Acaricidal efficacies of Lippia gracilis essential oil and its phytochemicals against organophosphate-resistant and susceptible strains of Rhipicephalus (Boophilus) microplus

Abstract

Plant-derived natural products can serve as an alternative to synthetic compounds for control of ticks of veterinary and medical importance. Lippia gracilis is an aromatic plant that produces essential oil with high content of carvacrol and thymol monoterpenes. These monoterpenes have high acaricidal activity against Rhipicephalus (Boophilus) microplus. However, there are no studies that show efficacy differences of essential oils between susceptible and organophosphate resistant strains of R. (B.) microplus. The aim of the present study was to compare acaricidal effects of essential oils extracted from two different genotypes of L. gracilis and the main monoterpenes on larvae of both susceptible and organophosphate resistant R. (B.) microplus larvae. The efficacy of the essential oil of two genotypes of L. gracilis (106 and 201) and their monoterpenes carvacrol and thymol was measured using the larval immersion test on coumaphos-resistant and susceptible strains of R. (B.) microplus. Lethal concentrations were calculated using GraphPad Prism 6.0. Chemical analysis was performed by GC–MS and FID. Thymol and carvacrol were observed to be major constituents in 106 and 201L. gracilis genotype essential oils, respectively. Essential oils of both genotypes were more effective against organophosphate-resistant tick strain than susceptible tick strain. Carvacrol was 3.2 times more toxic to organophosphate resistant strain than to susceptible strain. Thymol was equally toxic to resistant and susceptible tick strains. The significantly higher efficacy monoterpene carvacrol against resistant ticks may lead to development of new natural product acaricide formulations for use to control organophosphate resistant R. (B.) microplus populations.