Screening of volatile compounds of essential oils with acaricidal activity in Rhipicephalus microplus larvae using a new in vitro evaluation method

The study assessed the acaricidal properties of 20 compounds derived from essential oils against the Rhipicephalus microplus tick, a significant menace to tropical livestock with considerable economic losses. A novel methodology was devised, involving the determination of mortality times for R. microplus larvae upon direct contact with these compounds. Mortality times were compared against a negative control (soybean oil) and two positive controls (commercial pour-on products). Tests were conducted in 48-well polyethylene plates, with around 100 larvae immersed in 200 mu l of oil (at a 100% dose). The 10 compounds exhibiting the shortest mortality times were subsequently diluted in soybean oil at varying concentrations. Notably, thymol exhibited potent acaricidal activity, with the lowest effective dose (1.56%) resulting in larval mortality within 15 minutes, whereas positive controls necessitated over 66 minutes. A correlation was established between the molecular structure of the compounds and their acaricidal efficacy, indicating that cyclic compounds containing 10 carbon atoms demonstrated heightened lethality compared to acyclic counterparts. This innovative methodology enables the rapid identification of substances with substantial acaricidal potential in vitro, yielding results within minutes, in stark contrast to the traditional larval immersion test, which demands a minimum of 24 hours for conclusive outcomes.