Formic acid, oxalic acid and thymol are widely used to combat Varroa mite infestation. The effect is that the mites fall off the bees. When formic acid is administered, there is considerable mite fall within 24 hours, after which mite fall ceases. When thymol is administered the mite fall lasts longer.
How formic acid, oxalic acid and thymol work is based on changes to the microbial systems which the bees and mites carry. However, the operation of each of these substances is different. Even though these acids are applied externally, the bees cannot avoid ingesting some anyway. Once absorbed by the bees, the substances cause all minerals (including iron) to be released into the bees' hemolymph. This changes the relative availability of the various minerals. The changed microbial composition in the bees could also have a favorable effect on the digestion of food.
After administering formic acid, oxalic acid and thymol the conditions are also changed for the microorganisms that help the mite to infect the bee. The chemicals don't necessarily have an effect on the mites, but they do affect the microbial system that bees and mites carry.
How formic acid and oxalic acid work
The effect of formic acid and oxalic acid on the mite fall is based on a reduction of the pH value. The system becomes more acidic. This increases the availability of minerals including that of iron. The pH reduction affects the microorganisms in the bees as well as in the mites. The microbial biofilm at the infection site changes.
The University of Wageningen publication ‘Effectieve bestrijding van varroa’ [Effective Varroa control] (WUR, 2010) states that the formic acid kills the mites by affecting the respiratory system. Perhaps this is deduced from the way in which the organic solvents chloroform and ether work. For oxalic acid, the publication states that it is not exactly known how this works. The high acidity of oxalic acid is described as the most likely effective mechanism.
How thymol works
Thymol is known for its antibacterial effect. See http://en.wikipedia.org/wiki/Thymol. It is not an antibiotic, but it does affect the microbial composition. The operation of thymol is based on the chelate effect. Chelates have the property to bind metals (minerals), as well as to extract metals and to make them soluble. Essential oils work in the same manner. These types of substance are also known as feeding stimulants, for example, in Honey-B-Healthy and a range of other products.
The brochure ‘Effectieve bestrijding van varroa’ [Effective Varroa control] (WageningenUniversity, 2010) describes, among other things, the operation of thymol, that is poisoning the mites and blocking neuroreceptors. It is highly unlikely that that information is correct.