Abstract: To explore the defense responses of Plutella xylostella (Lepidoptera：  Plutellidae) hemolymph against the infection of different strains of entomopathogenic fungus Isaria fumosorosea (syn. Paecilomyces fumosoroseus) (Hypocreales：  Cordycipitaceae), cellular immune responses of the infected 4th instar larvae was investigated with optics microscope and Giemsa stain. The results indicated that the invasion of P. xylosetlla by I. fumosorosea resulted in changes in the characteristics of host hemolymph, including the initial increase of the total numbers of hemocytes, and the change of the proportion of differential hemoeytes in association with fungal infection processes. During 8-45 h post inoculation, the peak response reached to 6 250/mm³ at 24 h after invasion by the highly pathogenic strain PFCF-001. The highest total hemocyte counts (3 000/mm³) were observed at 36 h when P. xylostella larvae were inoculated with the least pathogenic strain PFCF-D58, which was 12 h later than the treatment of strain PFCF-001. The major defense hemocytes were plasmatocytes and granulocytes. Different types of cellular immune responses were observed at the early stage of infection including adhesion, phagocytosis, hemocytic aggregation, encapsulation and nodule formation. However, the defense responses of the P. xyolstella against the highly pathogenic strain of I. fumosorosea just inhibited the pathogen temporarily but could not clear it completely.

Key words: Isaria fumosorosea, Plutella xylostella; hemolymph, physiological defense responses, pathogenic strain