Bioactivity-guided isolation of toosendanin and salanninolide from Melia volkensii and their antifeedant activity against economically important insect pests

The fall armyworm (Spodoptera frugiperda), sweet potato weevil (Cylas puncticollis) and red flour beetle (Tribolium castaneum) are insect pests of economic importance that negatively impact production of maize, sweet potato and stored grain, respectively. The continued use of synthetic insecticides to control these insect pests can cause undesirable effects to the environment, human, and other non-target organisms, leading to renewed interest in botanical pesticides as a safer alternative. In this study, bioactivity-guided strategy was applied to isolate insect antifeedants from Melia volkensii, a native tree species to the drylands of East Africa. Initial laboratory screening of M. volkensii nut and pulp crude extracts showed antifeedant activity against S. frugiperda, C. puncticollis and T. castaneum in our previous publication. Fractionation of the nut and pulp crude extracts using various solvents of increasing polarity showed that the antifeedant activity was retained in the dichloromethane fraction, which was subsequently purified by column chromatography. The resultant bioactive fractions were further purified by preparative-HPLC, preparative-TLC and recrystallization, which yielded two pure bioactive compounds char-acterized as toosendanin and salanninolide using NMR and LC-MS. Toosendanin and salanninolide showed strong activity against T. castaneum and S. frugiperda, with EC50 values of 0.11% and 0.14%, and 0.15% and 0.10%, respectively. In contrast, there was no antifeedant activity by both isolated compounds against C. puncticollis. This study provides new leads for development of insect control products from M. volkensii.