Evaluation of synergism and development of a formulation with thymol, carvacrol and eugenol for Rhipicephalus microplus control

Abstract The acaricidal activity of combinations of thymol, carvacrol and eugenol was evaluated on larvae and engorged females of the cattle tick Rhipicephalus microplus. The first step assessed the compounds separately, in concentrations of 3.125, 6.25, 12.5 and 25 mg/mL. Then tests were performed with the compounds combined in the ratio of 1:1 at concentrations of 3.125 and 6.25 mg/mL, along with the control group treated with the solvent (3% DMSO). In the second step, combinations were tested incorporated in a formulation at the concentration de 6.25 mg/mL, using the larval packet and female immersion tests. The associations carvacrol + thymol (3.125 mg/mL), carvacrol + eugenol and thymol + eugenol (6.25 mg/mL) presented synergism, while the other associations had an additive effect. In the experiments with formulation, all combinations caused 100% larval mortality, but the efficacy was under 15% against engorged females. Therefore, the combinations of thymol + carvacrol (3.125 mg/mL) as well as carvacrol + eugenol and eugenol + thymol (6.25 mg/mL) had a synergistic effect on engorged females, but when incorporated in the formulation, the acaricide activity was strong against larvae but weak against engorged females.