Studies On Insecticidal Activity And Mechanisms Of Several Insecticidal Mixtures

For decades, many environmental and ecological problems have been increasing by using of chemical pesticides. The hazards coused by pests also become more and more serious. Meanwhile, the resistance has been developing by long term use of single insecticide. Therefore, it is a hot of research to find more effective, lower toxic and lower residual pesticides. Currently, one of the important study measures is to use pesticide mixtures. Four mixtures, YSEW, YSWG, YSME and DSWG were prepared by mixing acephate and thiamethoxam or mixing chlorpyrifos with thiamethoxam at different ratios and different formulations. Joint actions of them against Plutella xylostella L., Pieris rapae L., Spodoptera litura (Fabricius) and Brevicoryne brassicae L. were evaluated by bioassays. Besides, in order to know mechanisms of synergism, the relationship of synergisms of these mixtures to metabolic enzymes, target enzymes and midgut cells were studied by biochemical tests of enzyme activity and paraffin section method. The main results are as follows:1. The activities of three new mixtures of YSEW, YSWGO and DSWGO were evaluated against P. xylostella L., Pieris rapae L., Spodoptera litura(Fabricius) and Brevicoryne brassicae L. by spray and immersion application bioassays. The results indicated that:the three new mixtures were all showed obvious synergisms against Pieris rapae L.. In addition, YSEW also showed obvious synergism against P. xylostella L. and Brevicoryne brassicae L. after 72h treatments, the Co-toxicity coefficient(CTC) were 276.35 and 213.69. However, the three mixtures were all showed antagonism against Spodoptera litura(Fabricius).2. Influences of the three new mixtures(YSEW, YSWGO and DSWGO) on three enzymes in P. xylostella L. and Pieris rapae L.were studied. Biochemical analysis results indicated that:the three new mixtures inhibited the activity of carboxylesterase(CarE) in the two test insects, and the inhibitions were 90.04%.12.11%,71.16% and 88.57%,72.54%,66.19%, respectively. YSEW and YSWGO also significantly inhibited the activity of glutathione S-transferase(GSTs) and acetylcholine esterase(AChE) in the two test insects. But DSWGO inducted the activity of GSTs in P. xylostella L. and the activity of AChE in Pieris rapae L. were induced by 5.14% and 67.95%.3. Influences of the three new mixtures (YSEW, YSWGO and DSWGO) on midgut cells in Pieris rapae L. were studied by paraffin section method. The morphological changes of midgut epithelial cells were observed by optical microscope. The results showed that:the hierarchical structure and the shape and structure of goblet cells of intestinal cells in Pieris rapae L treated by three mixtures were changed.4. By changing the ratios and formulations, six kinds of YSME, there kinds of YSWG and three kinds of DSWG were developed. The joint actions of them on P. xylostella L. were studied by spray application bioassays. The results demonstrated that they were all show high insecticidal activities against P. xylostella L. Of which, YSME2, YSME3, YSWG2, YSWG3, DSWG1, DSWG2 and DSWG3 showed significant synergisms. The results of biochemical determination of enzyme activity showed that: with rare exceptions, most of the mixtures showed inhibition on the three enzymes.Summary, the new mixtures mixed acephate with thiamethoxam or mixed chlorpyrifos with thiamethoxam all showed good insecticidal activity, and all showed synergisms against P. xylostella L. and Pieris rapae L.. To inhibit the activities of CarE,GSTs and AChE and to destruct the shape and structure of intestinal cells may be the key factors of synergisms.