Effects of brewery by-products on growth performance, bioconversion efficiency, nutritional profile, and microbiota and mycobiota of black soldier fly larvae.

Brewery by-products are recognised as suitable rearing substrates for Hermetia illucens, better known as black soldier fly (BSF) but information about the impact of different ratios of brewer's spent grains (BSG) and brewer's spent yeast (BSY) are still scarce. This study evaluated the effects of BSG-BSY-based diets on BSF larval growth, survival, bioconversion efficiency, nutritional profile, and microbiota and mycobiota. A total of 3 000 6-day-old BSF larvae were allotted to five dietary treatments (six replicate boxes/diet, 100 larvae/box): (i) BSY2.5 (25 g/kg of BSY+975 g/kg of BSG), (ii) BSY5 (50 g/kg of BSY+950 g/kg of BSG), (iii) BSY7.5 (75 g/kg of BSY+925 g/kg of BSG), (iv) BSY10 (100 g/kg of BSY+900 g/kg of BSG), and (v) control (Gainesville diet). Larval weight and substrate pH were recorded every 4 days. At the end of the trial (5% of prepupae), bioconversion efficiency corrected for residue (BER), reduction rate (RR), and waste reduction index (WRI) were calculated, and the larval proximate composition, microbiota and mycobiota characterised. At 10 and 14 days of age, BSY7.5 and BSY10 larvae displayed higher weight than BSY2.5 and BSY5 (P < 0.05), with BSY10 larvae showing the highest weight among the BSG-BSY-based diets at the end of the trial (P < 0.05). The BSY7.5 and BSY10 larvae also displayed a better BER than BSY2.5 and BSY5 (P < 0.01), whereas similar RR, WRI, survival and development time, as well as pH, were, however, observed among the BSG-BSY-based diets (P > 0.05). The BSY10 larvae displayed lower ether extract content than the other BSG-BSY-based diets (P > 0.001). The use of BSG-BSY-based diets did not influence the alpha diversity of larval microbiota and mycobiota (P > 0.05), but a specific microbial signature was identified per each dietary treatment (Porphyromonadaceae [BSY5], Sphingomonas [BSY7.5], Bacillus [BSY10] and Ruminococcus and Myroides [BSG-BSY-based diets]; P < 0.05). Co-occurrence and co-exclusion analysis also showed that Saccharomyces cerevisiae and Pichia excluded and favoured, respectively, the presence of Streptomyces and Fluviicola, while Clavispora lusitaniae was associated with Myroides (P < 0.05). In conclusion, BSG-BSY-based diets are suitable for rearing HI in terms of larval performance, nutritional profile, and microbiota and mycobiota, with 7.5 and 10% of BSY inclusion levels being able to improve larval growth and bioconversion efficiency.

Wheat starch processing by-products as rearing substrate for black soldier fly: does the rearing scale matter?

Rearing scale may influence black soldier fly (BSF) larvae traits when they are fed on a single diet, but different feeding substrates have not been tested yet. This study evaluated the effects of wheat starch processing by-products-based diets on growth performance, bioconversion efficiency, and nutritional profile of BSF larvae reared in different scales. Four diets (D1 and D2 [isonitrogenous, isolipidic and isoenergetic]; D3 and D4 [displaying 1:1 and 1:2 as protein to carbohydrate ratios, respectively]) were tested at 3 rearing scales (4 replicate boxes/diet, with a constant volume [0.84 cm3]/larva and feed [0.7 g]/larva): 1) small (S; 12 × 12 cm, substrate height: 4 cm, 686 6-day-old larvae (6-DOL)/box), 2) medium (M, 32 × 21 cm, substrate height: 7 cm, 5 600 6-DOL/box), and 3) large (L, 60 × 40 cm, substrate height: 7 cm, 20 000 6-DOL/box). Larval weight was recorded at the beginning of trial and every 4 days, and growth rate (GR), specific growth rate (SGR), feed conversion ratio (FCR), survival, bioconversion efficiency corrected for residue (BER), reduction rate (RR), and waste reduction index (WRI) calculated at the end of larval growth (frass DM ≥ 55%). Substrate pH, T and height were measured at the beginning, every 4 days, and end of trial. Larval proximate composition was analysed at the end of trial. Data were analysed by generalised linear mixed model (SPSS software, P < 0.05). The D1 larvae showed higher weight, GR, SGR and WRI (along with higher substrate T) than D2 at M scale, while increased SGR and FCR - as well as decreased survival, RR and WRI - were observed in D2 larvae at S scale (P < 0.05). Larval CP and ether extract (EE) contents were influenced by M and L scales only, being higher in D2 group than in D1 (P < 0.001). Differently, decreased ash was recorded in D2 larvae when reared at S and M scales, while L scale revealed higher ash in D2 group than D1 (P < 0.001). The D3 larvae displayed greater weight, SGR, survival, RR and WRI (along with greater substrate T) than D4 at M scale, with increased survival and substrate T being also highlighted in L scale (P < 0.05). The D3 larvae also showed lower DM and EE - as well as higher CP - than D4 at all the rearing scales (P < 0.001). In conclusion, D1 and D3 led to better BSF larval growth performance, bioconversion efficiency and nutritional profile mainly at M and L scales, as a consequence of their ability to facilitate larval aggregation and, in turn, allow achieving a higher substrate T.