In vitro assessment of the acaricidal activity of carvacrol, thymol, eugenol and their acetylated derivatives on Rhipicephalus microplus (Acari: Ixodidae)

This study reports the comparative evaluation of the activity of carvacrol, thymol, eugenol and their respective acetylated derivatives (carvacrol acetate, thymol acetate and eugenol acetate) on Rhipicephalus microplus, to verify the possible influence of the acetate group. The acetylated derivatives were prepared from reactions of the phenolic compounds with acetic anhydride/pyridine. The formation of the products was confirmed by analysis of hydrogen and carbon-13 nuclear magnetic resonance (H-1 and C-13 NMR) spectra. The larval packet test was used to evaluate the acaricidal activity, with concentrations of 0.312, 0.625, 1.25, 2.5, 5.0, 7.5, 10.0 and 15.0 mg/mL. Thymol and carvacrol resulted in 100% mortality starting at the concentration of 2.5 mg/mL, while the same was observed for carvacrol acetate starting at the concentration of 5.0 mg/mL. For the other treatments, 100% mortality was only achieved in the groups treated with the highest concentration (15.0 mg/mL). The LC50 and LC90 values (mg/mL) of carvacrol acetate (2.49, 4.21), thymol acetate (2.97, 8.52) and eugenol acetate (4.25, 13.10) were higher than those for the corresponding precursor molecules carvacrol (0.83, 2.02), thymol (1.26, 2.21) and eugenol (2.77, 5.35). The acetylation process did not enhance the activity of these substances on unengorged larvae of R. microplus, since the precursor substances (carvacrol, thymol and eugenol) had greater efficacy.