Abstract: The polydnavirus of Cotesia plutellae (Hymenoptera: Braconidae) was characterized and its physiological effects on the host larvae of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) were studied in this paper. The results showed that the virion of polydnaviruses (PDVs) in the calyx region of female ovary had a tail and multiple nucleocapsids (up to 16) enveloped; the nucleocapsid measured 40-168 nm in length and 39-40 nm in diameter; PDVs replicated only in calyx cells that underwent lysis after replication completed; the calyx fluid was injected into the host larval haemocoel while female wasps laid eggs; large numbers of PDV virions were observed along the sheaths of the host tissues; it appeared that PDVs emerged from their envelopes first, and then infected the host tissues. When 2nd, 3rd and early 4th instar host larvae were parasitized by female wasps irradiated with Co⁶⁰ (resulting in pseudoparasitism), almost all of them were not able to pupate, but the period of the last instar larvae was significantly prolonged and a pair of brown short wing buds appeared on the thorax of host larvae at the end of the last instar; however, when host larvae of the late 4th instar were pseudoparasitized, and even if super-pseudoparasitized 9 times, they were still able to normally pupate, but not able to emerge. These results suggest that the PDVs and venom could prolong the developmental period of host larvae under conditions where there were no parasitoid eggs hatching, no teratocytes released and no parasitoid larvae present, but they seemed to have limited effect on the late 4th instar larvae. The numbers and micromorphology pattern of fat bodies of host larvae pseudoparasitized by the irradiated female wasps were distinctly different from those of normally parasitized hosts, suggesting that the teratocytes and parasitoid larvae may have some effects on the fat bodies of host larvae.

Key words: Cotesia plutellae, calyx region, polydnavirus, Plutella xylostalla, pseudoparasitism