Abstract: Suppression of host immune response after parasitization is essential for the survival of endoparasitoids. In Plutella xylostella (L.) (Lepidoptera: Plutellidae) larvae parasitized by Cotesia vestalis (Haliday) (Hymenoptera: Braconidae), how the parasitoid egg avoids encapsulation by the host’s immune attack is still not well understood. In this study, the ultrastructure of C. vestalis egg surface was investigated systematically using electron microscopes. The results showed that the egg possesses a fibrous layer, which is coated by an overlying sheath of virus-like filaments. During oviposition, C. vestalis lays an egg into the hemocoel of the host larvae, together with calyx fluid mainly including polydnavirus. Both the outer virus-like filaments and the fibrous layer of parasitoid eggs seem to be the first passive protection barrier to host immunity during the initial stages of parasitism. They may delay encapsulation until a more permanent means of suppressing encapsulation is established. Afterwards, polydnavirus plays a great role in the immune suppression of recognition and encapsulation by host hemocytes. Within eight days post pseudoparasitization by C. vestalis, the encapsulation response of P. xylostella larval hemocytes to the irradiated eggs was strongly suppressed, indicating that the polydnavirus has a permanent immunosuppressive capability. The results suggest that in the system of C. vestalis-P. xylostella, C. vestalis avoids the host immune reaction by a combination of both passive and active mechanisms.

Key words: Cotesia vestalis, polydnavirus, fibrous layer, virus-like filaments, immunosuppression