Abstract: The western conifer seed bug, Leptoglossus occidentalis Heidemann (Heteroptera: Coreidae), is an invasive pest that severely threatens the health and production of conifer orchards since its arrival to Europe about two decades ago. Despite its economic and ecological relevance, little is known about the potential of native entomopathogenic fungi (EF) for its biological control. In this study, we sampled native fungal isolates from L. occidentalis and surrounding soils in coniferous forests. Pathogenicity bioassays were conducted to assess mortality rates and median lethal time (TL₅₀) across different fungal species and environmental conditions. We specifically examined the effects of water availability, insect sex and potential host defense responses. Beauveria bassiana was found naturally infecting L. occidentalis, with an 18% incidence in quarantined individuals. All tested fungal isolates induced high mortality. The lowest TL₅₀ was observed with Cordyceps farinosa (5.9 d, 100% mortality) under dry conditions, and with Metarhizium anisopliae (8 d, 91.6% mortality) when a moisture source was present. Native B. bassiana strains exhibited pathogenicity comparable to commercial formulation. Mortality accelerated under water stress, except in treatments with M. anisopliae, where mortality and TL₅₀ were reduced. Sex differences in susceptibility were detected with females showing higher resistance. Limb loss was observed during infection and postmortem metallic grey discoloration of compound eyes was detected exclusively in insects treated with M. anisopliae. These findings suggest that native fungal isolates are promising candidates for integrated control strategies against L. occidentalis in forest systems.

Key words: Biocontrol, Pesticide, Pest, Invasive species, Host–Pathogen interaction