Investigating safety aspects of using insect farming to reduce pig and chicken wastes at semi-commercial and lab-scale

Increasing global demand for food is driving the need to reduce wastes produced by agriculture to minimise environmental impacts. Black Soldier Fly, Hermetia illucens, can reduce livestock wastes, but research into the safety and scalability of the system is required. Insect bioconversion concerns include potential bioaccumulation of pathogens, antimicrobial resistance genes and heavy metals in larvae and substrates. Here, a semi-commercial-sized insect rearing facility was used to rear larvae on pig slurry, alongside a lab-based experiment using chicken manure. Larval microbiome composition was impacted by substrate, with increased Clostridia in larvae reared on slurry and manure. Pathogens largely decreased in the larvae from starting levels. Both slurry and manure substrates showed time-related changes regardless of insect presence or absence except for E. coli in chicken manure which was reduced in substrates with larvae added (-2.840 LFC vs -1.168 LFC; p<0.05), suggesting that time-associated alterations in the substrate could be more significant than larval presence. Antimicrobial resistance gene changes were dependent on the substrate and gene, with increases found for tetM in chicken manure after larval bioconversion (9.000 vs 10.370 LFC; p<0.001), and for sul2 in larvae reared on chicken manure (3.509 vs -0.985 LFC; p=0.001). In pig slurry-reared larvae, tetM decreased (-1.578 LFC; p<0.001) but there was no difference in sul2. Heavy metal contents generally met permissible standards for animal feed and organic fertilizers. However, there was some non-significant evidence for bioaccumulation of cadmium in slurry-reared larvae 0.18 to 0.70 mg/kg) compared to starter larvae (0.25mg/kg) requiring further study.