Towards Functional Fertilisers: Feed Composition Shapes Microbial Community Structure and Viability in Black Soldier Fly ( Hermetia illucens ) Frass

ABSTRACT Black soldier fly larvae (BSFL) are increasingly valued as a sustainable protein source for aquaculture and can be reared on local industrial side streams, enhancing their environmental and economic benefits. The resulting frass—a byproduct of larval excreta and residual feed—shows promise as an organic fertiliser. In addition to its nutrient content, frass contains microbial communities that may enhance plant growth through phytohormone production, nitrogen fixation, and organic matter turnover. Yet, the roles of feed composition and thermal hygienisation in shaping these communities remain underexplored. This study examined the impact of five feed substrates, including industrial side streams and a control diet, on frass microbial composition, and assessed responses to thermal treatment. Feed nutrients were characterised, and microbial communities profiled using amplicon sequencing. Viable populations were quantified via culture‐based methods, with bacterial isolates taxonomically classified. Feed type was the dominant factor influencing frass microbiota, with distinct communities reflecting substrate nutritional profiles. High‐fibre diets promoted fungal diversity and abundance, while high‐protein feeds enriched specific bacterial taxa. Thermal hygienisation had a heterogeneous effect on viable counts but minimal impact on overall community structure. These findings support microbiome‐informed feed design to tailor frass microbial profiles for enhanced biofertiliser function in sustainable agriculture.