Effect of probiotic-based fermentation on transcriptional profiles of black soldier fly during watermelon waste bioconversion

The growing demand for food production has led to substantial agro-industrial waste generation, particularly from fruit residues such as watermelon peels. Conventionally, these wastes are managed through landfilling, on-site combustion, and direct mulching methods that pose serious risks to human health and contribute to environmental pollution. Black soldier fly larvae (BSFL; Hermetia illucens) have emerged as a sustainable solution for organic waste valorization by converting low value-biomass into high-value protein. In this study, we developed a probiotic-based fermentation strategy for the first time to utilize watermelon waste as feed for BSFL and explored the underlying biological mechanisms. Results revealed that fermentation with probiotic consortium (Bacillus subtilis, Enterococcus faecalis, and Aspergillus oryzae) over 14 days significantly improved BSFL growth performance, biomass yield, and nutritional composition, particularly enhancing protein and fat content. Transcriptomic profiling showed upregulation of key genes involved in growth and development (CK1, HIB, and PDK1), protein and fat biosynthesis (DVL, GSK3, and Lipin), and immune defense and stress tolerance (PGRP-SA, Spz, Toll, and Cactus). Functional enrichment analysis of these genes further confirmed their participation in critical signaling pathways, including Hedgehog, Wnt, mTOR, Toll and Imd, and MAPK. These findings demonstrate that probiotic fermentation of watermelon waste not only improves BSFL bioconversion efficiency and nutritional quality but also enhances physiological resilience, offering a promising approach for integrating insect-based protein production into sustainable food and waste management systems.