The Pathogenicity Of Metarhizium Rileyi And Toxin Against The Garden Pest Spodoptera Liutura

Pathogenicity of Metarhizium rileyi 5772 isolate（Nr 5772） against larvae of Spodoptera litura was determined by conidial dipping method,and the physiological response of host was also monitored to explore the pathogenic mechanism.Furthermore,virulence of crude toxin produced by Nr 5772 was assayed against the S.litura larvae,the composition of toxin was analyzed by High Performance Liquid Chromatography（HPLC） and Thin-Layer Chromatography（TLC）,in the end,the structure of toxin was identified by mass spectroscope method.Results listed as below:1 Dipping method was adopted to test the LC₅₀ and LT₅₀ of the 3^rd,4^th and 5^th instar larvae in the conidia suspension of M.rileyi Nr 5772 isolate.Results showed that LT₅₀ of the 3^rd,4^th and 5^th instar larvae were 4.44 d,4.48 d and 4.85 d respectively when exposed to 5×10⁹conidia/ml suspension;the LC₅₀ of the 3^rd,4^th and 5^th instar larvae were 2.71×10⁶ conidia/mL,5.61×10⁶ conidia/mL and 1.75×10⁷ conidia/mL respectively.In conclusion,M.rileyi Nr 5772 isolate has strong pathogenicity to S.litura larvae and the virulence increased along with the increase of conidia concentration and the decrease of the larva instar.2 In vivo Development of M.rileyi and the immune response of host was monitored by inoculating the hyphal bodies（Hb）into the hemocoel of the larvae of S.litura with microinjection method,to illustrate the pathogenic mechanism of M.rileyi on insect pest.Results showed that the hyphal body reproduction fit the power function model 64 h post inoculation,without evoking the immune response of host,has no affection on the hemocytes number in larvae.48 h post inoculation,PO activity of infective larvae was the same as control,while 48 h later,yeast-like hyphal bodies transformed into mycelia and the phenoloxidase activity begins to decreased and eventually disappeared 72 h later.It is concluded that M.rileyi will not evoke the immune response of host,conversely,it will inhibit the immune response at later stage.3 Mycosed larvae of S.litura and broth cultured mycelium of M.rileyi were used as materials to extract the toxin.results showed that both toxins extracted from different sources had obvious toxic effects on the larvae of the S.litura.The LC₅₀ value of toxin extract from mycosed larvae（larvae-source toxin）against the 3^rd,4^thand 5^thinstar larvae were 39.1342mg/mL,52.1034mg/mL and 68.6457mg/mL,respectively.The LC₅₀ value of the toxin extracted from mycelium（mycelium-source toxin） was 60mg/mL,63.9976 mg/mL and 75.3265 mg/mL,respectively.It was concluded that both toxins have obvious toxicity to the tested larvae,and the larvae-source toxin is stronger than mycelium-source toxin.4 After purification,the composition of toxins was analyzed by HPLC and TLC.Results showed that the toxin mainly composed by three substances.Two components were separated by Semi-preparative chromatography.By mass spectrometry identification,one was confirmed as C₈H₁₀O₆,and the molecular weight is 202.0372.