lmdorf Anton, Charrière Jean-Daniel, Maquelin Charles, Kilchenmann Verena,

Bachofen Boris


Apicultural Department

Federal Dairy Research Institute, CH-3097 Liebefeld, Switzerland




Alternative varroa control methods have occasionally been applied with success since the appearance of varroa. Their advantage is, that no acaricides causing persistent residues have to be used. In the long term a good quality of the bee, products can be guaranteed only under this condition. It is important therefore, that as many manufacturers of bee products as possible decide to use alternative varroa control in the near future. The treatment concepts described below will support them in taking their decision.


The rapid and almost world-wide spreading of the bee parasite Varroa jacobsoni in the last years has changed beekeeping practice. In order to avoid colony losses annual varroa treatment is required under Central European conditions. Up to now the highly effective pyrethroids have mainly been used. The application is simple and the amount of work involved is reasonable. However, the future use of these products is questionable due to the formation of resistant mites, as in Italy. The remaining varroacides (Perizin, Apitol, Folbex) are efficient in broodless colonies only, during November and December. If they are applied earlier, a reduced efficiency has to be taken into account, and multiple treatments are needed. The repeated application of the products mentioned above causes increasing residues in wax and to a lesser extent also in honey. Under these circumstances the long term quality of bee products will deteriorate.


Alternative varroa control leads the way out of this dead end; The active substances used here are either organic acids, such as formic, lactic and oxalic acid or components of essential oils, e.g. thymol. They are widespread in nature and some of them also occur in honey. A treatment concept has to be followed when using these substances. Biotechnological measures for the reduction of the varroa population have to be combined with an optimum application of the products and control of treatment efficiency. As for the organic acids no residue problems should arise, when applied properly. Accumulation in wax does not occur. During treatment with essential oils, however, residues accumulate in the wax and subsequently evaporate to a great part. The residues in honey are small and not important from a toxicological point of view.


The following treatment concepts have been developed and tested in extensive investigations by Swiss and European apicultural institutes and by various beekeepers during the last years. Various of the described methods are applied successfully in large and small apiaries. A number of large apiaries with several hundred colonies have in the last years refuted the claim, that alternative varroa control be unprofitable. The key to success is the full integration of varroa treatment into the system of hive management.



All important information about carrying out the different methods and the application of the various active substances is summarised in Tables 1 and 2. The methods are described briefly below.



The experience with this method has shown, that short term treatment alone often does not reduce the varroa population sufficiently, and the varroa increase in the following year, combined with small re-invasions, may reach a dangerous level until the following treatment. It is therefore necessary to reduce the increase of the varroa population in spring by 2 to 3 removals of drone brood or by forming a nucleus. In order to keep the amount of work involved within reasonable limits, these biotech­nological measures have to be combined with other beekeeping activities.


During the short-term treatment small amounts of formic acid evaporate relatively uncontrolled within 6 to 10 hours. At the beginning of the treatment the formic acid concentration in the hive air increases rapidly. Six hours later most of it has already evaporated. The time and the dosage of the treatment depend a lot on the temperature and on the hive system. When applied from above, 60% formic acid is used, whereas 85% is necessary from below. The treatment in two blocks of two to three applications within a week in August after the honey harvest and in the end of September has proved efficient. The treatment efficiency obtained under these circumstances is approximately 95%. This result is possible, because formic acid also kills a part of the mites in the brood. The efficiency may be controlled two weeks after the last treatment by measuring the natural mite fall, using a bottom board with a metal lattice covering the entire hive bottom. It is sufficient to count the mites once a week. If the natural mite fall is above one varroa per day, another treatment with oxalic or lactic acid must follow. Six years’ experience with this method has shown, that follow-up treatments are necessary only after re-invasions in October.


If formic acid is applied after the honey harvest in late summer, there will be no residue problems. In order to avoid bee and queen losses, the indications concerning temperature and application must be followed. In August it is an advantage to feed at the time of the treatment.




The amount of work involved is reduced a lot by long term treatment. The soft fibre plates (Pavatex) may be prepared in advance, during times of less work. Joint manufacturing by several apiarists is worthwhile. The plates are impregnated with formic acid and sealed in plastic bags of 0.15 mm thickness. If thinner plastic is used, the plates should be kept frozen until their use in August or stored in an airtight plastic container. Before application the necessary evaporation holes have to be made in the plastic with a round punching tool of 1 .5 cm diameter. The number of holes varies according to the hive system and climate (the microclimate of the apiary is also important) and has to be tested beforehand by small experiments. For the treatment in August the plates are hung into the empty honey chamber by means of a honey comb frame for 7 days. After the treatment several plates are wrapped up together in a plastic bag and frozen. In the September treatment the same board is provided with additional evaporation holes and put horizontally on wooden strips of 2 cm height during 14 days. Between the cover and the upper side of the board another two wooden strips of 2 cm are placed. An insulating cover is advantageous. For medium-size one ­storey hives a distance of 5 cm between the brood combs and the board should be kept.


