However, of more than 150 essential oils and components of oils tested, only very few have proven effective when applied in hives in field trials. This is most probably due to the fact, that the screenings tests used were incapable of predicting the acaricidal effect under field conditions. Difficulty in obtaining standardised essential oils also affects treatment predictability. Only a combination of wintergreen oil and thermal treatment, an aerosol treatment of a thyme-sage oil mixture, and the passive evaporation of thymol, oregano oil and marjoram oil in combination with diluted formic acid have been used successfully for mite control. For different reasons, however, none of these treatments have been widely adopted by beekeepers, with the exception of thymol.
Different thymol containing products are available on the market.
Because of insufficient predicting capacity of the screening tests used until now, we devised an assay, in which the dose-response relationship of an airborne acaricide and the corresponding mite and bee mortalities can be assayed under laboratory conditions. Identifying compounds with acceptable acaricidal activity but with low toxicity to honey bees is essential for providing candidate compounds for field trials. After finding suitable substances under laboratory conditions we will measure the air concentration under field conditions to test their efficacy in a bee colony. This procedure can serve as a powerful screening technique because it guides subsequent field research into the most productive avenues. The development of effective delivery systems for essential oils remains one of the greatest obstacles to their implementation as mainstream control measures.