Acaricidal activity of Mentha suaveolens subsp. timija, Chenopodium ambrosioides, and Laurus nobilis essential oils, and their synergistic combinations against the ectoparasitic bee mite, Varroa destructor (Acari: Varroidae)

The main objective of the present study was to evaluate under laboratory conditions the toxicity of essential oils (EOs) extracted from Mentha suaveolens L. subsp. timija (Briq.) Harley, Chenopodium ambrosioides L. and Laurus nobilis L. against Varroa destructor adult mites. Also, various combinations were prepared by mixing equal concentrations of any two and all three studied EOs and used to evaluate the combinational acaricidal effects of mixtures against adult Varroa mites. The GC/MS analysis indicated that the main chemical compounds of M. suaveolens subsp. timija EO were menthone (40.42%) and pulegone (19.22%), whereas C. ambrosioides EO is rich in alpha-terpinene (34.08%), isoascaridol (13.6%), p-cymene (10.95%), thymol (10.26%), ascaridole (10.25%) and carvacrol (7.75%). L. nobilis EO is mainly composed with 1,8-cineol (37.5%) and linalool (14.09%). The fumigation bioassays showed that the three tested EOs showed varroacidal toxicity, whereas no mortality has been observed on honey bees that were exposed to their higher concentrations. The highest acaricidal potency was observed with M. suaveolens subsp. timija (LD50 = 3.360 mu l/L-air and LD90 = 7.300 mu l/L-air). According to the combination index (CI) calculated by CompuSyn software, the ternary blend combination exhibited high synergistic effectiveness with a CI value of 0.343. All combinations have shown favorable dose reduction with DRI indexes greater than one. Our findings suggest that EO combinations tested could present a promising solution to reduce the effective doses of EOs and constitute a promising alternative to synthetic acaricides.