Endoparasitoid insects introduce maternal factors into the body of their host at oviposition to suppress cellular defences for the protection of the developing parasitoid. We have shown that transient expression of polydnavirus genes from a hymenopteran parasitoid Cotesia rubecula (CrPDV) is responsible for the inactivation of hemocytes from the lepidopteran host Pieris rapae. Since the observed downregulation of CrPDV genes in infected host tissues is not due to cis-regulatory elements at the CrV1 gene locus, we speculated that the termination of CrPDV gene expression may be due to cellular inactivation caused by the CrV1-mediated immune suppression of infected tissues. To test this assumption, we isolated an imaginal disc growth factor (IDGF) that is expressed in fat body and hemocytes, the target of viral infection and expression of CrPDV genes. Time-course experiments showed that the level of P. rapae IDGF is not affected by parasitization and polydnavirus infection. However, the amount of highly expressed genes, such as storage proteins, arylphorin and lipophorin, are significantly reduced following parasitization. [copyright] 2004 Elsevier Ltd. All rights reserved. Biochemistry; Circulatory system; Haemolymph; Genetics; Molecular genetics; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Immunology and repair mechanisms Pieris rapae (Pieridae); Proteins; Haemocytes; Gene expression; Hymenopteran parasites; Cotesia rubecula; Immune like reactions; Suppression by hymenopteran parasitism, mechanism determination Cotesia rubecula (Braconidae); Molecular genetics; Lepidopteran hosts; Pieris rapae; Host immune function suppression mechanism determination none