The treatment efficiency of this method depends on the formic acid concentration in the hive air and on the duration of the treatment. It can be controlled during the autumn treatment on the basis of evaporated formic acid amount. For this purpose the board has to be weighed before and after the treatment. If there is an evaporation of more than 7g of formic acid per day, a treatment efficiency of more than 95% may be expected, at 1 Og per day it will exceed 97%. If less than 7g evaporate, the treatment efficiency will be insufficient. In that case drone brood has to be removed 2 to 3 times in spring or a 5-days treatment has to be carried out in the period between mid-March and mid-April. The used plates may be applied again for this treatment. They must be re­impregnated with formic acid up to a gross weight of 250g. The way of application and the number of holes is the same as in September. Residue problems should not arise, if the treatment is carried out 4 weeks before the nectar flow.



A single application of lactic acid in broodless colonies has an efficiency of approximately 80%. In colonies with brood. the efficiency lies between 20 and 40% About 4 treatments per year are needed, in order to keep the varroa population below the tolerance threshold. One application has to be carried out in November or December, when the colonies are broodless. The time of further treatments depends on the development of the varroa population, the nectar flow and the management system. If the natural mite fall amounts to 5 to 10 mites a day, another treatment has to be considered soon. During nectar flow no treatments should be made.


All bees of a colony must be sprayed by an atomiser with a fine jet with 5 to 8 ml of 15% lactic acid per each comb side. Depending on the type of atomiser and the adjustment of the jet 4 to 6 pumping strokes are needed. In order to avoid overdoses, the sprayed quantity per pumping stroke should be determined. The amount of work involved is relatively high. Therefore the use of lactic acid is mainly suitable for small apiaries with just a few colonies.


There are no residue problems and queen losses. Overdoses must be avoided, above all in November, otherwise there may be losses of bees.



Oxalic acid is used in the same way as lactic acid. The concentration (30g oxalic acid dehydrate added to 11 of water) and the dosage (3-4 ml per comb side), however, are different. In broodless colonies oxalic acid has an excellent efficiency of approximately 98%. The differences in treatment efficiency between the colonies are small. In colonies with brood an efficiency of 30-40% may be expected. The treatment in November is therefore the most efficient one. If the natural mite fall in July is above 1 varroa per day, a treatment should be carried out after the honey harvest in August. If there are more than 5 varroa per day in September, a subsequent treatment is needed. In most cases two treatments should be sufficient. According to the present knowledge no residue problems can be expected after the treatment in late summer and autumn, If the necessary precautions are met during the treatment, by protecting the respiratory system (mask), the eyes (glasses) and the hands (gloves), there should be no risk for the applying person according to toxicologists. Inhaled mist irritates the mucous membranes and causes a dry cough. When the treatment is carried out in a bee house, it must therefore be well aired. As for the bees, no. noticeable side effects have been detected with the dosage described above.


The amount of work involved is quite reasonable in one-storey hives. Under these conditions three persons treat approximately 25 colonies per hour in November.



The amount of work can be reduced by the combination of a one-week formic acid treatment or three short term treatments in August with one oxalic acid treatment or two lactic acid treatments in November. This method is also suitable for medium-size apiaries. It combines the advantages of good efficiency: of formic acid in August in colonies with brood (approximately 80%) and of oxalic and lactic acid in broodless colonies in November (approximately 98 resp. 96%). “Kramer” plates may be used for the formic acid treatment in August. In this case the same plates may be used for the treatment of two subsequent groups of colonies.



Apilife VAR consists mainly of thymol (76%), besides eucalyptol, camphor and menthol. A vermiculite tablet, impregnated with approximately 20g of this mixture, is placed on the brood combs from mid-August for 3 to 4 weeks. Subsequently, the tablet is replaced by a fresh one for another 3 to 4 weeks. With application from below, the treatment efficiency is insufficient. The efficiency strongly depends on the thymol concentration in the hive air, which is greatly influenced by the bee behaviour and other factors, as e.g. the comb position (warm or cold position). If the average daily temperature falls below 120C for long periods, the efficiency decreases.


