The Japanese beetle (Popillia japonica) is an invasive scarab beetle originating from Japan. In the European Union, it is listed as a priority quarantine pest. Currently, it is mainly controlled using synthetic insecticides. Here, we tested an environmentally friendly control alternative. We investigated whether Japanese beetle adults can be used as vectors to autodisseminate lethal doses of the European native entomopathogenic fungus Metarhizium brunneum ART 212 within adult populations. Additionally, we tested whether infested females could carry conidia into the soil environment during oviposition, increasing neonate larval mortality. We showed that inoculated adults can indeed transmit the fungal conidia horizontally for up to two days, significantly reducing the survival of both donor and recipient beetles in same-sex and opposite-sex couples. Furthermore, horizontal transmission among adults was verified under semi-field conditions. Another set of laboratory tests showed that beetles carried the inoculum to their oviposition sites, where larval survival was reduced at high concentrations (≥ 1.11 × 105 conidia/g substrate). However, the release of inoculated beetles in semi-field cages resulted in soil fungal concentrations more than ten times lower, failing to provide larval control. Thus, carriage of M. brunneum ART 212 into the soil by female vectors does not seem to provide control of larvae outside the laboratory setup. However, our results suggest that lethal conidial doses can be autodisseminated among the more susceptible adults. This may be the basis for an environmentally friendly control strategy against invasive Japanese beetle adults, applicable in both agricultural and non-agricultural areas.