Efficacy of Permethrin as a Repellent and Toxicant for Personal Protection Against the Pacific Coast Tick and the Pajaroello Tick (Acari: Ixodidae and Argasidae)

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Abstract
The repellency and toxicity of permethrin as a clothing impregnant was tested against adults of the Pacific Coast tick, Dermacentor occidentalis, and nymphs and adults of the pajaroello tick, Ornithodoros coriaceus, under field and laboratory conditions. When offered a choice of permethrin-treated (5.7–12.8 μg AI/cm2) vs. untreated cotton surfaces to crawl on, most individuals of both ticks gravitated toward the untreated surface within 1–2 min. This apparent initial repellency wore off gradually (8–15 min) in D. occidentalis and more rapidly (4–8 min) in O. coriaceus. Exposure of D. occidentalis to cotton surfaces treated with ca. 3.8–5.4 μg AI/cm2 for 75 s caused 92% and 100% morbidity/mortality within 1 and 22 h, whereas exposure of O. coriaceus to similar concentrations for only 45 s produced 48% and 100% morbidity/mortality within 1 and 24 h. A comparison of the number of D. occidentalis ticks collected from humans wearing treated vs. untreated overalls as they walked through tick-infested chaparral-grassland revealed that 14% fewer ticks were found on the treated clothing, but this difference was not significant. The difference in the morbidity/mortality 1 day later of ticks removed from the treated and untreated overalls (60% vs. 3%) was highly significant. When D. occidentalis adults (n = 40) were placed on the midshin region of untreated overalls (worn by standing subjects) or those containing μg permethrin/cm2, 35–45% of the ticks crawled up to various points between the knee and waistline within 15 min, and all were still healthy 1 h later. In contrast, 90% of the adults (n = 20) similarly placed on overalls treated with 4 μg AI/cm2 did not crawl up to the knee or beyond within 15 min, and 94% (n = 18) were morbid 1 h later. Preliminary attempts to determine the effectiveness of permethrin for personal protection against O. coriaceus ticks under field conditions were unsuccessful, apparently because of unfavorable weather conditions.