The Inhibitory Effect of Toosendanin on the Growth and Development of Spodoptera litura

This study aimed to investigate the inhibitory effect of toosendanin (TSN) on the growth and development of Spodoptera litura (Fabricius, 1775), especially in terms of feeding preference, larval growth and development, nutritional indices, and enzyme activities. The feeding preference of S. litura, larval growth and development, and nutritional indices were evaluated using the diet incorporation method. The activities of three detoxification, three digestive, and three antioxidant enzymes were determined using spectrophotometric, micro-assay, and microplate methods. The feeding preference of S. litura exposed to TSN indicated that the larvae had a significant preference for the untreated (control) feed compared with the TSN-supplemented feed. The growth and development results showed a prolongation of the larval developmental period by approximately one instar in S. litura larvae fed TSN-supplemented feed. At the highest concentration (100 μg/g), larval weight was 0.76 times that of the control group, with a 27.34% reduction in the pupation rate and a 23.58% reduction in the emergence rate. All differences were statistically significant compared with the control group. In addition, the pupal and adult deformity rates were significantly higher than those in the control group, being 14.68 and 5.67 times higher, respectively, at this concentration. Growth inhibition increased with increasing TSN concentration. In terms of nutritional efficiency, TSN treatment reduced food intake and decreased food conversion rate, thereby inhibiting the relative growth rate of the 4th instar larvae. Further, TSN treatment significantly increased the activities of trypsin, α-amylase, glutathione S-transferase, carboxylesterase, superoxide dismutase, peroxidase, and catalase, and had no significant effect on lipase activity, and significantly reduced cytochrome P450 enzyme activity. TSN can significantly inhibit the digestion and absorption of nutrients in S. litura larvae, slow down their growth and development, and increase the pupal and adult deformity rates, thereby protecting plants against infestation by S. litura.