Dose-response effects of starch in sterilised and unsterilised chicken excreta on Musca domestica larval growth and bioconversion efficiency

Housefly (Musca domestica L.) larvae can convert animal manure into valuable protein and fat but little is known about their nutritional requirements. We assessed the effects of differential starch content in unsterilised (UE) and heat sterilised (SE) chicken excreta on larval performance and bioconversion. Gelatinized corn starch was added to excreta resulting in 11 diets with 0 to 50% starch of dry mass. Individual larval mass increased from (mean ± SD) 12 ± 0.2 (UE) and 13 ± 0.4 (SE) mg at 0% starch up to 18 ± 0.4 (UE) and 18 ± 0.5 (SE) mg at 15% starch and decreased to 5 ± 0.4 (UE and SE) mg in 50% starch. Higher starch concentrations resulted in non-normal distributions of individual larva mass. Larval wet yield showed a curvilinear relation with starch content, with maxima of 39 ± 2.3 g in UE at 10% starch, and 43 ± 2.8 g in SE at 15% starch. The decrease in yield after the maxima was faster in UE compared to SE. Larval survival was highest in 0% starch, 80 ± 8% (UE) and 72 ± 13 % (SE), linearly decreasing to 14 ± 7% (UE) and 18 ± 3% (SE) in 50% starch. Dry matter bioconversion increased from 4 ± 0.1% (UE) and 4 ± 0.3% (SE) in 0% starch to 6 ± 0.2 in UE with 10% starch and 7 ± 0.3 in SE with 15% starch, decreasing to close to 0 in 50% starch. Highest nitrogen bioconversion was 15 ± 0.5% in UE with 10% starch and 15 ± 0.5% in SE with 15% starch. Optimal starch inclusions increased nitrogen bioconversion with 40% (UE) and 100% (SE) compared to pure excreta. Finally, around 20 to 25% starch uric acid and ammonia contents, pH, and diet residue C/N decreased, indicating a tipping point. In conclusion, adding starch to chicken excreta can substantially increase larval yield and bioconversion while reducing ammonia content in the substrate residue.