Maternal factors introduced into host insects by endoparasitoid wasps are usually essential for successful parasitism. This includes polydnaviruses (PDVs) that are produced in the reproductive organ of female hymenopteran endoparasitoids and are injected, together with venom proteins, into the host hemocoel at oviposition. Inside the host, PDVs enter various tissue cells and hemocytes where viral genes are expressed, leading to developmental and physiological alterations in the host, including the suppression of the host immune system. Although several studies have shown that some PDVs are only effective when accompanied by venom proteins, there is no report of an active venom ingredient(s) facilitating PDV infection and/or gene expression. In this study, we describe a novel peptide (Vn1.5) isolated from Cotesia rubecula venom that is required for the expression of C. rubecula bracoviruses (CrBVs) in host hemocytes (Pieris rapae), although it is not essential for CrBV entry into host cells. The peptide consists of 14 amino acids with a molecular mass of 1598 Da. In the absence of Vn1.5 or total venom proteins, CrBV genes are not expressed in host cells and did not cause inactivation of host hemocytes. Biochemistry; Circulatory system; Haemolymph; Associations; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Diseases and disorders; Microbial diseases Pieris rapae (Pieridae); Haemocytes; Hymenopteran parasites; Cotesia rubecula; Parasite novel venom peptide isolation & role in viral associate genes expression in host haemocytes; Viral diseases; Polydnavirus Cotesia rubecula (Braconidae); Toxins and venoms; Novel venom peptide; Isolation & role in viral associate genes expression in lepidopteran host; Associations; Polydnavirus; Lepidopteran hosts; Pieris rapae none