Synergistic effect of tea saponin combined with toosendanin and azadirachtin on the antifeeding activity against Spodoptera litura

In order to study the synergistic effect of tea saponin mixed with toosendanin and azadirachtin (two plant-derived insecticides) on the antifeeding activity against Spodoptera litura, this experiment used leaf immersion method to determine the antifeeding rate and synergistic ratio of tea saponin, toosendanin, azadirachtin, and their combinations (tea saponin + toosendanin and tea saponin + azadirachtin) against third-instar larvae of S. litura. Through pot efficacy trials, the antifeeding rate changes of S. litura were determined under different treatment times after smearing the three single agents and the combinations (tea saponin + toosendanin, tea saponin + azadirachtin) to Chinese cabbage leaves. The results showed that the medium antifeeding concentrations of tea saponin, toosendanin, and azadirachtin were 64.02 μg/mL, 29.90 μg/mL, and 3.98 μg/mL, respectively. The combination of tea saponin and toosendanin exhibited synergistic effect at ratios ranging from 9∶1~3∶7, with a maximum synergistic ratio of 1.39 at a ratio of 5∶5, showing additive effect. The combination of tea saponin and azadirachtin showed synergistic effect at ratios ranging from 9∶1~1∶9, with a maximum synergistic ratio of 1.87 at a ratio of 8∶2, showing synergistic effect. The highest antifeeding rate of tea saponin + toosendanin (5∶5) was 2.05 times that of tea saponin alone and 1.21 times that of toosendanin alone; the highest antifeeding rate of tea saponin + azadirachtin (8∶2) was 2.47 times that of tea saponin alone. In summary, the results showed that tea saponin mixed with toosendanin and azadirachtin had synergistic effect on the control of S. litura by the two plant-derived pesticides, providing scientific basis for the scientific prevention and control of S. litura and the development of environmentally friendly synergists.