Utilizing agro-industrial by-products such as sugar beet pulp (SBP) in black soldier fly (Hermetia illucens L.) rearing is an attractive strategy for circular feed production; however, its consequences on larval chemical composition and cuticular mineralization require detailed characterization. This study evaluated the effects of SBP inclusion, with or without Trichoderma sp. inoculation or beet molasses addition, on the proximate composition, macro- and trace mineral profiles, and gravimetric chitin yield of whole H. illucens larvae. Five-day-old larvae were reared across three independent biological cycles (n = 3 rearing containers per treatment) on four diets: Control (100% wheat bran), Option 1 (60% wheat bran + 40% SBP), Option 2 (60% wheat bran + 40% SBP with Trichoderma sp. at 1 × 10⁶ CFU/g), and Option 3 (60% wheat bran + 30% SBP + 10% molasses). Larvae were harvested without a pre-harvest fasting period to reflect commercial whole-larva processing. Dietary inclusion of SBP significantly decreased larval crude protein (44.66 ± 2.88% DM in Option 1, 44.36 ± 3.12% DM in Option 2, and 45.25 ± 0.97% DM in Option 3 vs. 50.85 ± 3.23% DM in Control; p = 0.038), reflecting the lower protein density of SBP. Crude fat (32.18–36.32% DM; p = 0.774) and water-soluble carbohydrates (1.80–2.20% DM; p = 0.388) did not differ significantly among treatments. SBP inclusion markedly increased the crude ash content (10.59–13.68% DM vs. 5.75 ± 0.52% DM in Control; p < 0.001) and whole-body calcium (7.46–8.84% DM vs. 2.43 ± 1.39% DM in Control; p = 0.003), driving the larval Ca:P ratio from 2.45:1 to 8.04:1. Manganese concentrations were elevated in all SBP diets (661.2–700.6 mg/kg vs. 589.9 ± 71.7 mg/kg in Control; p = 0.041), while iron was significantly higher in Option 1 (286.9 ± 71.5 mg/kg vs. 168.9 ± 34.4 mg/kg in Control; p = 0.024). The gravimetric chitin yield was significantly higher on 40% SBP diets (7.75 ± 0.22% DM in Option 1; 7.82 ± 0.18% DM in Option 2) than in the Control (7.34 ± 0.15% DM; p = 0.004), whereas Option 3 yielded the lowest chitin content (7.14 ± 0.12% DM). Fungal inoculation produced no significant differences over SBP alone. These results demonstrate that SBP-based diets substantially increase whole-body ash and calcium levels while altering chitin yields, providing critical compositional benchmarks for tailoring insect meal to non-ruminant livestock feeding.

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.