<p>Insect frass, a nutrient-rich byproduct of insect farming, is gaining attention as an organic fertilizer; however, its microbial composition and impact on plant growth remain underexplored. This study examined the nutrient profiles, plant growth-promoting microorganisms, and seedling growth effects of frass from four edible insect species: <i>Gryllus bimaculatus</i>, <i>Gryllus testaceus</i> Walker, <i>Acheta domesticus</i>, and <i>Patanga succincta</i>.&#xa0;Bacterial communities in frass were characterized via high-throughput 16S rRNA gene sequencing targeting the V3–V4 region. Nutrient content, enzymatic activity, pH, and electrical conductivity were measured. Seed germination assays with morning glory (<i>Ipomoea aquatica</i>) and rice (<i>Oryza sativa</i>) assessed early plant responses.&#xa0;Frass was dominated by Proteobacteria, Firmicutes, and Bacteroidetes, with Cyanobacteria unique to locust frass. Cricket frass showed higher nitrogen (2.88–5.05%), phosphorus (1.06–1.23%), neutral to slightly acidic pH (6.41–6.97), and moderate EC (3.71–5.25 dS m^− 1). Locust frass had the highest organic matter (79.73%), lower nitrogen (2.08%), acidic pH (5.22), and high EC (10.66 dS m^− 1). Key plant growth-promoting microorganisms included <i>Klebsiella</i> (42%) and <i>Bacillus</i> (12%). The combined effects of nutrient richness and active microbial communities correlated with improved seedling vigor, with cricket frass enhancing rice germination above 97% and morning glory germination index up to 176%.&#xa0;Nutrient content, pH, electrical conductivity, and microbial activity jointly shape the agronomic potential of insect frass. Cricket frass promotes rapid early growth in crops such as rice and morning glory, while locust frass supports long-term soil fertility. Selecting frass types based on crop requirements provides a sustainable alternative to synthetic fertilizers.</p> Graphical Abstract <p></p>