Acaricidal activity of essential oil-derived components from Thymus schimperi Ronninger against Varroa destructor Anderson and Trueman

Abstract

The ectoparasitic mite Varroa destructor has become a major worldwide pest of the honey bees (Apis mellifera L). Intensive use of chemical acaricides has led to the development of acaricide resistance in mites, thereby reducing the efficacy of acaricides and causing contamination of hive products. Thus, natural products such as essential oils can become an important alternative to synthetic chemicals in controlling Varroa mites. In this study, essential oil from Thymus schimperi grown in the Ethiopian highland was investigated for its chemical composition and toxicity against Varroa mites through complete exposure tests. Hydrodistillation of T. schimperi leaves yielded a pale yellow essential oil (0.8% v/w) with a strong and pleasant odor. Analyzed by GC-MS, 29 compounds, comprising 96.67% of the oil, were identified. Carvacrol (34.84%), thymol (23.03%), and p-cymene (14.20%) were found to be the main constituents. In the toxicity assay, T. schimperi essential oil showed high acaricidal activity against Varroa mites, with a 4 h-LC50 value of 109.16 µg/ml (5.46 mg oil/l air volume). Among the main constituents, thymol and carvacrol induced high toxicity to Varroa mites, with 4 h-LC50 values of 70.71 µg/ml (3.54 mg thymol/l air volume) and 106.10 µg/ml (5.31 mg carvacrol/l air volume), respectively. However, p-cymene was less effective in toxicity testing (4 h-LC50 > 5000 µg/ml). The results show for the first time that the essential oil of Thymus schimperi and its two main constituents, thymol and carvacrol, are highly acaricidal and can be used to control Varroa mite infestations.