In vitro evaluation of the acaricidal activity of linalool alone and in association with ivermectin against Rhipicephalus microplus larvae

Rhipicephalus microplus is one of the important ectoparasites affecting cattle production and is responsible for significant economic and health losses, especially in tropical and subtropical regions. The intensive use of synthetic acaricides, such as ivermectin, has favored the selection of resistant populations, reinforcing the need for alternative control strategies. In this context, the present study aimed to evaluate the acaricidal activity of linalool, alone and in association with ivermectin, against ivermectin-resistant R. microplus populations. Four field populations from three municipalities in the semiarid region of Paraíba, Brazil, were evaluated. The bioassays were performed using the larval packet test, with determination of the lethal concentrations for 50% and 90% of the larvae (LC50 and LC90). As a result, linalool tested alone showed acaricidal activity in all evaluated populations, with LC50 values ranging from 0.941% to 1.865% and LC90 values ranging from 1.244% to 4.850%. Ivermectin, when tested alone, showed high levels of resistance, with LC50 values ranging from 345.637 ppm to 14,622.967 ppm and resistance ratios ranging from 13.70 to 579.70 in relation to the POA strain. On the other hand, when associated with the LC50 of linalool, ivermectin showed a marked reduction in its lethal concentrations, suggesting a possible synergistic effect between the compounds in all tested populations, with LC50 values ranging from 0.643 to 8.998 and a potentiation factor (PF) ranging from 1.625 and 537.53. These results demonstrate that the association between linalool and ivermectin represents a promising strategy for the control of resistant R. microplus populations and may contribute to the development of more sustainable management methods.