasp’, I collected a number of barely alive mites from the floor insert and placed them on a glass plate. This plate was exposed to direct sunlight in order to observe them more clearly. The mites responded to the exposure to the heat and light by suddenly beginning to actively run around on the plate. The plate was turned vertically and then upside down to test if the mites were capable of holding on. Finally I bumped the edge of the plate with palm of my hand and then using a goose quill attempted to dislodge them from the glass. Only using considerable force were the mites able to be dislodged from the plate.

I then took the plate into my Bee House and tried to turn the mites onto their backs. However no sooner had a flipped a second mite over than the first mite was already right way up. The mites rocked strongly on their backs and after a number of attempts landed again on their legs. Ultimately using the gummed strip of an envelope I was able to anchor them on their back. Using a 50 times magnification I was able to examine the mites closely. Every mite had clumps of oxalic acid crystals on the outermost feet, which looked like tiny snowballs glistening in the light as the mites moved their legs. I wanted to show my friend Paul Menzi this interesting phenomenon, however by the following day around 2 o’clock the opportunity had passed. The mites were dead and strangely the glistering crystal clumps had disappeared. The feet were completely clean, only fragments of these clumps were to be seen glittering on the body of the mites. I rechecked the floor inserts and found a few other dying mites. These mites showed only weak signs of life and they had no crystal clumps on the feet, merely dull funnel shaped formations whose sharp ends showed where the sticky pads of the feet were located. These mites by constantly twisting the foot tips around tried to dislodge these uncomfortable deposits.

Diffusion of the Acid

Over time using the microscope I was able to observe how these oxalic acid deposits slowly dissolved and disappeared into the foot members. Occasionally a tiny drop remained in the hair of the foot members lightly joined to the foot member tip. Simultaneously with this procedure the signs of life became steadily weaker. Many mites were already dead before all of the substance had dissolved. The residue of the funnel shaped deposits later re-crystallised on the feet.

There is a question to be answered from these observations. What exactly happens during this diffusion process?

The oxalic acid crystals appear to collect on the mites’ feet from the oxalic acid crystals on the hairs of the bees and from the combs. These crystals may then be dampened by the moisture on the sticky pads on the mite feet and bind continuously with other dry crystals. In this way the crystal are caused to form relatively large clumps. Ultimately the acid penetrates the body of the mite through the feeding organ and damages the vital organs of the mite, killing it. Or the acid crystal enters the mite by osmosis and this cocktail of poison is then ‘by default’ taken up by the mouth parts of the mite.

The bees also collect oxalic acid crystals on their feet, however due to the glands on the bee’s feet which exude a waxy secretion and the protective feet hair, the acid crystals do not form clumps.

Therefore the aqueous secretion on the mites feet is responsible for its death and the waxy secretion on the bees feet is the decisive factor for the bees survival. These are only speculations on my part. But doubtless an overdosing with oxalic acid will also present a danger to the bees.

Some readers will say that there are already ‘a sufficiency’ of theories out there which cause the beekeeper ‘grief’. For this reason I will finish this article with the recommendations of the Liebefeld experts:

“Carry out an oxalic acid sublimation winter treatment. Check the mite fall every seven days over a period of three weeks (check a minimum of ten colonies). Carry out a subsequent check treatment using a tried and tested procedure and repeat the three weeks ‘drop’ check. Using such a regime the success rate of the treatment may be double checked.

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