Foliar‐Feeding Weevils Recruit Phyllosphere‐Derived Tannin‐Degrading Bacteria to Increase Their Fitness

The gut microbiota plays a crucial role in shaping animal life history, particularly by mediating interactions between herbivorous insects and their host plants. Phytophagous insects that feed on oak leaves must overcome the challenge of detoxifying tannins. While gut microbes contribute significantly to this detoxification process, the mechanisms by which insects acquire tannin-degrading bacteria remain poorly understood. In this study, we investigated tannin-degrading bacterial communities in Nothomyllocerus illitus, a phytophagous weevil that primarily consumes oak leaves. Through 16S rRNA sequencing, bacterial isolation, enzymatic activity assays, and fluorescence tracing experiments, we identified Bacillus, Acinetobacter, and Enterobacter as key tannin-degrading genera associated with N. illitus. These bacteria are predominantly acquired from the oak phyllosphere rather than from soil sources. Fluorescently labelled Bacillus and Acinetobacter strains were shown to successfully colonise the weevil gut, underscoring their functional relevance in tannin degradation. Furthermore, supplementation of oak leaves with these bacterial strains enhanced feeding behaviour, larval growth, and reproductive performance, whereas antibiotic treatment diminished these effects, thereby confirming the critical contribution of phyllosphere-derived bacteria to the overall fitness of foliar-feeding weevils N. illitus.