Analysis of synergism exhibited by essential oil combinations in controlling the brown dog tick Rhipicephalus sanguineus (Acari: Ixodidae)

This research investigated the acaricidal activity of essential oils (EOs) derived from Ocimum gratissimum (OO), Monarda citriodora (MO), and Cymbopogon nardus (RO), both individually and in binary and ternary mixtures, against the brown dog tick, Rhipicephalus sanguineus using the larval packet test. Seven concentrations (0.1, 0.2, 0.4, 0.8, 1.2, 1.6, and 2.0%) were evaluated for each oil and their combinations. Lethal concentration values (LC50, LC95, and LC99) with 95% confidence limits (CL) were determined, and interaction effects were assessed through the Combination Index (CI) and Dose Reduction Index (DRI) to identify additive, synergistic, or antagonistic responses. Among the tested individual oils, OO demonstrated the greatest acaricidal effectiveness, exhibiting the lowest LC50 value (0.19%). The binary mixture of OO + MO showed enhanced efficacy (LC50 = 0.12%) with a CI of 0.464, indicating the combination achieves the same biological effect with less drug exposure. Furthermore, the ternary blend (OO + MO + RO) produced the strongest synergistic interaction (CI = 0.359) and achieved substantial dose reductions, with DRI values of 10.66, 7.85, and 7.21 for OO, RO and MO, respectively. Low CI value with higher DRI values in ternary mixture indicated enhanced efficacy with reduced required doses. These results demonstrate the potential of EOs, both alone and in combination, against the larvae of R. sanguineus, and support their integration into pest management strategies.