Binary combinations of thymol, carvacrol and eugenol for Amblyomma sculptum control: evaluation of in vitro synergism and effectiveness under semi-field conditions

Abstract

This study aimed to investigate the in vitro acaricidal effect of thymol, carvacrol and eugenol (binary combinations) and their synergistic effect on unfed larvae and nymphs of Amblyomma sculptum, estimate the cost to prepare a formulation with these compounds, and evaluate the formulations under semi-field conditions against A. sculptum larvae. To evaluate the acaricidal effect of the compounds alone or their binary combinations (1:1 ratio) the larval packet test was performed using six concentrations (0.3125 to 10.0 mg/mL) for larvae and seven concentrations (2.5 to 20.0 mg/mL) for nymphs. The cost to produce 1 L of thymol, carvacrol and eugenol solutions, combined or alone, was estimated. In the semi-field test, formulations were sprayed on Panicum maximum plots experimentally infested with unfed A. sculptum larvae. In the in vitro tests with larvae, a synergistic effect (synergism and moderate synergism) was observed for thymol + eugenol at all concentrations tested, while thymol + carvacrol and eugenol + carvacrol showed a synergistic effect at the concentration of 5.0 mg/mL. In the in vitro tests with nymphs, thymol + eugenol, thymol + carvacrol and carvacrol + eugenol showed a synergistic effect (synergism and moderate synergism) at a concentration of 5.0 mg/mL and higher. Combining the compounds reduced the concentrations required to obtain mortality rate above 95%, resulting in lower costs. Under semi-field conditions, the treatments with thymol + eugenol and carvacrol + eugenol showed efficacies of 63.2% and 42.6%, respectively. We conclude that, depending on the concentrations, the binary combinations of thymol, carvacrol and eugenol have a synergistic acaricidal effect on unfed A. sculptum larvae and nymphs, and under semi-field conditions the associations between thymol + eugenol and carvacrol + eugenol were effective against A. sculptum larvae.