Impact of nursery-to-fattening dietary transitions on gut microbiome composition and growth performance in black soldier fly larvae.

Black soldier fly (BSF) (Hermetia illucens) larvae are used in large-scale bioconversion due to their capacity to convert diverse organic substrates into high-quality biomass. To improve circularity, rearing practices must be optimized, especially when nutritionally poor, low-cost substrates are used. Production systems often adopt a two-phase feeding strategy in which larvae first receive a nutrient-rich nursery diet before transitioning to a fattening diet. The effects of such a diet shift on the gut microbiome and larval performance remain unclear. This study explores how a diet shift between the nursery stage (0-7 days after egg harvest; DAH) and the fattening stage (DAH 8-15) influences larval performance and microbiome composition. Chicken feed (CF) and artificial supermarket food waste (SFW) were used as contrasting diets across four conditions: continuous feeding (CF to CF and SFW to SFW) and diet shifts (CF to SFW and SFW to CF). At the end of the nursery phase, there was no significant difference in larval weight between the two diets. Immediately after the diet shift (DAH 8-9), CF-nursed larvae were heavier than SFW-nursed larvae, but SFW-nursed larvae gradually reached comparable weights over time. This convergence occurred more rapidly when larvae were maintained on SFW. Notably, survival within both fattening diets was lower for SFW-nursed larvae than for CF-nursed larvae, which may have reduced larval density and feed competition during the fattening phase. Microbiome profiling through 16S rRNA gene amplicon sequencing revealed distinct bacterial communities associated with CF- and SFW-based rearing: CF-reared larvae maintained a consistently diverse community, whereas SFW-reared larvae exhibited low diversity. Following a diet transition, microbial composition shifted toward a fattening-driven profile. However, the rate and extent of this shift depended on the fattening substrate. Under SFW fattening, microbiome differences between nursery treatments disappeared by the end of the experiment, whereas several biomarkers persisted under CF fattening, suggesting a stronger legacy effect of the nursery diet. Overall, these findings demonstrate that interactions between diet, microbiome, and larval physiology shape performance in two-phase rearing systems, highlighting that early-life nutrition can influence microbiome trajectories and affect responses to subsequent feeds, an important consideration for optimizing BSFL production.