Sub-lethal Effects And Mechanisms Of Tea Saponin Levels On The Diamondback Moth, Plutella Xylostella

Plant natural products are regarded as an effective way to control insect pests or to delay their evolution of resistance to insecticides. To fully understand the bioactivities of natural chemicals on insect pests, it is most important to examine the sub-lethal effects on the pests. Tea saponin is an important biological resource not only for its insecticidal activities but also for its characteristics as a synergist and its excellent surfactant properties. However, the sublethal effects of tea saponin on insect pests, as well as the synergism between sublethal tea saponin and host plants, have not been explored. In this paper, the development, fecundity and nutritional effects, and the activities of digestive enzymes, endogenous enzymes of protective systems and juvenile hormone esterase (JHE) of the diamondback moth (DBM), Plutella xylostella (L.), were examined following its feeding on different host plants treated with the LC20 and LC50 concentrations of tea saponin. This research will contribute to clarifying the toxicity mechanism for tea saponin and promoting the development and utilization of tea saponin. The results were as follows:1. The toxicity of tea saponin was different to the third larval stage of Plutella xylostella following feeding on different host plants, of which rape made the insect more sensitive.2. Sublethal treatments of tea saponin resulted in reduced feeding consumption, frass, pupal weights, pupation rates, emergence rates and fecundity for DBM moths. However, prolonged larval lifespans and pupae durations were achieved in the experiment. The rape enhanced the above effect.3. The mean relative growth rates (MRGRs) of Plutella xylostella following feeding on three host plants treated with tea saponin were decreased. The efficiency of conversion of ingested food (ECI) and the efficiency of conversion of digested food (ECD) were increased. However, the tea saponin treatments did not cause significant differences in the approximate digestibility (AD). The rape enhanced the above effect for MRGR, and cabbage enhanced the effects for ECD and ECI.4. The activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) of Plutella xylostella following its feeding on three host plants treated with tea saponin were increased. The rape could inhibit this trend. There were significant differences in malondialdehyde (MDA) activity among host plants.5. Expressed as hormone titer per insect, both juvenile hormone (JH) and molting hormone (MH) contents increased after the larvae fed on the three host plants treated with tea saponin. The rape could enhance tea saponin interference with JH, and the radish seedling enhanced tea saponin interference with MH.