Posted 05 Feb 2008

Alternative anti-Varroa ‘Hive Cleansing’ Treatments Re-visited


Eric McArthur

The Varroa mite has hopefully made us all better beekeepers, because if we have not become better beekeepers many of us will become non-beekeepers very soon or will have already become non-beekeepers. The mite is a hard task mistress and will mercilessly root out the ‘let alone’ beekeepers and also the ‘in denial’ folk. Beekeepers can fool themselves in the short to medium term about the mite being ‘somebody else’s problem’ but they can’t fool the mite – it will always have the last laugh! Being ‘in denial’ does not necessarily only mean that the beekeeper is of the opinion that the mite has passed him/her by, it implies also that the beekeeper being aware of the mite’s presence in the colonies will not deviate from the accepted wisdom of the potentially less than effective treatments handed down by the ‘powers that be’, who for whatever reason are themselves ‘in denial’ and have steadfastly for many years refused to discuss, let alone authorise well tried mainland European alternative methods.

The drip, drip of the continuous reports of the unusually high colony losses world wide is beginning however to have effect and the new ‘hive cleansing’ substances like oxalic and formic acid are now being viewed with a less jaundiced eye.

There is an encouraging number of beekeepers now expressing an interest in treating their bees with oxalic acid, which when applied at the correct time of the year has an astonishing ‘cleansing’ effect on the colonies, considering the negative reaction of many, in the mid 90s, when the Scottish Beekeeper magazine was ‘slated’ for its coverage of the positive reports and translations about the substance from the German language ‘bee press’. It is now time however, in my opinion to move on.

The ‘cleansing effect’ of oxalic acid is unfortunately only felt by the phoretic mites. There is an imperative need for a much deeper ‘cleansing effect’ if the colonies are to remain healthy with their immune systems intact and clear of the debilitating viruses which appear to act as catalysts for colony demise.

Formic acid is such a ‘deep cleanser’, which penetrates the membrane of the sealed brood cell and kills the mites before they can debilitate the developing bees. The sealed brood of colonies not treated with formic acid are completely at the mercy of the mites, which have ‘free access’ to the hapless developing bees as they suck the larval/nymph haemolymph causing the lesions into which the lethal viruses penetrate, By applying formic acid at the correct time and at the correct dosage the potential devastation caused by a heavy viral infestation can be largely avoided. The ideal time to apply formic acid is when there are large numbers of newly sealed brood in the hive, in Scotland, this will be initially from around the 10th April as the colonies accelerate the rate of spring build up in anticipation of the first major nectar flow, which occurs throughout May from; sycamore, gean, OSR, hawthorn, chestnut, dandelion, whitebeam and other sources. Another critical time where formic acid application will work wonders is June. The dreaded ‘June Gap’ in Scottish beekeeping lore can become a ‘grace’ instead of a ‘sin’, if the beekeeper removes the early summer honey and the bees are treated from the end of the first week in June. The colonies can also be ‘swarm prevented’ at the same time by splitting the strong colonies and making up queenless nuclei with unsealed brood, which may be treated by the oxalic acid ‘Trickle Method’, which is able to be applied , before the brood is sealed, with impunity in the active season. The bees, queen and the remaining sealed brood can be treated with formic acid as carried out in early April, resulting in colonies which will work on the late summer flows unhindered by heavy mite infestations, which will otherwise be developing in colonies which have had untreated/inadequately treated mite infestations over a 2 -3 year period.

On mainland Europe a most effective method against the mite is the re-queening of all colonies with vigorous current year queens during the late summer/early autumn and then carrying out the chosen anti Varroa treatment using the organic acids to ‘cleanse’ the hives.

Oxalic Acid Trickle Method

Oxalic acid treatments may be applied at temperatures as low as 5 C.

Oxalic acid as a Spray Treatment was pioneered in Russia and perfected by Radetzki et al and reported in the May ’94 issue of the Schweizerische Bienen Zeitung having proved to be extremely effective, however the method is quite labour intensive since the procedure entails withdrawing all the bee covered combs and spraying both sides of the comb with the acid solution. A less labour intensive procedure, the Trickle Treatment was pioneered by Italian scientists using a 10% aqueous solution in 1995. This solution strength was found however to be too strong for use in Northern Europe. Swiss researchers carried out exhaustive trials in autumn 1999, which have been well documented, using different solution strengths (see S.B. November 2000, page 266) and ultimately recommended a 3.5% oxalic acid/aqueous solution (or oxalic acid/sugar syrup solution) applied at a dosage of 4 – 5 mls of the solution trickled slowly onto the bees clustering between the combs, for Central Europe. The conventional wisdom dictates that a maximum of 50 mls of the solution be applied in the case of a colony covering 10 frames, thus 10 x 5 = 50 mls; a colony covering perhaps 6 frames would be dosed with a maximum of 6 x 5 mls = 30 mls and so on. I used the Trickle Treatment as a prophylactic measure from December 2000, despite not having Varroa in my colonies at that time – to prove that the recommended dosage was tolerated by the bees. This treatment applied around the end of December will kill some 95 - 98% of the mites on the adult bees and dramatically improve the over-wintering survival chances of the colonies.

Mite fall should be monitored before and after treatment. The Trickle method may only be applied ONCE as a winter treatment.

When using the Trickle method it is imperative that safety spectacles and acid proof rubber gloves are worn.

