Acaricidal activity of carvacrol and thymol on acaricide-resistant Rhipicephalus microplus (Acari: Ixodidae) populations and combination with cypermethrin: Is there cross-resistance and synergism?

This study evaluated the acaricidal activity of thymol and carvacrol on Rhipicephalus microplus populations with different resistance profiles and investigated the synergistic effect of combinations of these monoterpenes with cypermethrin. The adult immersion test (AIT) was used to characterize the susceptibility of tick populations (45 field populations) to synthetic acaricides: deltamethrin, amitraz and chlorfenvinphos. The larval packet test (LPT) was used to determine the LC₅₀ values for thymol (25 tick populations) and carvacrol (20 tick populations). The susceptible strain Porto Alegre (POA) was used as a reference for calculating the resistance ratio (RR). Subsequently, larval immersion tests (LIT) were performed with combinations of cypermethrin with thymol or carvacrol to assess a synergistic effect. In the AIT, deltamethrin showed efficacy > 90% in one (2.2%) population tested (mean: 12.1 and 11.1 for populations 1–25 and 26–40, respectively), whereas amitraz and chlorfenvinphos showed efficacy > 90% for two (4.4%) populations (mean: 61.3 and 47.3 for populations 1–25 and 26–40, respectively) and eight (17.7%) populations (mean: 69.7 and 59.7 for populations 1–25 and 26–40, respectively). In the LPT, the LC₅₀ values for thymol and carvacrol varied from 0.67 to 2.12 mg/mL and 0.55–3.21 mg/mL, with an average LC₅₀ for populations of 1.49 and 1.75 mg/mL, respectively. For thymol, no resistance was observed in any of the populations, values of RR₅₀ > 1.5. There was no correlation between the LC₅₀ values for thymol and the efficacy of the chemical acaricides tested. Regarding carvacrol, for only one tick population had the value of RR₅₀ > 1.5, indicating an incipient resistance. No correlation was observed between the LC₅₀ values for carvacrol and the efficacy of tested acaricides. The combination of thymol and carvacrol with cypermethrin showed a synergistic effect in the resistant population (Jaguar – thymol 4.19 and carvacrol 3.67), and no synergistic interaction were showed in the susceptible population. Answering the questions we conclude that: 1 - The comparison between the LC₅₀ values for thymol and carvacrol in field populations and the susceptible strain POA suggests the absence of cross-resistance (ticks and terpenes), and the differences between the LC₅₀ values for thymol and carvacrol in the different R. microplus populations are inherent to the characteristics of each population tested; 2 - the combination of thymol or carvacrol with cypermethrin showed a synergistic effect with different activity according to the population of ticks.