Abstract: In order to determine the effects of parasitism by Diadromus collaris (Hymenoptera: Ichneumonidae) on structure of fat body of host Plutella xylostella (Lepidoptera: Plutellidae) pupae, the morphology and ultrastructure of pupal fat body and adipocytes of the diamondback moth, P. xylostella parasitized by D. collaris were observed using light and electron microscopy. After 72 h of parasitism, the structure of fat body became loose; the adipocytes were dissociated from fat body; the membrane of adipocyte was torn, its entocyte was consumed passively, the number of apparatus of cell reduced, and the morphology of nuclear chromatin was changed. It is so concluded that parasitism by parasitoid can make host fat body and adipocytes fell to pieces, and this is advantageous for development of parasitoid.

Key words: Diadromus collaris, Plutella xylostella, fat body, adipocytes, ultrastructure, parasitism