Study On Insecticidal Activity Of Symbiotic Bacteria From Entomopathogenic Nematodes And Screen Of Highly Virulent Strains

In this study, 80 strains of symbiotic bacteria were isolated from 9 strains of entomopathogenic nematodes. Insceticidal activities of these strains were examined by bioassays. The highly virulent strains were selected through a large number of bioassays, and the substance of insecticidal activity was studied. This work is helpful to provide new insecticidal toxins and genes.In this paper, bioassay results showed that the oral insecticidal activity of Steinernema sp. A24-1B, A24-19B, and A24-6R, which distinctly inhibited the growth of Ostrinia larvae, were higher than other strains. Base on the result the three highly virulent strains were selected and the substance of insecticidal activity was studied.By the injection bioassays of the three highly virulent strains, it was found that they were lethal to Galleria mellonella when given injected only lOOcell per larva. Observed from microscope, I phase was longer bacilli than II phase. The relative factors of insecticidal activity showed that the corrected mortality of Ostrinia larvae decreased at 50℃ 10 min, but the inhibiting rate of larval weight kept high. Corrected mortality of Ostrinia larvae was lost at 100 ℃ 10 min, and just a little inhibiting rate of larval weight. Toxicity of three highly virulent strains gradually decreased by ultraviolet rays from 5 min to 60 min, and after the ultraviolet rays 120min, the insecticidal activity was lost. The oral toxicity of A24-19B increased wavily with the culture time delaying. LC50 of the strains to the first-instar Ostrinia larvae was 7.46 X 109cell/ml. Growing curves of the three highly virulent strains were compared. It was found that A24-6R (II phase) was more eugonic than A24-1B and A24-19B (I phase). The three highly virulentstrains also had highly oral virulence to Plutella xylostella, Spodoptera exigua, and Mythimna separata larvae.The insecticidal toxins of the three highly virulent strains were purifications primary. Insecticidal activities of their extracellular secretions were examined by the means of oral bioassay in every purified process. The insecticidal toxin inhibited the growth of Ostrinia larvae, and was the same as the original insecticidal activities.