Bioconversion of fresh chicken excreta by housefly larvae (Musca domestica L.) in relation to excreta sterilisation and carbohydrate addition

Housefly larvae can be reared on manure and used as animal feed. Larvae can utilise proteins and easily digestible carbohydrates and have a largely unknown relation with microorganisms. The hypothesis addressed in this study was that larvae compete with microorganisms for easily digestible carbohydrates such as starch. This was tested by adding starch or (non-digestible) fructo-oligosaccharides (FOS) to unsterilised or heat-sterilised fresh chicken excreta. Experimental substrates were unsterilised excreta (E), sterilised excreta (sE) with either starch (E + star, sE + star) or FOS (E + FOS, sE + FOS). The highest wet yield (9.7 g) and heaviest larvae (13.2 mg) were on sE + star, followed by E (7.2 g and 8.1 mg). Both E + FOS and sE + FOS had minimal yields and larval weights (0.3 and 0.2 g, 2.9 and 1.7 mg) with E + star intermediate (3.3 g and 7.5 mg). Survival differed between diets, sE and E averaged 70%, sE + star 57%, E + star 33% with 9% for E + FOS and sE + FOS. DM bioconversion decreased from E (3.5%), sE + star (3.1%), sE (2.6%), E + star (1%) to E + FOS and sE + FOS (0.1%). Nitrogen bioconversion was highest on sE + star (9.9%) and E (9.3%), lower on sE (6.6%), E + star (4%) and the lowest on E + FOS and sE + FOS (0.5%). Different substrate temperature profiles during the larval growth period and different larval size distributions were found for the different substrates. The results support the hypothesis that larvae compete with microorganisms for an easily digestible carbohydrate (starch).