Oxalic Acid Fumigation Method

This extremely effective method was first tested in aerosol form at Fischermühle, Germany in 1994. Later, in 1999 again at Fischermühle, the method was improved by applying the oxalic acid in gaseous form using sublimation. The method was thoroughly tested ‘in the field’ during 2000 and 2001 and proved to be highly efficient (95%+ success rate, see S.B. December 2001, pp 295 – 298). The sublimating device used however was rather labour intensive and not cheap.

Further improvements in the sublimation procedure from that time on occurred in mainland Europe and in 2003 the first DIY device appeared, published in the February 2003 issue of “Bienenmütterchen” ( see S.B. February 2004, pp 40 – 41). This was a 0.7 metre long, 16mm dia., copper pipe, which was charged with 3 g oxalic acid dihydrate crystals and the tube heated using a blow lamp. This idea appealed to my Scottish thrift and I experimented with it, however I felt that the pipe as designed was too long – most of the oxalic crystals condensed at the relatively cold top end of the tube. I modified the design to a short bend (see S.B. February 2004, pp 40 – 41) which was charged with 1.5 g of oxalic acid dihydrate crystals. I used this device in one of my apiaries on October 16th 2003, fumigating into the top of the hive above the clustering bees and joined the “Varroa Members Club” on that day. This bend was subsequently improved upon, in my opinion, and another similar pipe bend was designed suitable for fumigating at the hive entrance (see S.B. December 2004 pp 323 -324.). I fumigate from above when there is no fondant or sugar bag feed on the hives, otherwise fumigation is done at the entrance. Fumigation is effective and quick and since there are many ways to ‘skin the proverbial cat’ improved designs for sublimation will continue to be produced. I fumigate every year in mid to late October and continue to monitor mite fall. Mite fall starts the following day and will continue for perhaps a couple of weeks depending on the infestation levels. Fumigation, as opposed to the Trickle Method may be applied two or three times at three to four week intervals. If more than one mite falling per 2 days is recorded on the Varroa floor insert during late November /early December another cycle of fumigation around the end of the 3rd week in December will be necessary.

When using the Fumigation method it is imperative that a properly designed gas mask suitable for use with oxalic acid gas is worn, safety spectacles and acid proof rubber gloves are also prerequisites.

Formic Acid

Formic acid is a spring/summer treatment. The optimum evaporation temperature is 18 – 24 C. This temperature will be achieved above the brood nest at any time when brood is being raised

There are numerous devices which are recommended for the application of formic acid and the acid solution strength varies according to the appliance being used. This variety of application methods and solution strengths, which incidentally have all been scientifically tested and found to be safe for the bees and effective against the mite and leave no residues, demonstrates the flexibility of this substance as an anti-Varroa treatment.

A simple method of application which has been used for the past 3 years and has been reported as achieving quite dramatic and reassuring results is a modified version of an extremely simple German design. The formic acid used is a 60% aqueous solution applied by a plastic veterinary syringe using a 20 ml quantity at 3 day intervals over a period of 10 days. The acid is applied to an ordinary flat synthetic kitchen sponge measuring 5”x 5”, supported on a sheet of plastic garden mesh, which is fixed to the underside of a plywood carrier, approximately 8” square and ½” thick with a 6” square hole cut out of it. The plywood carrier is placed directly onto the brood frame tops, the sponge is laid in place, the 20 mls of acid gently trickled onto the sponge and another flat square of plywood placed over the sponge to ensure the vapour is driven down into the brood nest as the acid evaporates. An empty super may be used to house the device. Three days later the 20 ml amount is repeated and three days after that the dose is repeated again making 3 - 20 ml doses in all. Mite fall will continue until all the sealed brood from this period has emerged; 12+ days from initial application. If more than five mites /day drop after this period the treatment must be repeated. Thereafter the device is removed and stored for future use. The simplicity of the design belies its effectiveness, which is due to the fact that when brood rearing is underway in the colony the brood nest temperature is at or close to 35 C. The optimum temperature range for formic acid evaporation is 18 – 24 C. Even in mid April the heat rising from the brood nest impinging on the sponge is said to be sufficient to evaporate the formic acid within the necessary time scale. During the brood rearing period around 85% of the mites infesting a colony of bees are in the brood cells thus by the use of formic acid around Mid April the developing larvae and nymphs are saved from serious damage by the parasitisation and the colony is ‘swept’ virtually clean of mites. Combining oxalic acid and formic acid treatments within the correct time scale as an IPMS (Integrated Pest Management System) appears to be a winner. Makes sense to me!. Formic acid kills the mites in the brood cells and also the phoretic mites on the adult bees and at this present time is the only substance, which performs this double function. The substance used correctly and on the right time scale dramatically reduces the incidence of bees exhibiting viral conditions like ‘cloudy wing’ and ‘deformed wing’ viruses and will certainly eliminate any ‘jockey’ mites seen on the bees – a condition which should never be allowed to happen, but which unfortunately seems to be quite common according to reports. Bees should be treated against the mite long before they are able to be seen on the adult bees – this level of infestation will usually result in colony death despite being treated.

My own colonies indicated negligible mite drop in late December 2007 and still do, as do the colonies being fostered for the ‘Save the Scottish Honey bee Gene Pool’ project in the Clyde Area Beekeepers’ Association apiary. But that is another story!

This formic acid procedure is extremely easy to apply, however it is recommended that the user practice with water initially until the procedure is understood and perfected.

When using the formic acid treatment it is necessary to wear industrial quality acid proof gloves, safety spectacles, a suitable mask to protect against the acid fumes and have a bucket of clean fresh water immediately available in case of accidents.

An important additional ‘bonus effect’ of the formic acid treatment is the eradication of acarine disease, which is caused by the microscopic spider, Acarapis woodi Rennie.

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