Abstract: Diadegma semiclausum Hellen is a larval endoparasitoid of the diamondback moth, Plutella xylostella L. The parasitoid regulates its host physiology and development mediated by its maternal secretions, such as venom and polydnavirus (PDV), but the possible functions of venom of this PDV-producing endoparasitoid are unknown. The SDS-PAGE analysis of venom showed that the most protein bands ranged from 35 to 220 kDa with a few bands less than 15 kDa, but the most abundant bands were within 35-70 kDa, which were similar to venom proteins in other parasitoid-host systems. By in vitro primary-culture of host hemocytes with parasitoid venom fluid or Escherichia coli, the changes of spreading and phagocytic abilities of hemocytes from the parasitized or the non-parasitized host larvae indicated that the venom of D. semiclausum alone was harmless to the spreading and phagocytic abilities of plasmatocytes and granulocytes in larvae of the diamondback moth. However, the spreading ability of hemocytes from host larvae was obviously inhibited in the early stage of parasitism by D. semiclausum, but these hemocytes could still recognize the pathogens with a lapse of endocytosis; at 24 h post-parasitization, the hemocytes recovered their spreading ability and then granulocytes acted as phagocytes to engulf bacteria successfully. This study demonstrates that the parasitism of D. semiclausum can affect the phagocytosis of hemocytes from its host larvae through inhibiting the spreading ability of granulocytes.

Key words: Diadegma semiclausum, Plutella xylostella, parasitism, venom, polydnavirus (PDV), hemocyte, hemocyte spreading, phagocytosis