Abstract
Ground beetles of the genus Poecilus (Carabidae) play key ecological roles in pest control and soil health. However, their gut microbiome remains largely unexplored despite increasing interest in insect-associated microbiota and its environmental implications. This study used next-generation sequencing and qPCR to characterise the gut microbiome of Poecilus beetles collected from organic and conventional tomato fields. Core microbiota were identified through prevalence-abundance filtering, revealing dominant genera including Gilliamella, Weissella, Enterobacter, and Enterococcus, alongside several low-abundance but consistently present taxa. Notably, Carnobacterium was detected for the first time in an insect host, and Nosema ceranae was identified for the first time in Carabidae, expanding the known host range of this microsporidian pathogen. Functional predictions based on 16S rRNA data and comparative genomic analysis showed enrichment in pathways related to amino acid synthesis, protein degradation, and monosaccharide metabolism. Significant inter-individual variation in microbial diversity and predicted functionality was observed, with lowest diversity and metabolic potential in beetles from conventionally managed fields, potentially indicating dysbiosis and environmental stress. The detection of Nosema and Serratia pathogens in some individuals adds new insights into pathogen dynamics within carabid beetles. Our findings reveal that the gut microbiome of Poecilus may be shaped by environmental factors and agricultural practices, influencing host health and ecological performance. These insights support the use of Poecilus as a bioindicator for soil ecosystem health and highlight the potential of microbiome-based metrics in agroecological monitoring.