Ocimum basilicum essential oil controls Haemaphysalis longicornis via multi-modal toxicity, enzyme inhibition, and synergy with hexythiazox

Abstract

Haemaphysalis longicornis is an important ectoparasite infesting livestock, particularly cattle and sheep, causing significant economic losses and serving as a vector for multiple zoonotic pathogens. The widespread development of acaricide resistance necessitates the exploration of alternative control strategies with novel mechanisms of action. The essential oil (EO) of Ocimum basilicum L. demonstrates potential as a multi-target botanical acaricide against the hard tick Haemaphysalis longicornis. GC-MS analysis identified eugenol (47.75%) as the major constituent. Bioassays revealed significant, dose-dependent contact toxicity against larvae (LC₅₀ = 18.47 mg/mL), nymphs (LC₅₀ = 34.93 mg/mL), and adults (LC₅₀ = 27.75 mg/mL). Purified eugenol exhibited 1.3–1.9-fold greater potency than the whole EO. The EO also showed strong fumigant activity and significantly suppressed fecundity by inhibiting oviposition and reducing egg hatchability. Mechanistic studies indicated that the acaricidal effect may be associated with the inhibition of key metabolic and neurological enzymes, including acetylcholinesterase (AChE), carboxylesterase (CarE), and glutathione S-transferase (GST). Furthermore, a 2:8 (hexythiazox: oil) formulation exhibited significant synergism against nymphs (Co-toxicity coefficient = 128.35), enabling a reduction in synthetic acaricide dosage. This work elucidates a multi-modal action mechanism of O. basilicum EO against H. longicornis through contact toxicity, fumigation, reproductive suppression, and enzyme inhibition. The synergistic interaction with hexythiazox provides a sustainable, plant-based strategy for controlling tick infestations in livestock, showing potential for addressing the critical challenge of acaricide resistance in animal production systems.