Abstract: Harmonia axyridis (Coccinellidae, Coleoptera) is an invasive ladybird that is known to outcompete native species, inflict economic harm, and act as a significant pest species. Harmonia axyridis is parasitized by the ectobiotrophic fungus Hesperomyces virescens (Ascomycota, Laboulbeniales). Recently, integrative taxonomy has shown that He. virescens is a species complex of which delimitation is determined by ladybird host. The fungus associated with Ha. axyridis was recently formally described as He. harmoniae. Here, we review the effects of He. harmoniae on its invasive host. During winter, He. harmoniae increases mortality in male ladybirds by exhausting their energy reserves despite a limited effect on their immune system. Co-infection experiments have had varying results. When Ha. axyridis was infected with He. harmoniae and either Beauveria bassiana or Metarhizium brunneum, there was no significant increase in mortality compared to infection by He. harmoniae-only infection. This may be explained by the enemy release hypothesis, which states that invasive species in new geographic areas have reduced regulatory effects from natural enemies. When co-infected with either Spiroplasma or Wolbachia bacteria and He. harmoniae, both fecundity and hatchability were significantly lowered in female ladybirds, but there was no increase in mortality. Hesperomyces harmoniae thus shows potential as a biocontrol agent for Ha. axyridis, especially when co-infected with other natural enemies.