The potential role of pathogens or parasites in maintaining species coexistence is well documented. However, the impact of vertically transmitted symbionts, that can markedly modify their host's biology, is largely unknown. Some females of the Drosophila parasitoid Leptopilina boulardi are infected with an inherited virus (LbFV). The virus forces females to lay supernumerary eggs in already parasitised hosts, thus allowing its horizontal transmission. Using two independent experimental procedures, we found that LbFV impacts inter-specific competition between L. boulardi and the related L. heterotoma. While L. boulardi rapidly outcompetes L. heterotoma in the absence of the virus, L. heterotoma was able to maintain or even to eliminate L. boulardi in the presence of LbFV. By forcing females to superparasitise, LbFV induced egg wastage in L. boulardi thus explaining its impact on the competition outcome. We conclude that this symbiont whose transmission is L. boulardi-density-dependant may affect the coexistence of Leptopilina species.