As I mentioned the use of organic acids to combat varroa is known from almost the very beginning of this problem in Europe. The mechanism of action of volatile acids (formic acid, oxalic acid sublimation) on mites is well understood and reduced to dust mites damage respiratory organs, penetrate into its interior and cause fatal disorders of metabolic processes. Liquid acids (oxalic, lactic, citric) used in the form of spraying the bees work similarly and cause damage to the mouthparts, antennae, legs, and as a result causes the death of the mites.
The situation is complicated in the case of trickling mixtures of organic acids and sugar syrup on bees in beespaces between frames. The first question that arises is how it is possible that trickling the solution just on the top of the beespaces we get high varroa mortality?
The answer is relatively simple, the mixture of acid and the syrup is gradually spread by the bees through their social behaviour onto the whole colony and comes in the form of micro-droplets for each bee in the family. It is a long process and in winter the distribution of the mixture in a colony can last up to two weeks. That is why we observe the effect of increased varroa mortality over such a long time
The second question which arises is why we use the sugar syrup and not pure acid? Beekeeping practice and research on the use of oxalic acid made by the European Working Group for Integrated Varroa Management over 10 years ago showed that the addition of sugar (30-60%) causes a significant increase in the effectiveness of the treatment up to 100%.
The researchers called this the result of synergy between the sugar syrup and oxalic acid. The mechanism of this effect has never been explained scientifically. The following will present the hypothesis resulting from my own and other beekeepers’ experience explaining the essence of synergy.
Observing the bottoms inserts of white sticky paper on the application of lemon juice in sugar syrup I noticed a lot of falling living varroa, many mite viewed through a microscope showed injury that could not be an effect of acid. The most logical explanation is the activity of bees themselves, which, after application of lemon juice make clean themselves of varroa mites removing them from their bodies. On the inserts after the application of lemon juice a lot of minor garbage can also be noticed, for example scraps of wax, wood pieces, etc. Again, it is concluded that this is a result of increased activity due to which irritated bees clean the hive environment as a whole.
Since I had already had to deal with the use of oxalic acid and the preparation BEEVITAL before, I observed similar effects and attempted to formulate a general hypothesis:
Sugar syrup trickled over bees in beespaces strengthens hygienic behavior of bees. This results in removal of mites by bees themselves. This constitutes the essence of hitherto inexplicable synergy. The role of acid is reduced to the role of a buffer to prevent the consumption of sugar syrup by bees.
And so according to this hypothesis different cases can be explained:
1) Oxalic acid + sugar syrup
Approximately 50% -70% of varroa falls on through the direct toxic effects of oxalic acid, the rest is self-cleaning effect of bees.
2) Citric acid (lemon juice or natural) + sugar syrup
A small percentage (10-20%) of varroa drop as a direct effect of citric acid (which is much weaker than oxalic acid) while a vast majority drop as a result of bees’ self-cleaning.
3) Acetic acid + sugar syrup (such as sweetened wine vinegar)
Very weak acid, does not cause varroa death. Used at a concentration of 10% is a weak buffer against consumption of syrup by bees..
4) Formic acid (60%) + sugar syrup
Formic acid is too volatile, evaporates from the syrup practically within hours, mite drop is increased but also a lot of dead bees on the insert. Complete lack of the effect of stimulation of hygienic instinct.
The review cited may be used in combating varroasis through organic acids in sugar syrup trickled over bees in beespaces, citric acid (present in lemon juice) seems to be the safest for bees while providing highly effective treatment. Oxalic acid, although working faster and more efficiently is not indifferent to the health of bees. The rule is that (on a winter bee) you can use it only once.
Italian experiments as well as my own have shown that repeated application of citric acid in concentrations up to 5% on the winter bee is harmless. But we should avoid repeating this procedure. Each winter application of (whatever) in sugar syrup has a stimulating effect. For about two weeks the activity of bees is increased, including the rise of temperature by ca 2st., which costs the colony consumption of about 1.5 kg of food extra. The guts of bees become overloaded which in the case of a late spring may cause known problems. The optimal time for application is late November / early December, just before a predicted period of several warmer days to allow bowel evacuation by wintering bees.