Defense Function Of Spodoptera Litura Epidermis Against Nomuraea Riley Infection

As an important natural control factors of insect population, entomopathogenic fungus kill the insect by penetrating through cuticle, therefore, besides the infectivity, infection efficiency is influenced strongly by cuticle structure and composition. We isolated a strain from infected Spodoptera litura, and our previous research showed the strain was highly pathogenic to young S. litura larvae, while lethal effect to old larvae was quite weak. In order to explore the development and utilization potentiality of the strain, we studied on its morphology, cultural conditions, while pathogenicity of Nomuraea riley against the 2nd and 4th instar larvae of S. litura with infusion method or by injection, the expect of cuticular lipid on germination of CQNr01 conidia was investigated. Those were useful to establish the pathogenesis of N. riley against S. litura and the interaction between each other, which provide the basis for the application of the pathogen in the integrated control of S. litura and provide new cxemplum between host and pathogens. Here below are the results:We successfully isolated a strain. Morphology-based and molecular identification were used to definite that CQNr01 is Nomuraea riley.The biological research showed that the SMAY media is the proper basic cultural conditions for hypha growing and sporulation. The environmental conditions about temperatures and pH values were studied in details, which was 25℃and pH 6.5; The extracellular protease and chitinase of CQNr01 were studied at different times and temperatures. The strain produced most protease at 25℃, while it had little chitinase output at any temperature.Pathogenicity of Nomuraea riley against the 2nd and 4th instar larvae of S. litura with infusion method or by injection was studied. Results indicated the mortalities of the 2nd and the 4th were 86.58% and 89.91% respectively at the concentration of 1×108 spores·mL-1 with infusion method, while the mortality was 100% and 88.89% by injection. Those proved that the epidermis play an important role in fungal infection prevention and the role of the 4th instar larvae was larger.Under fluorescence microscopy, spores of CQNor01 were found adhered to isolated epidermis of the 2nd and the 4th instar larvae, and spores germinated within 24 h post inoculation, while adhesion strength of the 2nd instar larvae was stronger. The germ tubes grew longer and the germination rate became larger as the time of inoculation longer. However, under the same inoculation time, the germination rate and the number of appressorium on the 2nd instar larvae significantly exceeded to those on the 4th instar larvae. We found spores germinated on the epidermis of the 2nd instar larvae within 24 h post inoculation, and little spore geminated on the epidermis of the 4th instar larvae.Extracting cuticular lipid from the 2nd and the 4th instar larvae, and based on the expect of cuticular lipid on germination of CQNr01 conidia on Water Agar and SMAY, we found cuticular lipid could be used for germinating when lacking of nutrition, while cuticular lipid would inhibit germination when nutrition was enough, and cuticular lipid of the 2nd instar larvae may contain more inhibitory substance.Conclusion: the optimum condition for hypha growing and sporulation was 25℃and pH 6.5. The epidermis of S. litura larvae was the main reason for fungal infection prevention. It was easy to see spores germination and appressorium formation on the epidermis of the 2nd instar larvae, but it hardly happened on on the epidermis of the 4th instar larvae. Cuticular lipid could be used for germinating when lacking of nutrition, while cuticular lipid would inhibit germination when nutrition was enough, and cuticular lipid of the 2nd instar larvae may contain more inhibitory substance.