Repellency of Cassia Bark, Eucalyptus, and Star Anise Oils and Their Major Constituents to Leptotrombidium pallidum (Acari: Trombiculidae)

ABSTRACT Leptotrombidium pallidum (Nagoya, Miyagawa, Mitamura & Tamiya) is a primary vector of Orientia tsutsugamushi (Hyashi), the causative agent of scrub typhus. An assessment is made of the repellency to L. pallidum larvae (chiggers) of cassia bark, eucalyptus, and star anise oils and major constituents (E)-cinnamaldehyde, 1,8-cineole, and (E)-anethole of the corresponding oils. Results were compared with those of conventional repellents DEET (N,N-diethyl-3-methylbenzamide), IR3535 [(ethyl 3-[acetyl(butyl)amino]propanoate)], and permethrin. Based on the median repellent concentration (RC50) values, (E)-cinnamaldehyde, (E)-anethole, cassia bark oil, and star anise oil (RC50, 0.95–1.52 mg/cm2) exhibited significantly more potent repellency than DEET (3.85 mg/cm2). (E)-cinnamaldehyde, (E)-anethole, cassia bark oil, 1,8-cineole, and star anise oil were ≈43, 16, 11, 8, and 4 times more effective than IR3535 (CC50, 6.51%) as judged by the median climbing distance-disturbing concentration (CC50) values. The ...