Role of antioxidant activity of essential oils in their acaricidal activities against Rhipicephalus annulatus

Essential oils of Origanum majorana and Satureja thymbra as well as carvacrol are natural products that are known to have potent antioxidant activities. The current study was designed to investigate the role of the antioxidant properties of these natural products in their acaricidal activities against Rhipicephalus annulatus larvae. The synergistic and/or antagonistic effects of the addition of vitamins E and C and hydrogen peroxide (H₂O₂) to these natural products were also evaluated. Larval packet tests were used to evaluate the acaricidal activities against the larvae of R. annulatus. The antioxidant effectiveness of these products was determined by a DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging assay. The addition of vitamin E at 100 mg/mL to O. majorana and S. thymbra decreased the concentrations required to achieve the death of half of the larvae (LC₅₀) to 0.44 and 0.47%, respectively. The combination of O. majorana and S. thymbra attained the LC₅₀ at 1.54% which was decreased to 0.69% after addition of vitamin E. Also, the addition of vitamin E to carvacrol reduced the LC₅₀ to 0.27%. The total antioxidant activity of these natural products increased significantly in presence of vitamin E. The addition of H₂O₂ inhibited the acaricidal activity of all tested materials, especially at low concentrations. All treatments induced an increase in lipid peroxidation, whereas carvacrol-treated larvae revealed the lowest values for the superoxide dismutase. Glutathione peroxidase and catalase activity decreased in larvae treated with S. thymbra combined with vitamin E. In conclusion, the addition of vitamins E and C increased the acaricidal activities of the tested compounds, whereas the addition of H₂O₂ decreased these activities. The antioxidant activities of essential oils and their active components may play an important role in mediating their acaricidal activities.