Insecticidal and repellent activities of thymol from the essential oil of Trachyspermum ammi (Linn) Sprague seeds against Anopheles stephensi

Essential oil of seeds of Trachyspermum ammi (Linn.) Sprauge and its pure constituent thymol showed promising results when evaluated for larvicidal, oviposition-deterrent, vapor toxicity, and repellent activity against malarial vector, Anopheles stephensi. Thymol was 1.6-fold more toxic than the oil toward fourth-instar larvae of A. stephensi with LD₅₀ values of 48.88 and 80.77 μg/ml, respectively. Egg laying by female adults of A. stephensi was much significantly reduced when exposed to vapors of thymol compared to the oil of T. ammi seeds, and similar effects were recorded for subsequent egg hatching and larval survival. Vapor toxicity assay showed LC₅₀ value of 79.5 mg/mat for thymol against adults of A. stephensi, whereas the crude oil exhibited the LC₅₀ value of 185.4 mg/mat. Thymol provided complete repellency toward A. stephensi adults at the dose of 25.0 mg/mat after 1 h duration, whereas same degree of repellency was obtained by the oil at the dose of 55.0 mg/mat, indicating its double-fold activity than the oil.