INVESTIGATIONS ON THE VINE-FEEDING SPECIES COMPLEX: INSECT-MICROORGANISM INTERACTIONS AND THEIR IMPLICATIONS FOR VINEYARD MANAGEMENT

The spread of invasive insect pests and the reduction in available plant protection products are major challenges for sustainable viticulture. New pest management strategies require a better understanding of the ecological interactions between insects, host plants and microorganisms. In this context, the insect holobiont concept considers insects and their associated microbiota as a single biological system. This perspective provides new opportunities to investigate insect adaptation, invasion processes and the development of sustainable control strategies. This thesis applies the holobiont approach to two economically important grapevine-associated invasive insects, Scaphoideus titanus, the vector of Flavescence dorée phytoplasma, and Drosophila suzukii. The overall objective was to investigate the role of microbial communities in insect ecology and to evaluate their potential application in sustainable viticulture. The first part of the thesis investigates the influence of host plant species on the biology and bacterial microbiome of S. titanus. Survival and developmental analyses showed that American Vitis species represent more suitable hosts than Vitis vinifera, although insect development required a longer time. High-throughput sequencing revealed that the bacterial community was consistently dominated by the obligate symbionts ‘Candidatus Karelsulcia muelleri’ and ‘Candidatus Cardinium’. Host plants mainly affected the less abundant bacterial taxa, whereas the core microbiome remained stable. These results suggest that host plants contribute to shaping bacterial diversity while preserving the dominant symbiotic community. The second part focuses on the recent invasion of Madeira Island by S. titanus. Mitochondrial haplotypes and bacterial communities were analysed to investigate population differentiation. The Madeiran population showed higher mitochondrial diversity than long-established European populations. Differences in the bacterial community were mainly detected during the nymphal stage, whereas adults retained a microbiome comparable with that of Italian populations. No association was found between mitochondrial haplotypes and bacterial composition. These findings indicate that microbiome variation is more closely associated with ecological factors than with host mitochondrial diversity. The final part evaluates grape-associated yeasts as sustainable tools for the management of D. suzukii. Behavioural assays identified Starmerella bacillaris as a strong attractant for adult flies, highlighting its potential use in attract-and-kill strategies. Fermentation trials demonstrated that this yeast maintains its bioprotective activity during winemaking and can limit the growth of undesirable microorganisms without compromising the fermentation process. These results support the development of multifunctional microbial applications that can simultaneously contribute to pest management and wine production. Overall, this thesis demonstrates that microorganisms are integral components of insect biology and play an important role in adaptation, invasion and host interactions. The integration of microbial ecology, invasion biology and applied entomology provides a broader understanding of grapevineassociated insect pests and identifies beneficial microorganisms as promising resources for sustainable vineyard protection. The findings also support the adoption of the holobiont concept as avaluable framework for future studies aimed at developing environmentally friendly pest management strategies