Effects of Herbal Distillation Residues on Growth Performance, Bioconversion Efficiency and Nutritional Composition of Black Soldier Fly Larvae (Hermetia illucens)

Abstract The increasing generation of herbal distillation residues (HDRs) from the hydrosol (aromatic water) production industry poses both environmental challenges and opportunities for resource recovery within a circular bioeconomy framework. This study investigated the phytochemical-mediated effects of three HDRs—chicory (Cichorium intybus), fumitory (Fumaria parviflora), and green cumin (Cuminum cyminum)—on the growth performance, bioconversion efficiency, and nutritional composition of black soldier fly larvae (Hermetia illucens L.). Each HDR was used as the sole substrate in a 14-day rearing trial under controlled conditions (28 ± 1°C, 65 ± 5% RH). Substrates were characterized for proximate composition (protein, fat, fiber, ash), physical properties (bulk density, specific surface area, particle size distribution), and inferred phytochemical profiles. Larval performance was assessed via final weight, length, survival rate, bioconversion efficiency (BE), and proximate composition of harvested larvae. Statistical analyses included one-way ANOVA (Duncan's post-hoc), Pearson correlation heat map, and principal component analysis (PCA). Green cumin residue exhibited the highest crude protein (9.12%), crude fat (7.88%), specific surface area (21.4 m²/g), and smallest particle size (D50 = 145 µm), leading to superior larval performance: final weight (0.165 g), BE (21.3%), survival (96.4%), and larval protein (43.6%) and lipid (29.4%) content (p < 0.05). Chicory residue, despite the highest crude fiber (17.41%) and largest particle size (312 µm), supported moderate performance (weight = 0.112 g, BE = 14.6%, survival = 89.2%) through compensatory feeding. Fumitory residue, despite intermediate chemical and physical properties, resulted in the poorest outcomes (weight = 0.087 g, BE = 9.8%, survival = 72.5%, larval protein = 36.2%), revealing a "fumitory anomaly" attributed to isoquinoline alkaloids. PCA explained 100% of variance (PC1 = 85.4%, PC2 = 14.6%), clearly separating green cumin from chicory and fumitory, while heat map correlations confirmed strong positive relationships between protein/fat/surface area and growth (r = 0.70–1.00), and negative correlations for fiber/particle size (r = -0.42 to -0.98). A conceptual mechanistic model (substrate encounter → digestive hydrolysis → metabolic allocation) was proposed, illustrating optimal (green), compensatory (yellow), and inhibited (red) pathways. These findings demonstrate that green cumin residue is a promising substrate for BSFL bioconversion, while fumitory is unsuitable due to alkaloid toxicity. This study provides a framework for integrating phytochemical assessment into substrate selection for insect-based valorization of hydrosol production by-products within a circular bioeconomy.