Enhanced efficacy of carvacrol and thymol in combination with benzalkonium chloride for the control of the cattle tick Rhipicephalus microplus

The control of Rhipicephalus microplus represents a major challenge for livestock production due to the widespread emergence of tick populations resistant to conventional acaricides, highlighting the need for alternative strategies capable of improving control efficacy. Bioactive constituents of essential oils, such as the phenolic monoterpenes carvacrol and thymol, have demonstrated acaricidal activity, although their performance may vary depending on formulation and exposure conditions. In this study, we evaluated the effects of carvacrol and thymol in combination with the cationic surfactant benzalkonium chloride (BAC) against R. microplus, aiming to determine whether BAC enhances the biological activity of these monoterpenes. In vitro larval and adult immersion assays were performed, and lethal concentrations were determined for individual compounds and their combinations. In larval assays, the addition of BAC significantly increased the efficacy of thymol. In assays with engorged females, the association of BAC with either monoterpene resulted in a marked, dose-dependent increase in acaricidal efficacy compared with the compounds tested individually, leading to pronounced reductions in reproductive parameters and efficacies exceeding 90%. Overall, the results demonstrate that the enhancement of acaricidal activity by BAC is both life-stage-dependent and dose-dependent, supporting the use of BAC as an effective adjuvant in formulations containing phenolic monoterpenes. This approach represents a promising strategy to improve the efficacy of alternative acaricidal formulations for the control of R. microplus, particularly in the context of acaricide resistance.