Formic acid, oxalic acid and thymol give insufficient and also widely varying results in the fight against winter mortality. This is because the amount that reaches the bees and the mites is highly dependent on the temperature. In addition, microorganisms also rapidly decompose the acids. In the majority of cases where bee colonies are exposed to low doses of formic acid for prolonged periods there is no reduction whatsoever in winter mortality. In the brochure ‘Effectieve bestrijding van varroa’ [Effective Varroa control] (WageningenUniversity, 2010), only the effectiveness against the Varroa mite in the various applications is mentioned, and not the results after the winter.

Limited influence on winter mortality

Formic acid, oxalic acid and thymol have been used for many years already, for approximately 30, 25 and 15 years respectively. Yet bee decline, including winter mortality, in this same period has only increased. This should lead to the conclusion that these substances are not so very effective in preventing such problems.

From large-scale research conducted by the USDA it transpires that with the use of products containing thymol, winter mortality was 8 percentage points lower, dropping for instance from 32% to 24%. At the same time, it was found that where bee decline was very high (e.g. > 40%) thymol did not have any effect at all in preventing winter mortality.

Many studies into how formic acid, oxalic acid and thymol work have only examined the effect on the number of mites still present in the bee colony after treatment. The influence of the treatments on winter mortality was not examined.

Side-effects of formic acid, oxalic acid and thymol

The application of these substances weakens the bees' immune system. For further information, see

Also the effects on bee larvae are documented, see