Assessment of the contact toxicity of methyl palmitate on tetranychus viennensis (Acari: Tetranychidae)

Previous work demonstrated that methyl palmitate possesses acaricidal activity against Tetranychus viennensis Boisduval (Acari: Tetranychidae) via an unknown mechanism. Here, the symptoms of methyl palmitate toxicity to T. viennensi were studied to determine the acaricidal mechanism of action of this fatty acid methyl ester. Methyl palmitate caused concentration-dependent mortality of T. viennensis, with a moderate concentration (5 mg/ml) eliciting excitement and premature oviposition without spinning shortly after exposure. Tremors of the appendages were subsequently observed, followed by quiescence after approximate to 5 h. Mites developed dorsal fluid exosmosis at 15-20 h posttreatment with reduced egg production, followed shortly thereafter by death. Some typical neurotoxic symptoms such as excitement and convulsions were observed in methyl palmitate-exposed mites, suggesting that methyl palmitate may be a neurotoxin. Compared with other neurotoxic acaricides, methyl palmitate poisoning is a slow process in mites. Transmission electron microscopy revealed serious ultrastructural damage in response to 5 mg/ml methyl palmitate exposure. Autolysis of membranous structures was also observed, especially in the mitochondria, suggesting a novel mode of action for methyl palmitate-induced toxicity.