In Swisstype and one-storey hives (Zander or Langstroth) a treatment efficiency of approximately 97% may be expected under optimum conditions In Dadant and two storey hives the efficiency is often lower (90-95%) and the efficiency variation from one colony to another is considerable. Therefore the treatment efficiency must be controlled by measuring the natural mite fall during the two weeks after the end of the treatment. If it is above one varroa per day, another treatment with oxalic or lactic acid must follow. If the drone brood is removed in the following spring, a treatment will be necessary only at a mite fall rate of more than 3 varroa per day If the tablet is placed too near the brood, the latter will be removed within a radius of a few cm. Therefore, the colonies should be fed before the treatment. During the treatment essential oil components accumulate in the wax. A great deal of them evaporates after the treatment. In honey residues are mostly in the range of 0.1 to 0.2 mg/kg.. Concentrations above 1.1 mg per kg alter the honey taste. From the point of view of human toxicology, these residues are not important.


Pure thymol is applied in the same way as Apilife VAR. 15 g of thymol are dissolved in approximately 20 ml of ethanol and poured on a viscose sponge. After the alcohol has evaporated, the sponge is put on the brood combs like the tablets of Apilife VAR.


Alternative varroa control is not restricted to the methods described in this account Various other combinations are possible. We do not describe further methods such as trap combs, brood rearing stop and thermo-treatment, because a great deal of work is involved in their application.


It is essential for alternative varroa control, that each method be carried out correctly. The chosen method first has to be adapted to the actual conditions (managing system, climate and nectar flow) and must be tested with a few colonies. Control of treatment efficiency is of great importance. Some modifications of the described methods may be necessary. If the treatment is successful, no controls are needed any more. Biotechnological measures reducing the growth of the varroa population are welcome in any hive management. Thus, the number of treatments is reduced, and the formation of varroa populations near the tolerance threshold is prevented. Once the beekeeper has decided on switching to alternative varroa control, he should make sure to use wax for the production of comb foundations, which is free from residues.


Before switching to alternative varroa control, the beekeeper should be well informed on the alternatives. Courses on alternative varroa control organised by the beekeepers associations might be helpful.



A number of institutes as well as many beekeepers have contributed a great deal to the present knowledge concerning alternative varroa control Therefore we sincerely thank especially Gerhard Liebig, Kurt Kramer, M. Krasnik, Alois Wallner, Johann Weiss and many others for sharing with us their experiences in various discussions and many publications. Special thanks to the many beekeepers, who have contributed to the present knowledge by collaborating in the experiments of our studies during the past decade. Thanks also to Barbara Bogdanov, who translated this report into English.



A reference list may be requested from the Apicultural Department, Federal Dairy Research Institute, CH­3097 Liebefeld, Switzerland.



The Use of Drone Brood Comb in Anti-Varroa Treatment


Translated from the Schweizerische Bienen Zeitung, March 1999, page 131



From the time it was possible to purchase drone foundation from bee keeping equipment supply houses, I have used them wired into my frames with stainless steel wire. These frames are exactly the same as the frames used in the super and brood box. This frame is more than a ‘building frame’. The wiring firms up the comb. The drone foundation is drawn more readily than the strip of foundation and therefore more rapidly available as a ‘trapping’ comb for the mites.


Exchange of Drone Comb

Each colony receives two drone combs in the spring, one on each side of the brood nest. As soon as either one or both combs are around 2/3 sealed, they are removed with adhering bees. At the same time another two drone combs fitted with foundation are inserted.


Reinforcement or Nucleus

The drone combs with the adhering bees are used either:

a) Individually as reinforcement together with sealed worker brood.


b) Used to form Nucleus stocks with up to 10 of these removed combs.


They serve as trapping combs, since they contain as yet unsealed drone brood and the Varroa mites on the adult bees on the sealed worker brood have nowhere else to go, other than into the unsealed drone brood. Three or four days later these drone combs are removed from the nuclei and placed in the solar wax extractor to retrieve the wax. This method of operation removes the mites safely from the nucs. The nucs are then given comb with eggs or unsealed larvae using either the method developed by Götz or Jenner to encourage them to produce a new queen.


Drone Comb Sacrifice Cycle

The strong colonies draw up to five drone combs each. Their removal is a mild form of ‘shook swarm’ and reduces the urge to swarm and limits the Varroa populations considerably. I feel myself completely justified by the work done by Anton Imdorf at Liebefeld: that drone removal reduces the potential Varroa population by as much as 50%.




Using drone combs for nucleus building on their own or together with worker brood combs I am able to achieve a dramatic reduction in the levels of Varroa infestation and in so doing I am able to give each of my strong stocks either a nucleus stock as a reinforcement or a new current year queen in the autumn.


The use of drone foundation reduces the construction of drone cells in the corners of the worker brood frames. The former practice of inserting foundation strips has now been superseded as has also the use of expensive anti-Varroa treatments which at worst leave unpleasant residues in the wax and at best result in resistant mites.