Repellency of cassia bark, eucalyptus, and star anise oils and their major constituents to Leptotrombidium pallidum (Acari: Trombiculidae).

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, cassiabark oil, 1,8-cineole, and star anise oil were approximately 43, 16, 11, 8, and 4 times more effective than IR3535 (CC5, 6.51%) as judged by the median climbing distance-disturbing concentration (CC50) values. The median residual duration time of repellency (RT50) was significantly more pronounced in DEET (RT50, 323 min) than in all essential oils and constituents (108-167 min). In the light of global efforts to reduce the level of highly toxic synthetic repellents, the three essential oils and their major constituents described merit further study as potential biorepellents for the control of L. pallidum populations.