Host Species Identity Is Associated With Distinct Frass-Associated Bacterial Communities in Spodoptera exigua (Hübner) and Pyrausta panopealis (Walker) Reared on a Shared Diet

Abstract

Whether insect gut-associated microbial communities are structured by host identity or diet remains unresolved. Using a common-garden design, we reared two lepidopteran pests of contrasting host breadth, the polyphagous Spodoptera exigua (Hübner) and the Lamiaceae-associated Pyrausta panopealis (Walker), exclusively on perilla (Perilla frutescens [L.] Britton var. frutescens), and characterized frass-associated bacterial communities by 16S rRNA metabarcoding of three pooled replicates per species (240 larvae). The two hosts showed apparent separation in community composition (PERMANOVA R² = 0.857), which did not reach conventional significance (exact-permutation p = 0.100, the smallest value attainable with three replicates per group). Alpha diversity did not differ significantly (Wilcoxon rank sum, all p ≥ 0.10), although S. exigua showed numerically greater richness (median observed ASVs: 55 vs. 25) and P. panopealis showed higher evenness. Because the two species differ in taxonomic family, body size, and life history beyond host breadth, these patterns reflect a host species–associated difference under a shared diet rather than general evidence that dietary breadth shapes microbiome architecture. To our knowledge, this is the first description of the bacterial community of P. panopealis, and shows that two co-occurring pests reared on an identical diet were numerically dominated by mutually near-exclusive genera (Enterococcus, 72.2%, in S. exigua vs. Enterobacter, 71.8%, in P. panopealis), with complete rank separation in five genera. Species-level names are descriptive because the V3–V4 region gives limited subgenus resolution. These patterns are consistent with host-associated filtering contributing to frass community assembly, a hypothesis warranting confirmation with larger samples and additional diet treatments.