Post-Hatch Starvation in Black Soldier Fly Larvae: Implications for Growth, Nutritional Quality, and Multi-Omics Profiles Across Rearing Substrates

The black soldier fly (BSF, Hermetia illucens L.) has gained interest for its ability to convert organic waste into high-quality protein. The eggs of BSF are typically sold in fixed quantities, which often does not match the fluctuating waste input of processing facilities; post-hatch starvation offers a practical strategy to buffer this supply-demand mismatch. However, how post-hatch starvation affects larval development and nutrient accumulation remains unclear. After being reared to approximately 5 mg, neonate larvae were subjected to starvation periods of 0, 7, 14, and 21 days in wheat bran. After starvation, larvae were reared on two substrates: kitchen waste and chicken manure, to evaluate how starvation time influences larval nutrition, growth, microbiota and metabolome. With prolonged starvation, larvae reared on kitchen waste showed increased crude protein and total amino acids, while crude fat exhibited a biphasic pattern, decreasing initially before rising again. The proportion of C12:0 fatty acid consistently increased. In contrast, larvae reared on chicken manure showed an increase in crude fat content after 21 days, alongside decreased total amino acids and increased C12:0. After 21 days of starvation, the survival rate dropped to 68%. The final fresh body weight and length of larvae in both groups increased with prolonged starvation duration. Substrate conversion efficiency peaked at 7 days of starvation across both substrates, whereas substrate reduction rates progressively declined with extended starvation, reaching the lowest values at 21 days. Microbiota analysis of the larval intestines revealed that the gut bacterial community underwent selective remodeling in response to substrate type and starvation duration. Specifically, kitchen waste, rich in fermentable carbohydrates and proteins, promoted an increased relative abundance of Ignatzschineria to decompose nitrogenous organic matter and supply energy. Conversely, chicken manure, which is higher in refractory organic carbon, enriched Moheibacter to degrade complex compounds such as chitin and cellulose. Metabolomic changes included enrichment in tryptophan, linoleic acid, and α-linolenic acid metabolism pathways. Ultimately, the studies revealed that 7 days is the optimal fasting duration to maximize bioconversion efficiency and larval production, while fasting for up to 14 days remains a safe preservation limit before survival rates significantly decline. These findings clarify how post-hatch starvation shapes BSF larval composition, growth, and molecular profiles, aiding the optimization of preservation duration and the enhancement of bioconversion efficiency and larval nutritional quality of BSF.