Economic and ecological values of frass fertiliser from black soldier fly agro-industrial waste processing

The sustainable utilisation of black soldier fly (BSF) for recycling organic waste into high-quality protein feed and organic fertiliser with a low environmental footprint is gaining momentum worldwide. Although BSF farming is becoming a rapidly growing agribusiness, studies on the BSF farming’s economic aspects are limited. This study analysed the economic benefits of farming BSF for animal feeds and composted frass, called frass fertiliser (FF) production using experimental data. The BSF larvae were fed on brewery spent grain amended with sawdust, biochar, and gypsum to determine the cost-effective feed and other by-products production. The agronomic performance of FF on the maize crop was assessed using field experiments. Our results demonstrated that sourcing and preparing the waste substrate for rearing the BSF larvae accounts for 81-90% of the total BSF production cost. The utilisation of FF as an additional value-added product would increase farmer’s net income by 5-15 folds compared to BSF farming alone. Feedstock amended with 20% biochar increased net income by 10-64% for BSF larvae and FF production than other feedstocks. Production of one megagram (Mg) of dried BSF larvae (USD 900) would generate 10-34 Mg of FF worth USD 3,000-10,200. Maize grown on plots treated with FF yielded 29-44% higher net income than maize harvested from plots amended with commercial organic fertiliser. Furthermore, smallholder insect farmers’ direct use of FF for maize production would generate 30-232% higher net income than farmers purchasing similar FF. Our results demonstrate for the first time the role of insect farming in circular economy and justify the opportunities for future investments that would lead to enhanced sustainability for agricultural and food systems, especially for smallholder farmers in low- and middle-income countries.