Substrate and stage effects on growth, composition, frass traits, and economic efficiency in black soldier fly ( <i>Hermetia illucens</i> )

The black soldier fly (BSF) (<i>Hermetia illucens</i>) has gained global economic relevance as a sustainable resource for animal feed, organic waste bioconversion, and the production of nutrient-rich frass for agricultural use. Its ability to convert low-value waste streams into high-value proteins, lipids, and fertilisers makes BSF a key component of emerging circular-economy models. However, the influence of vegetable and fruit waste (VFW) substrates on insect performance, nutrient composition, frass quality, and economic efficiency remains poorly defined. This study evaluated five substrate levels obtained by mixing laying hen feed with VFW at 0%, 25%, 50%, 75%, and 100%, assessing their effects on the growth performance, body measurements, chemical composition, frass characteristics, and economic traits of BSF larvae and prepupae. The results showed that up to 75% VFW can be used to rear BSF up to 23 days of age without compromising growth traits. Substrate reduction ranged from 67.2% to 89.4%, and larval-to-prepupal conversion increased with feed intake. VFW level significantly affected larval and prepupal nutrient profiles, including dry matter, crude protein, ether extract, nitrogen-free extract, and chitin. Frass from the 100% VFW substrate exhibited the highest nutrient concentration, although this inclusion level impaired feed conversion due to elevated fibre content. Economic analysis indicated that using VFW can reduce substrate costs without negatively affecting revenue. Overall, a VFW inclusion level of up to 75% is recommended as the optimal balance between insect growth efficiency, nutrient recycling, and frass fertilising potential.