Microstructural and ultrastructural changes in the muscle cells of the oriental armyworm Mythimna separata Walker (Lepidoptera: Noctuidae) on treatment with wilforine

•Wilforine showed oral toxicity against both M. separata larvae and adults.•The typical toxicity sign of M. separata on treatment with wilforine was paralysis leading to death.•Wilforine could induce pathological changes in the muscle cells of oriental armyworm.•This is the first report of insecticidal mechanism of wilforine on M. separate.

This study investigated the mode of action of wilforine, an alkaloid with insecticidal properties, extracted from Tripterygium wilfordii Hook f., on the microstructure and ultrastructure of the muscle cells of larvae and adults of the oriental armyworm Mythimna separata Walker. The bioassay results showed that wilforine had oral toxicity against both M. separata larvae (LC50 = 63 μg/mL) and adults (LC50 = 36 μg/mL). The typical toxicity sign was paralysis leading to death. Both light and electron microscope observations revealed that damage to the muscle cells increased with poisoning time in larvae and adults treated with the LC80 dose of wilforine. Histopathological examinations in the muscle cells of M. separata adults showed that there were large cytoplasmic spaces, disrupted Z-lines and swollen mitochondria in the muscle cells. Further, the sarcoplasmic reticulum was excessively dilated and fragmented; the nuclear membrane was ruptured; nuclear material was overflowing; and the myolemma was damaged. The similar pathological changes in the muscle cells of oriental armyworm larvae were observed, as above. In addition, a medullary sheath structure appeared and crystalline inclusion was also observed in the muscle cells of M. separata larvae. In conclusion, wilforine could induce pathological changes in the muscle cells of oriental armyworm larvae and adults, leading to their death; thus, the active site of action of wilforine maybe located in the muscle tissue of insects.

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