Laboratory and field studies on the efficacy of essential oils in controlling varroasis on Apis mellifera ligustica Spinola and Apis mellifera macedonica Rüttner

For about 50 years, Italian beekeeping has been plagued by the main obligate pest of Apis mellifera L., the mite Varroa destructor (Anderson & Trueman). This ectoparasitic mite is the main problem for beekeepers all over the world, as it leads to the colony collapse of honeybees in a short time. Effective control of V. destructor is achieved by choosing appropriate methods in relation to the conditions of the colony and the environment, and by a combination of methods. Years of research have examined numerous products to be used against the mite, but although beekeepers use different approaches to control varroa infestation, there is still no product that is 100% effective, sustainable, economical and does not lead to drug resistance. Depending on the country, methods may vary due to laws regulating the use of chemicals. The trials in this paper were carried out in Italy on A. m. ligustica Spinola and in Greece on A. m. macedonica Rüttner, where the use of chemicals is legislated by the phytosanitary Regulation (EC) No 1107 of 21/10/2009. Three classes of acaricidal compounds with different mode of action are currently used: pyrethroids, fluvalinate and flumethrin; organophosphate, coumaphos; formamidine, amitraz. Unfortunately, these compounds tend to persist in honey and wax, compromising bee health and contaminating hive products used by humans. Organic acids such as formic acid and oxalic acid are also used. The latter have been found to be very effective in controlling the mite but in the long run can be toxic and they report the same problems as synthetic products. The aim of this work was to search for new products that can effectively control the mite as there is a need for new biologically active, non-risk compounds. The starting point was the study of current evidence suggesting that resistance to pyrethroid insecticides in V. destructor initially emerged in Italy during a 4 year period of heavy application of the product. Subsequently, several cases of resistant populations have been reported worldwide. In the context of environmentally friendly treatments, natural products, such as plant extracts and essential oils, could represent an opportunity for mite control with a lower environmental impact and reduced risks to human and bee health. Moreover, due to the large number of components of essential oils and the multiple target sites involved in their mechanism of action, the emergence of resistance in treated mite populations is less likely. Essential oils are highly volatile plant compounds with a strong and characteristic odour, stored in glands located in various parts of aromatic plants such as leaves, flowers, fruits, seeds, barks and roots. Essential oils perform several functions in plants, including acting as an attractant for pollen-carrying insects or as a repellent to protect plants from phytophagous insects. Several essential oils and their components, such as thyme oil, oregano, garlic, turmeric, cloves, cinnamon, neem, castor oil, eugenol, have been shown to be effective in reducing both mite viability and hive contamination. Furthermore, it has been shown that the efficacy of essential oils against V. destructor is comparable to that of organic acids and their application but causes a lower degree of stress in bees than the application of organic acids. In this work the efficacy of the essential oils tested was compared to oxalic acid, which is the most widely used organic acid and has been shown in several studies to be more than 90% effective. Tests were performed with three essential oils selected from data obtained in previous field trials (De Cristofaro et al., unpublished data): Lavandula officinalis Chaix, Geranium robertianum L. and Melaleuca leucadendron L. var. cajeputi R. The application of these oils at various doses and with different methodologies was evaluated with in vitro toxicity bioassays on V. destructor adults and A. m. ligustica and A. m. macedonica larvae, in the field on adults bees living in Dadant Blatt and Langhstroth hives, on adult bees reared under laboratory conditions, and finally, to study their olfactory perception, on foragers by electroantennography tests using 11 different essential oils (including the three selected ones). From the results of these trials that lasted about three years, according to the scientific literature, we conclude that essential oils could be an effective and sustainable tool to control V. destructor by integrating them with other means.