Bioconversion of Sugar Beet Pulp by Black Soldier Fly (Hermetia illucens) Larvae: Nutritional Balancing via Molasses and In-Situ Fungal Inoculation

Agro-industrial by-products such as sugar beet pulp (SBP) represent a substantial environmental and logistical challenge if not valorized within the circular bioeconomy. This study evaluated the bioconversion performance and waste reduction efficiency of black soldier fly (Hermetia illucens L.) larvae reared on SBP-based diets across three independent biological cycles (n = 9 replicate containers per dietary treatment, total N = 36 experimental units, each initially stocked with 6000 larvae). Four dietary formulations were compared on a dry matter (DM) basis: Control (100% wheat bran), Option 1 (60% wheat bran + 40% SBP), Option 2 (60% wheat bran + 40% SBP inoculated with Trichoderma sp. at 1 × 106 colony-forming units (CFU)/g), and Option 3 (co-diet: 60% wheat bran + 30% SBP + 10% industrial molasses). Dietary formulation significantly affected all measured biological growth and waste reduction parameters (p < 0.05). Inclusion of 40% unsupplemented or fungal-inoculated SBP (Options 1 and 2) suppressed wet daily larval growth rate (12.65–12.83 mg/d) and impaired the feed conversion ratio (FCR 5.75–5.87) compared to the Control (15.72 mg/d; FCR 4.68), which is attributed to dietary protein dilution and high structural fiber density. Direct in situ inoculation with Trichoderma sp. yielded a modest increase in substrate mass reduction (SMR 53.04 ± 2.49% vs. 47.52 ± 2.67% in Option 1) but did not improve larval growth or feed conversion. In contrast, the co-diet formulation combining 30% SBP with 10% molasses (Option 3) achieved the highest larval survival rate (93.0 ± 1.8%), elevated daily wet growth rate (16.32 ± 0.83 mg/d; 3.26 mg DM/d), lowest FCR (4.52 ± 0.23), maximum substrate mass reduction (SMR 78.60 ± 2.15%), and peak biomass conversion efficiency (BCR 22.12 ± 1.13%), performing comparably to or outperforming the wheat bran Control. These findings demonstrate that simultaneous soluble carbohydrate enrichment and fiber reduction represent an effective dietary optimization strategy for valorizing high-fiber agro-industrial side-streams in commercial insect rearing systems.