Acaricidal activity of five essential oils of Ocimum species on Rhipicephalus (Boophilus) microplus larvae

The aim of this study was to evaluate the acaricidal activity on the cattle tick Rhipicephalus (Boophilus) microplus of essential oils from three Ocimum species. Acaricidal activity of five essential oils extracted from Ocimum gratissimum L. (three samples), O. urticaefolium Roth, and O. canum Sims was evaluated on 14- to 21-day-old Rhipicephalus microplus tick larvae using larval packet test bioassay. These essential oils were analyzed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC/MS) showing great variations of their chemical compositions according to the botanical species and even within the O. gratissimum species; the acaricidal activity of their main compounds was also evaluated. The essential oils of O. urticaefolium and O. gratissimum collected in Cameroon were the most efficient with respective LC50 values of 0.90 and 0.98 %. The two essential oils obtained from O. gratissimum collected in New Caledonia were partially active at a dilution of 5 % while the essential oil of O. canum collected in Cameroon showed no acaricidal activity. The chemical analysis shows five different profiles. Whereas the essential oils of O. urticaefolium from Cameroon and O. gratissimum from New Caledonia contain high amounts of eugenol (33.0 and 22.3–61.0 %, respectively), 1,8-cineole was the main component of the oil of an O. canum sample from Cameroon (70.2 %); the samples of O. gratissimum oils from New Caledonia are also characterized by their high content of (Z)-β-ocimene (17.1–49.8 %) while the essential oil of O. gratissimum collected in Cameroon is mainly constituted by two p-menthane derivatives: thymol (30.5 %) and γ-terpinene (33.0 %). Moreover, the essential oil of O. urticaefolium showed the presence of elemicin (18.1 %) as original compound. The tests achieved with the main compounds confirmed the acaricidal activity of eugenol and thymol with residual activity until 0.50 and 1 %, respectively, and revealed the acaricidal property of elemicin, which was the most efficient compound with 100 % of acaricidal activity at a dilution of 0.25 % and could be a valuable acaricide for the control of the cattle tick R. microplus.