Black soldier fly larvae (BSFL) are fast-growing, resilient insects that can break down a variety of organic substrates and convert them into valuable proteins and lipids for applications in the feed industry. Decomposition is mediated by an abundant and versatile gut microbiome, which has been studied for more than a decade. However, little is known about the phylogeny, properties and functions of bacterial isolates from the BSFL gut. We therefore characterized the BSFL gut microbiome in detail, evaluating bacterial diversity by culture-dependent methods and amplicon sequencing of the 16S rRNA gene. Redundant strains were identified by genomic fingerprinting and 105 non-redundant isolates were then tested for their ability to inhibit pathogens. We cultivated representatives of 26 genera, covering 47% of the families and 33% of the genera detected by amplicon sequencing. Among these isolates, we found several representatives of the most abundant genera: Morganella, Enterococcus, Proteus and Providencia. We also isolated diverse members of the less-abundant phylum Actinobacteria, and a novel genus of the order Clostridiales. We found that 15 of the isolates inhibited at least one of the tested pathogens, suggesting a role in helping to prevent colonization by pathogens in the gut. The resulting culture collection of unique BSFL gut bacteria provides a promising resource for multiple industrial applications.

Black soldier fly larvae (Hermetia illucens, Diptera: Stratiomyidae) are used for the bioconversion of organic side products into valuable compounds such as proteins, lipids and chitin. However, the economic competitiveness of farmed insects compared to conventional protein production systems in agriculture and aquaculture depends on the availability of large quantities of inexpensive insect feed. Cottonseed press cake (CPC) is a side-stream of cotton production that is rich in proteins and lipids but unsuitable as feed for several farmed animals, except ruminants, due to the presence of the anti-nutritional sesquiterpenoid gossypol. Here, we tested CPC as a feed for black soldier fly larvae and studied the impact of this diet on the gut microbiome. Larvae reared on CPC developed normally and even showed a shorter life-cycle, but were smaller at the end of larval development than control larvae reared on chicken feed. The adaptability of the larvae to different diets is mediated by their versatile gut microbiome, which facilitates digestion and detoxification. We therefore used amplicon sequencing to analyze the bacterial and fungal communities associated with larvae reared on each diet, revealing differences between the larval guts and frass (residual feed substrate) as well as differences between the two diet groups. For example, Actinomycetaceae and Aspergillaceae were significantly enriched in guts of the CPC diet group and may help to metabolize compounds such as gossypol. Potentially probiotic yeasts and beneficial Enterobacteriaceae, which presumably belong to the core microbiota, were detected in high relative abundance in the gut and frass, indicating a functional role of these microbes, especially the protection against pathogens. We conclude that CPC may be suitable as an inexpensive and environmentally sustainable feed for the industrial rearing of black soldier flies.

Insects offer a promising alternative source of protein to mitigate the environmental consequences of conventional livestock farming. Larvae of the black soldier fly (Hermetia illucens; Linnaeus, 1758) efficiently convert a variety of organic side streams and residues into valuable proteins, lipids, and chitin. Here, we evaluated the suitability of two palm oil industry side streams—empty fruit bunches (EFB) and palm kernel meal (PKM)—as larval feed, and their impact on the larval gut microbiome. Among 69 fungal species we screened, Marasmius palmivorus, Irpex consors, and Bjerkandera adusta achieved the fastest growth and lignin degradation, so these fungi were used for the pretreatment of 7:3 mixtures of EFB and PKM. Larvae reared on the mixture pretreated with B. adusta (BAD) developed significantly more quickly and reached a higher final weight than those reared on the other pretreatments or the non-fermented reference (NFR). Amplicon sequencing of the BAD and NFR groups revealed major differences in the larval gut microbiome. The NFR group was dominated by facultatively anaerobic Enterobacteriaceae (typical of H. illucens larvae) whereas the BAD group favored obligately anaerobic, cellulolytic bacteria (Ruminococcaceae and Lachnospiraceae). We hypothesize that fungal lignin degradation led to an accumulation of mycelia and subsequent cellulolytic breakdown of fiber residues, thus improving substrate digestibility.

Black soldier fly larvae (BSFL) have attracted attention due to their ability to upcycle various biological side streams into valuable biomass, such as proteins, lipids, and chitin. In this study, we investigated the impact of high-fiber diets on larval growth performance and the shift of microbes in the gut. We tested empty fruit bunches (EFB), potato pulp (PP), and cottonseed press cake (CPC), with chicken feed (CF) used as a control diet. We found that larvae reared on the EFB, PP, and CPC were smaller than control larvae at the end of development due to the low nutritional value of the diets. However, survival rates of more than 90% were observed regardless of the diet. We used a cultivation-dependent approach to analyze the microbial community in the gut of BSFL, isolated, and identified a total of 329 bacterial strains. Bacillaceae were most frequently isolated from larvae reared on the high-fiber EFB diet. These isolates were predicted to degrade cellulose in silico and this was subsequently confirmed in vitro using the Congo Red assay. Whereas the members of Enterobacteriaceae and Morganellaceae were mostly found in guts of larvae reared on the high-protein diets CPC and CF. We conclude that the gut microbiome plays a crucial role in the digestion of fiber-rich plant organic material, thereby enabling the BSFL to successfully complete their life cycle also on substrates with low nutritional value. As a result, BSFL convert industrial side streams into valuable biomass, reducing waste and promoting sustainability. IMPORTANCE: Organic side streams from various industries pose a challenge to the environment. They are often present in huge amounts and are mostly discarded, incinerated, used for biogas production, or as feed for ruminant animals. Many plant-based side streams contain difficult-to-digest fiber as well as anti-nutritional or even insecticidal compounds that could harm the animals. These challenges can be addressed using black soldier fly larvae, which are known to degrade various organic substrates and convert them into valuable biomass. This will help mitigate agro-industrial side streams via efficient waste management and will contribute to the more economical and sustainable farming of insects.

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.

Background: Black soldier fly larvae (BSFL; Hermetia illucens ) are increasingly farmed as a sustainable source of animal protein and are capable of converting diverse organic material into high-value biomass. Agricultural side streams represent an abundant and underutilized feed resource within the European Union (EU). However, their influence on BSFL development and gut microbial communities remains insufficiently characterized despite the central role of the microbiota in host nutrition, immunity, and health. Result: We evaluated five feed treatments, namely chicken feed (control), apple pomace, potato pulp (industrial-scale and lab-scale), and rapeseed cake, for their suitability in supporting BSFL growth from hatching to pupation. All feed treatments supported larval development, with time to pupation ranging from 25 days (rapeseed cake) to 87 days (potato pulp industrial-scale). Larvae reared on chicken feed exhibited significantly (7.6-fold) higher biomass compared to those fed with apple pomace, while the performance of BSFL reared on rapeseed cake was similar to that of the control larvae. Variations in development performance correlated with differences in feed nutrient profiles, particularly protein, fat, and fiber content. Furthermore, gut microbiota profiling via 16S rRNA and ITS2 amplicon sequencing revealed that gut microbial communities varied depending on diet chemistry. This was supported by functional predictions indicating an enrichment of fiber-degrading and nitrogen-fixing bacteria in larvae fed fiber-rich, nitrogen-poor diets. Notably, no methane production of L5 larvae was detected across treatments under tested laboratory conditions. Conclusion: This study demonstrates the potential of agricultural side streams to serve as viable feed substrates for BSFL production, with associated shifts in gut microbiota likely supporting host adjustment to less-optimal diets. These findings underline the importance of microbiota plasticity in BSFL digestion and highlight the relevance of feed composition in optimizing large-scale insect farming for sustainable protein production.