Acaricidal activity of (E)-cinnamaldehyde and α-bisabolol on populations of Rhipicephalus microplus (Acari: Ixodidae) with different resistance profiles.

This study aimed to investigate the acaricidal activity of (E)-cinnamaldehyde and alpha-bisabolol on populations of Rhipicephalus microplus with different resistance profiles. The adult immersion test (AIT) was used to characterize the susceptibility of tick populations (50 field populations) to synthetic acaricides: deltamethrin, amitraz, and chlorfenvinphos. The larval packet test (LPT) was used to determine the LC50 values for (E)-cinnamaldehyde (populations 1-25) and alpha-bisabolol (populations 26-50) at the concentrations of 0.31, 0.62, 1.25, 2.0, 2.5, 5.0 and 10.0 mg/mL. The susceptible strain Porto Alegre (POA) was used as a reference for calculating the resistance ratio (RR). In the AIT, deltamethrin did not show efficacy >95 % for any of the populations, whereas amitraz and chlorfenvinphos have presented efficacy >95 % for three (6 %) and 15 (30 %) populations, respectively. In the LPT, the LC50 values of (E)-cinnamaldehyde and alpha-bisabolol varied from 0.23 to 2.36 mg/mL and 1.57-3.01 mg/mL, respectively. The RR50 for (E)-cinnamaldehyde showed 20 (80 %) populations with values <1.0 and no population with values>1.5. As for alpha-bisabolol, only two (8%) populations have presented RR50 <1.0, whereas three (12 %) populations showed incipient resistance to this sesquiterpene (RR50 between 1.5 and 2.0). The results indicate that all studied tick populations showed low susceptibility to at least one of the commercial acaricides tested. In addition, comparison between the LC50 values of (E)-cinnamaldehyde and alpha-bisabolol for the field populations and the susceptible strain POA suggests that there is no cross-resistance of (E)-cinnamaldehyde and alpha-bisabolol for the tick populations evaluated, and that the differences in the LC50 values are due to population variations.