Abstract:In order to clarify the pathogenicity of Isaria fumosorosea (Paecilomyces fumosoroseus) (Hypocreales: Cordycipitaceae) to Plutella xylostella (Lepidoptera: Plutellidae), The tissues and organs of P.xylostella larvae infected with an indigenous strain EBCL 03011 suspension of I.fumosorosea at two concentrations of 1×105 conidia/mL and 1×108 conidia/mL were investigated. The results showed that infected larvae cuticle gradually became dark brown, the whole body of the insect thickened and reduced in size. Paraffin sections of infected larvae organs observed under light microscope indicated that the conidia of I.fumosorosea attached with the integument of P.xylostella larvae penetrated through the cuticle by germ tubes after 4h postinoculation. After 16h, the endocuticle and gut wall cells were disintegrated, the germ tubes passed through the epithelial layers and gut cavity into the hemocoel, then invaded the muscle tissues and fat body nearby the area of penetration. After 24h, most tissues and organs of infected larvae had different degrees of disintegration with the multiplication of the hyphal bodies. The hyphae emerged through the cuticle and formed a hyphal layer on the surface of the cadaver after 48h postinoculation. Our results indicated that there were two different pathways of invasion after conidia of I.fumosorosea EBCL03011 inoculated to P.xylostella larvae. One was the cuticle way, the other was the digestive tract way.