Impact of Sugar Beet Pulp-Based Diets on the Chemical Composition, Mineral Profile, and Chitin Yield of Black Soldier Fly (Hermetia illucens L.) Larvae

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.