The endoparasitoid wasp Asobara japonica has highly poisonous venom: the host Drosophila larvae are killed by envenomation at a dose that is naturally injected by the female wasp at parasitism. This insecticidal venom is neutralized, however, because A. japonica introduces lateral oviduct components soon after venom injection at oviposition. Although the venom and lateral oviduct components of this parasitoid have been partially characterized, how the venom components favor successful development of wasp eggs and larvae in the host remains ambiguous. Here, we demonstrated that A. japonica venom did not affect host humoral immune responses, determined as expression of antimicrobial peptide (AMP) genes, but significantly diminished two cellular responses, spreading and phagocytosis, by host hemocytes. Moreover, venom components drastically elevated a serine protease‐like activity 4 h after its injection. The lateral oviduct components did not negate the detrimental effects of the venom on host cellular immunities, but significantly reduced the venom‐induced elevation of protease activity. Both active factors in venom and lateral oviduct components were roughly characterized as heat‐labile substances with a molecular mass of at least 10 kDa. Finally, venom of A. japonica, with a wide host range, was found to be much more toxic than that of Asobara rossica, which has a limited host range. These results reveal that A. japonica venom toxicity allows exploitation of a broader range of host insects because it is essential to overcome cellular immune responses of the host for successful parasitism.