In vitro assessment of synergistic combinations of essential oils against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

Rhipicephalus (Boophilus) microplus (Acari: Ixodidae), is one of the most important ectoparasite of cattle, responsible for causing severe economic losses in the tropical and subtropical regions of the world. The development of resistance to most of the commonly used chemical acaricides has stimulated the search for new herbal products as an eco-friendly tick control alternative. The aim of the present study was to evaluate the in vitro acaricidal activity of essential oils (EOs) of cloves (Syzygium aromaticum), bark of cinnamon (Cinnamomum zeylanicum) and leaves of lemon grass (Cymbopogon citratus) against unfed larvae of R. (B.)microplus by larval packet test. Also, various combinations were prepared by mixing of equal concentrations of any two and all three EOs and used to study the effects of interactions of mixtures against larvae of R. (B.) microplus. The lethal concentrations at 50% (LC50), 90% (LC90) and 99% (LC99) with 95% confidence limits (CL) were estimated by Probit analysis using PoloPlus. The computer software, CompuSyn, was used for determining the effects of interactions (synergistic, additive or antagonistic) of EO mixtures by calculation of Combination index (CI) and Dose-reduction index (DRI). Among the individual and mixture of two EOs, cinnamon EO and cinnamon EO + lemon grass EO combination showed highest acaricidal activity against R. (B.) microplus larvae. Further, the combination of cinnamon EO + lemon grass EO showed high synergism with CI value of 0.381, followed by cinnamon EO + clove EO showing moderate synergism whereas, clove EO + lemon grass EO showed only additive effect, with favorable dose reduction for each constituent drug in all three combinations. The combination of all three EOs (1:1:1) showed high synergism (CI value of 0.376) and favorable dose reduction (DRI index of 8.19, 25.64 and 4.64 for clove, cinnamon and lemon grass EOs, respectively) against R. (B.) microplus larvae.