Mode of infection on the cuticle of Spodoptera frugiperda(Lepidoptera: Noctuidae) by two entomopathogenic fungi species revealed by scanning electron microscopy

【Objective】In this study, based on the previous screening of the high pathogenic Beauveria bassiana strain GZSL-1 and Cordyceps fumosorosea strain IFCF01 of Spodoptera frugiperda, the purpose of this study was to better understand the mode of action of these entomopathogenic fungi on the cuticle of S. frugiperda, and provide a theoretical basis for elucidating the pathogenicity mechanism of these two strains in order to achieve a constant and high control efficacy.【Method】The attachment, germination, penetration and conidial reproduction of B. bassiana and C. fumosorosea to 2^nd instar larvae of S. frugiperda infected with strains GZSL-1 and IFCF01(1.0×10⁷ conidia/mL) were observed using scanning electron microscope(SEM).【Result】Germinated conidia of both strains formed appressoria, and surface topography of S. frugiperda larvae affected germination of conidia, attachment and the length of penetrating germ tube. The two strains showed similarities in terms of high germination rate(> 90.00%) and appressoria rate(> 80.00%). There were shorter germ tubes grew before penetrating on gentle surface topography than on strumae surface topography. Strain IFCF01 exhibited longer penetrating germ tubes compared to GZSL-1 on all surface topographies. Besides, strain GZSL-1 exhibited significantly shorter penetrating germ tubes compared to strain IFCF01 on gentle surface topography(P<0.05). Strains GZSL-1 and IFCF01 began to germinate within 16 and 8 h, and penetrations were commonly observed within 24 and 32 h. After 72 and 96 h post inoculation, primary and secondary conidiophore emerged and the mycelial covered the entire cuticular surface.【Conclusion】SEM observation reveals the infection process of the highly pathogenic B. bassiana strain GZSL-1 and C. fumosorosea strain IFCF01 against S. frugiperda, the results of this study will further provide the theoretical basis and reference for strain improvement and field application.