Parasitoid wasps have evolved complex strategies to exploit and regulate the physiology of their hosts and enhance their suitability for the development of their progeny, which are modulated by host regulation factors both of maternal (venom) and embryonic (teratocytes) origin. The pea aphid, Acyrthosiphon pisum (Homoptera, Aphididae), and its endophagous parasitoid wasp, Aphidius ervi (Hymenoptera, Braconidae), represent a unique model system to investigate the molecular mechanisms underlying the interactions between parasitoids, their hosts and primary symbionts. In this study, we focused on the functional role of Ae-γ-glutamyl transpeptidase (Ae-γGT), the most abundant component of A. ervi venom which is known to induce host castration. Using RNA interference technique, we knocked down Ae-γGT gene in newly emerged female wasps, using microinjections of double-stranded RNA (dsRNA) in the pupal stage of the parasitoid. These females were used to observe phenotypic changes in parasitized hosts and the parasitoid's progeny, as affected by a venom blend lacking Ae-γGT. We found that Ae-γGT silencing prevented the aphid castration and enhanced the growth of both the host and parasitoid, which was supported by a higher load of the primary bacterial symbiont Buchnera aphidicola. However, emerging adults exhibited reduced survival and fecundity, suggesting a trade-off with body size. This demonstrates the primary role of Ae-γGT in host ovary degeneration and suggests that this protein counterbalances the proliferation of Buchnera likely triggered by other venom components. Our study offers a new approach to unravel the complexity of aphid parasitoid venom in vivo and sheds light on a previously unknown role for Ae-γGT in the host regulation process.