Study On The Pathogenic Mechanism Of Metarhizium Rileyi To Spodoptera Frugiperda

Spodoptera frugiperda（J.E.Smith）,is considered a global cross-border migratory pests and has developed resistance to most chemical insecticides,making microbial resources a critical tool for pest management.Currently,the available fungal resources for the prevention and control of S.frugiperda mainly consist of Beauveria bassiana,Metarhizium anisopliae and Metarhizium rileyi.However,the pathogenic mechanism of M.rileyi against Noctuidae is not well understood.Therefore,the aim of this study was to identify a highly pathogenic strain of M.anisopliae against S.frugiperda,investigate the biological characteristics of M.rileyi,and analyze its pathogenic mechanism,to provide a theoretical basis for efficient use of M.rileyi in controlling S.frugiperda.The main results are as follows:1)An entomopathogenic fungus was isolated and purified from diseased S.frugiperda collected from a corn field near Minhou,Fuzhou City,Fujian Province.The strain was found to grow on common potato glucose agar medium,with a white upper side and a light brown underside of the colony.A dark green spore layer appeared in the later stages of culture.The conidia were spike-shaped and aggregated in light green.The conidiophores were short cylindrical.Sequence analysis showed that the strain had 100%homology with OK178862.1,and belonged to the same branch,identifying it as M.rileyi through morphological and molecular biological identification.After comparing the growth of M.rileyi in 8different media,the yeast agar medium with maltose extract was chosen as the optimal medium for M.rileyi.2)The indoor toxicity of M.rileyi against S.frugiperda showed that the corrected mortality rates of the 2nd,3rd and 4th instar larvae were70.63%,57.60%and 27.14%respectively,indicating significant insecticidal activity.Among them,the insecticidal ability was most significant against the 3rd instar larvae,with an LT₅₀ of 3.4 d.The biological characteristics of S.frugiperda infected with M.rileyi,were evaluated by spraying the eggs of S.frugiperda.The results showed that the developmental duration of the 3rd and 6th instar larvae was significantly prolonged,while the pupal stage was shortened.3)Pathological examination of tissue sections revealed that after 48h of treatment with M.rileyi,the fat body in 2^nd instar S.frugiperda larvae exhibited unclear edges and disordered structure.By 96 h,some of fat body had completely disintegrated,leading to the death of the larvae.Transmission electron microscopy（TEM）was used to evaluate the midgut of S.frugiperda larvae infected with M.rileyi.The results showed that the hyphae of the fungus were present in the midgut lumen,the inner wall tissue of the midgut was broken,the mitochondria and rough endoplasmic reticulum of the parietal cells were missing,and a large number of microvilli and fungal spores were visible.4)The activity of glutathione transferase was significantly increased in 2nd instar S.frugiperda larvae infected with M.rileyi at 24 h.At 48 h,the activity of phenoloxidase was significantly increased,while at 96 h,the activity of superoxide dismutase and catalase was significantly increased.These data suggest that glutathione-S-transferase activity rapidly increases in the early stage of infection to neutralize mycotoxins in S.frugiperda larvae.In the middle and late stages of infection,the larvae mainly rely on phenol oxidase,antioxidant enzymes superoxide dismutase and catalase to catalyze the production of immune active molecules in order to resist fungal infection.In conclusion,this study successfully isolated and identified the entomopathogenic fungus M.rileyi from S.frugiperda,and determined that SMAY was the optimal medium for the growth of M.rileyi.M.rileyi demonstrated significant pathogenicity against 2nd and 3rd instar larvae of S.frugiperda and its pathogenic mechanism was investigated,providing valuable for the potential application of M.rileyi as a biological control agent against S.frugiperda.