Management of adult and immature large pine weevil (Hylobius abietis [L.]) using novel local-provenance entomopathogenic fungi and commercial

The large pine weevil (Hylobius abietis) is one of the most economically significant pests in European and Asian coniferous forests, particularly in restocking areas where clear-felling creates optimal breeding conditions. Current control strategies heavily rely on synthetic insecticides, which, despite their efficacy, raise environmental concerns and face increasing regulatory restrictions. As a result, there is a growing interest in biological control agents (BCAs) such as entomopathogenic fungi (EPFs) and entomopathogenic nematodes (EPNs) as sustainable alternatives. This thesis investigates the potential of native EPFs and their interactions with commercial EPNs to improve H. abietis management. A large-scale bioprospecting study was conducted to isolate novel EPF strains from different habitats, using a dual baiting technique to maximize diversity. The most promising isolates were tested under controlled laboratory conditions to assess their virulence, infection effects, and impact on H. abietis feeding behaviour. Beauveria bassiana 35G emerged as a particularly effective strain, significantly reducing weevil feeding activity and inducing high mortality rates. Subsequent field trials evaluated its efficacy in standalone applications and in combination with Steinernema carpocapsae, demonstrating additive effects when applied at reduced operational doses. Additionally, an assessment of non-target impacts showed no significant effects on overall invertebrate abundance or taxon richness, supporting the environmental safety of the tested BCAs. Efforts to target adult weevils proved challenging, as they exhibited high resilience to fungal infection, and EPF delivery method was not sufficiently effective. However, billets successfully attracted emerging adults, suggesting potential for future refinements in attract-and-infect strategies. While field experiments confirmed the potential of EPF-EPN combinations to suppress H. abietis, site-specific variations in efficacy highlighted the complexity of multi-agent biocontrol strategies. Practical constraints on EPN application, such as geographical limitations and variable site conditions, indicate that biological control may not be universally applicable across all restocking sites. Future research should focus on refining formulation technologies, such as encapsulation and gel-based carriers, to enhance persistence and efficacy under diverse forestry conditions. Long-term ecological studies are also needed to assess the sustainability of EPF applications and their interactions with native soil microbial communities. These findings provide a foundation for integrating EPFs and EPNs into sustainable forestry pest management programs, offering a viable pathway to reducing reliance on synthetic insecticides. While further optimization and large-scale validation are necessary, the results of this research reinforce the potential of biological control as a key component in the future of environmentally responsible forest pest management.