Ambrosia beetle communities in NW Italian chestnut-growing areas: environmental drivers shaping species distribution and laboratory insights into Xyleborinus saxesenii (Ratzeburg)

Ambrosia beetles (Coleoptera: Curculionidae, Scolytinae and Platypodinae) are highly specialised wood-boring insects that spend their life cycle within woody tissues, protecting themselves from predators and climatic extremes. A defining trait of their ecology is their obligate nutritional dependence on mutualistic fungi, which they cultivate on gallery walls to extract essential nutrients from poor substrates, thereby enabling a high degree of polyphagy. While native species primarily target dead or stressed trees by detecting stress-induced olfactory cues like ethanol, certain species have turned into significant global pests. In recent decades, the invasion dynamics of non-native ambrosia beetles have accelerated dramatically. This global spread is driven by the expansion of international timber trade and the ecological traits of these insects. Concurrently, climate change is facilitating the establishment of tropical and subtropical species in Europe and the Mediterranean basin. Within this shifting landscape, sweet chestnut (Castanea sativa Miller) orchards represent highly vulnerable ecosystems. This thesis investigates the ecological dynamics of ambrosia beetles’ communities, bridging field surveys in chestnut-growing areas with highly controlled laboratory experiments. The field research monitored ambrosia beetles’ assemblage using ethanol-baited traps over a 2-year period in chestnut-growing areas, documenting the presence of alien species, such as Xylosandrus crassiusculus (Motschulsky) and Xylosandrus germanus (Blandford), and the first record of the nonnative Anisandrus maiche (Kurentsov) in NW Italy. Further investigations studied the community and the related environmental drivers in managed chestnut orchards and unmanaged mixed environments across the Pesio and Susa valleys (NW Italy). Findings revealed a community overwhelmingly dominated by the native species Xyleborinus saxesenii (Ratzeburg), which accounted for over 90% of all captured individuals. The structural composition of the beetles is primarily driven by macro-environmental filters, such as geographical gradients and regional climatic regimes. Local ecological factors, including habitat type, host plant availability, and the specific microclimates of traditionally managed orchards, acted as secondary determinants that facilitated the establishment of both native and invasive species. In order to comprehend the ecological success of ambrosia beetles, the laboratory component of the research addressed the historical methodological challenges of rearing these insects in highly controlled conditions, which are frequently hindered by unpredictable microbial contamination. The study introduces a novel, reproducible protocol for generating symbiont-free populations by using the model species X. saxesenii. By integrating a pupal surface-sterilisation technique with targeted fungal inoculation on artificial substrates, the trial successfully established clean, mono- and bisymbiotic laboratory lines. This controlled experimental manipulation assessed the specific roles of the beetle's core mutualistic fungi, Dryadomyces sulphureus (L. R. Batra) and Raffaelea cf. canadensis L. R. Batra. The results demonstrated that bi-symbiotic nests exhibited superior fitness, including higher offspring production, compared to mono-symbiotic lines, underscoring the functional complementarity of the fungal partners. In conclusion, this thesis provides a comprehensive overview of ambrosia beetle population dynamics under current environmental pressures. It highlights the vulnerability of chestnut-growing areas to biological invasions and emphasises the necessity for continuous surveillance in a context of climate change. Furthermore, the newly developed laboratory framework establishes a robust foundation for future behavioural ecology studies and the potential development of targeted symbiotic-disruption strategies for biological control