Ginsenosides, the main effective components of Panax ginseng C.A. Meyer and Panax quinquefolius L., are important allelochemicals of ginseng. Although many studies have targeted the pharmacological, chemical, and clinical properties of ginsenosides, little is known about their ecological role in ginseng population adaptation and evolution. Pests rarely feed on ginseng, and it is not known why. This study investigated the effects of total ginsenosides on feeding behavior and activities of acetylcholinesterase (AChE) and glutathione s-transferase (GST) in Mythimna separata (Walker) larvae. The results showed that the total ginsenosides had significant antifeeding activity against M. separata larvae, determined by nonselective and selective antifeeding bioassays. In addition, the total ginsenosides had inhibitory effects on the activities of GST and AChE. The antifeeding ratio was the highest at 8 h, then decreased, and was the lowest at 16 h. Both GST and AChE activities decreased from 0 h to 48 h in all total ginsenosides treatments but increased at 72 h. Total ginsenosides had antifeeding activity against M. separata larvae and inhibitory effects on the activities of GST and AChE.