Behavioral and Electrophysiological Evidence for Repellency of Phytolacca americana (Pokeweed) Fruit Extract in Plutella xylostella

Phytolacca americana L. (pokeweed) is an invasive alien plant in China that experiences little herbivore or disease pressure, suggesting the presence of effective chemical defenses. Although insecticidal activity of P. americana tissues has been reported, its potential repellent effects against the diamondback moth, Plutella xylostella (L.), remain unexplored. In this study, we further optimized distillation conditions to obtain fruit extract (FE) from P. americana. Subsequently, its chemical composition was analyzed. behavioral repellency, and antennal olfactory activity against P. xylostella adults. Gas chromatography-mass spectrometry (GC-MS) tentatively identified 11 compounds, accounting for 96–99% of the total distillate composition. The major constituents in the final distillate were palmitic acid ethyl ester (26.00%), 6,10,14-trimethyl-2-pentadecanone (25.75%), and ethyl oleate (14.19%). Behavioral assays using a Y-tube olfactometer demonstrated that the FE, as well as diethyl phthalate, palmitic acid ethyl ester, and ethyl oleate, exhibited strong repellent activity against adult P. xylostella, whereas 6,10,14-trimethyl-2-pentadecanone showed moderate repellency only at high concentrations (10 μg/μL). Electroantennogram (EAG) recordings revealed robust antennal responses to the three highly repellent compounds, with ethyl oleate eliciting the strongest response (0.58 mV at 100 μg/μL). Furthermore, Diethyl phthalate shows moderate repellency to P. xylostella. These results indicate that P. americana FE mediates repellency primarily through olfactory perception and that its major constituents contribute substantially to this effect. Overall, this study provides the first laboratory evidence that P. americana FE functions as an effective olfactory repellent against P. xylostella. The findings offer preliminary proof-of-concept for developing phytogenic repellents.