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

Authors: COSTA-JUNIOR, LIVIO - Federal University - Brazil; Miller, Robert; ALVES, PERICLES - Federal University - Brazil; BLANK, ARIE - Federal University - Brazil; Li, Andrew; Perez De Leon, Adalberto - Beto

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2016
Publication Date: 9/9/2016
Citation: Costa-Junior, L.M., Miller, R., Alves, P.B., Blank, A.F., Li, A.Y., Perez De Leon, A.A. 2016. Acaricidal efficacies of Lippia gracilis essential oil and its phytochemicals against organophosphate-resistant and susceptible strains of Rhipicephalus (Boophilus) microplus. Veterinary Parasitology. 228:60-64.

Interpretive Summary: The cattle tick is an economically damaging ectoparasite and disease vector causing billions of losses to cattle production worldwide. This tick species has been eradicated from the U.S., but tick outbreaks continue to occur along the Mexico-U.S. border. The use of coumaphos, an organophosphate pesticide, is one critical component of the Cattle Fever Tick Eradication Program that has so far prevented the spread of cattle ticks beyond the quarantine areas at the border with Mexico in Texas. In countries where cattle ticks are endemic, tick control relies heavily on chemical pesticides (acaricides). As a consequence of continuous exposure to chemical acaricides, cattle ticks developed resistance to many commonly used acaricides in Mexico and other countries. Alternative tick control measures, including plant-derived natural products, are being investigated to help manage acaricide resistance problems. Lippia gracilis is an aromatic plant that produces essential oil with high content of the natural chemicals carvacrol and thymol, which are shown to have high pesticidal activity against ticks, mosquitoes, and other pest species. In this study, we tested two different extracts of Lippia gracilis, and their active ingredients, for their toxicities against susceptible and resistant strains of the cattle tick. We also demonstrated enhanced toxicity of Lippia gracilis essential oil and its major components against resistant populations of the cattle tick. This could potentially lead to development of new pest control products for management of cattle tick populations that are resistant to conventional chemical acaricides. The work is of interest to researchers, chemical companies, pest control managers, and public health professionals concerned with veterinary and medical risks associated with ticks.

Technical 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. 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 201 L. 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.