Insecticidal Activities and Mechanism of Action of Steroidal Saponins from Trillium govanianum Wall. ex D. Don Against Plutella xylostella (L.) and Aphis craccivora Koch.

Diamondback moth, Plutella xylostella (L.), is a major pest of cruciferous crops, whereas Aphis craccivora is a major sucking pest of leguminous crops. Indiscriminate use of synthetic insecticides for the control of insect pests leads to insecticide resistance, harmful to natural enemies of pests, pollinators, the environment, and consumers' health. Therefore, it is necessary to screen/identify alternate strategies to control the target pests. Steroidal saponins, a group of specialized amphiphilic metabolites, are known for their various biological properties. There are a few reports on insecticidal activities of steroidal saponins. In the present study, insecticidal activities of nine diverse steroidal saponins isolated from Trillium govanianum were evaluated against Plutella xylostella and Aphis craccivora. Among the molecules, govanoside E (3) was found more effective against Plutella xylostella (LD = 0.91 µL/insect) after 96 h of treatment followed by govanosides D and F (1.44 and 1.56 µL/insect, respectively). Similarly, govanoside E was also found effective against Aphis craccivora (LD = 1.04 µL/insect) followed by dehydroxy-diosgenin and pennogenin-triglycoside (1.19 and 1.36 µL/insect, respectively). Furthermore, govanoside E (3) showed promising repellency (RC = 1043.20 µL/L) against Plutella xylostella. The molecules also showed a dose-dependent relationship with inhibitory effects on the reproduction of Aphis craccivora. In vitro enzyme inhibition/mechanism study confirmed that govanoside E (3) inhibited glutathione-S-transferase (GST) and carboxylesterase (CES1) in test insects. The present study suggested the potential utilization of steroidal saponins for the control of target pests based on field bio-efficacy results.