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

## TL;DR

This study shows how the feed given to black soldier fly larvae affects the microbes in their frass, which could be used as a better organic fertilizer.

## Contribution

The study reveals how feed composition and thermal treatment influence microbial communities in black soldier fly frass for biofertiliser applications.

## Key findings

- Feed type was the main factor shaping frass microbial composition, reflecting substrate nutritional profiles.
- High-fibre diets increased fungal diversity, while high-protein feeds enriched specific bacterial taxa.
- Thermal hygienisation had minimal impact on community structure but reduced viable microbial counts.

## 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.

Frass derived from black soldier fly larvae reared on various industrial side streams was investigated in both untreated and hygienised (1 h at 70°C) condition. Hygienisation efficacy was evaluated by determination of viable microbial counts, using both selective and non‐selective cultivation media. Microbial communities were analysed using culture‐independent amplicon sequencing and culture‐dependent Sanger sequencing of bacterial isolates.