Abstract: Xenorhabdus spp. And Photorhabdus spp.are entomopathogenic bacteria symbiotically associated with insect pathogenic nematodes of the families Steinernematidae and Heterorhabditidae. Six strains were isolated from their nematode symbiots.Crude insecticidal material was precipitated with ammonium sulfate at 85% saturation from cell-free culture.By the means of bioassay, Xenorhabdus bovienii strain A54 was found highly toxic to insects. Injection of the toxin killed 93.3% of Galleria mellonella larvae within 96 h. Feeding artificial diet with the toxin, the mortalities of larvae of Ostrinia furnacalis and Helicoverpa armigera were 100% and 90% within 120 h respectively. Isolated by DEAE-52 column,a penetrating peak and three NaCl eluting peaks were obtained from the crude extract.Materials from the penetrating peak had oral insecticidal activity against H.armigera,but no toxicity when injected into the hemolymph of G.mellonella. Materials from three NaCl eluting peaks had high toxicity when injected, but no toxicity when given orally. Meanwhile, feeding artificial diet with the toxin, the mortalities of 2-instar and 3-instar larvae of H.armigera were 65% and 30% within 96 h respectively.

Key words: Xenorhabdus spp., Photorhabdus spp., insecticidal activity