Combination of thymol and eugenol for the control of Rhipicephalus sanguineus sensu lato: Evaluation of synergism on immature stages and formulation development

The aim of this study was to investigate whether the combination of thymol with eugenol has a synergistic effect on the immature life stages of Rhipicephalus sanguineus sensu lato (s.l.), to evaluate the cost-benefit ratio of using these compounds in combination, and to develop a formulation combining thymol with eugenol with activity on immature stages of R. sanguineus s.l. To evaluate synergism, thymol and eugenol, combined (ratio 1:1) or not, were tested at concentrations of 2.5, 5.0, 10.0, 15.0 and 20.0 mg/mL on unfed larvae and nymphs using a larval packet test, and 0.625, 1.25, 2.5, 5.0 and 10 mg/mL on engorged larvae and nymphs using an immersion test. A cost estimate was calculated to produce 1 L of a solution containing a concentration of thymol and eugenol, combined or not, that could cause a tick mortality rate greater than 95 %. Finally, a formulation was developed, consisting of a micellar dispersion containing polymers (MDP), with thymol + eugenol (1:1), at concentrations of 2.5, 5.0, 10.0, 15.0 and 20.0 mg/mL, and the activity was evaluated on unfed and engorged larvae and nymphs. For unfed larvae and nymphs, concentrations of 2.5 and 5.0 mg/mL and 2.5, 5.0 and 10.0 mg/mL, respectively, presented synergistic effects. In tests with engorged larvae and nymphs, respective concentrations of 0.625, 1.25 and 2.5 mg/mL and 2.5 and 5.0 mg/mL had synergistic effects. The estimated costs for producing a solution of 1 L with efficacy greater than 95 % was $5.97 using only thymol (15 mg/mL), $ 5.93 using only eugenol (15 mg/mL), and $ 3.97 using thymol + eugenol (1:1 - 5,0 mg/mL). In tests with MDP, the combination of thymol + eugenol resulted in a mortality rate higher than 95 % at concentration of 10 mg/mL for unfed and engorged larvae and nymphs. Thus, the combination of thymol + eugenol, depending on the concentration, has synergistic effects and this combination lowers the cost for the active ingredients thymol and eugenol. The combination of thymol + eugenol in MDP had acaricidal activity against immature life stages of R. sanguineus s.l.