Culture-based survey of fungi from decaying Vespa orientalis colonies reveals Beauveria varroae as a virulent and horizontally transmissible entomopathogen.

The oriental hornet (Vespa orientalis) is increasingly recognized as a significant pest of honey bees in southern Europe and other dry sub-humid regions, where it causes colony losses, disrupts foraging activity, and may contribute to pathogen circulation among apiaries. Current control measures, based mainly on trapping and nest destruction, are labor-intensive and raise concerns for non-target organisms, highlighting the need for sustainable alternatives. Entomopathogenic fungi represent promising candidates due to their insecticidal activity and potential for horizontal transmission within social groups. Here we characterized the fungal community associated with decaying V. orientalis nests and identified Beauveria varroae as a naturally occurring entomopathogen. The nest-associated strain was compared with a commercial Beauveria bassiana strain (ATCC 74040; Naturalis®) in laboratory bioassays on adult hornets using contact, ingestion, and indirect transmission routes. Survival analyses revealed significant route-dependent mortality for both strains. Histological observations confirmed fungal penetration and systemic colonization, particularly of fat body tissues, and indicated host immune responses at lower inoculum levels. Indirect transmission assays demonstrated the capacity of infected individuals to disseminate conidia to conspecifics. These findings identify B. varroae as a virulent pathogen of V. orientalis and support the feasibility of exploiting nest-level horizontal transmission ("Trojan horse" strategy) for hornet management. Further research should focus on optimizing delivery systems and assessing field applicability within integrated pest management frameworks.