Methyl Palmitate, an Acaricidal Compound Occurring in Green Walnut Husks

Walnut, Juglans regia L., is known for its insecticidal activities to a range of herbivores and microbes. Isolation and identification of bioactive compounds from walnut is a potential approach for the development of new pesticides. Laboratory experiments were carried out to investigate the acaricidal activities of green husk extracts of walnut. Bioassay-guided fractionation of petroleum-ether extracts of walnut led to the identification of a common plant-borne fatty acid ester, methyl palmitate (MP), which produced strong acaricidal activity (62.8% mortality) at 1 mg/ml at 24 h. The structure of MP was characterized with infrared spectrum and NMR, and the identification of MP confirmed using an authentic standard on high-performance liquid chromatography. Based on a slide dip bioassay, 10 mg/ml MP provided 97.9% mortality against adults of Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae), whereas mortality against eggs was much lower (57.2%).