Acute toxicity of essential oils and other natural compounds to the parasitic mite, Varroa destructor, and to larval and adult worker honey bees (Apis mellifera L.)

Essential oils and other natural compounds were evaluated for their toxicity to the parasitic mite Varroa destructor and to larval and adult worker honey bees (Apis mellifera L.). A glass-vial residual bioassay was used to fast-screen the relative toxicity of 22 natural products to V. destructor. Results showed that menthol, clove oil, origanum oil, and thymol were the most toxic products, causing 87, 96, 100, and 100% mite mortality, respectively, at a dose of 0.75 mg / vial. The acute toxicity of these four products to the mites was further evaluated by estimating LC50 values. Thymol and origanum oil had the lowest LC50 (56.1 mu g / vial in both cases). In a subsequent experiment, topical applications of eight dilutions of the above four selected products were carried out on adult worker bees and larvae, using a newly-developed larval assay, to estimate their LC(50)s. Thymol had the lowest LC50 for adult bees (210.3 mu g / bee) and menthol had the highest (523.5 mu g / bee). Furthermore, thymol was the most toxic product to larvae, with a LC50 of 150.7 mu g / larva, while menthol was the least toxic with a LC50 of 382.8 mu g / larva. Tau-fluvalinate used as a control was significantly less toxic to mites and equally or more toxic to workers and larvae than the above four natural compounds. Results of this study suggest that besides thymol, which is already a registered varroacide, origanum oil, clove oil, and menthol, are potential candidate products to be further tested for the control of V, destructor.