Heat treatment of black soldier fly frass for soil amendment enhances Brassica rapa growth

Abstract Insect frass, the residual product of mass-rearing insects for food and feed can be used as a soil amendment to fertilise the soil for crop growth. Heating frass before applying it to the field could be beneficial to enhance microbial safety and is required to align with EU regulatory standards before market placement as organic fertiliser. This study aimed to assess the effect of heat treatment of black soldier fly (BSF) Hermetia illucens L. frass as a soil amendment, on crop growth. The frass was heat-treated at 70 °C for one hour in an oven, after which it was cooled, milled, and used as a soil amendment. Seeds of Brassica rapa were sown in soil amended with both heat-treated and untreated frass sourced from two different commercial BSF producers and applied at two doses. Control plants were grown in unamended soil. The experiment assessed the impact on seed germination, leaf area growth, shoot biomass production, and leaf number. Our results show that heat-treated frass significantly enhanced plant leaf area and boosted shoot biomass, outperforming non-heated frass. Specifically, heat-treatment increased plant leaf area by up to 24% and shoot biomass by up to 41%. The impact of BSF frass on plant growth did not differ between producers. Seed germination percentages remained unaffected by frass treatment, and were similar to those observed in unamended controls, ranging from 88% to 100%. This study demonstrates that thermal processing of BSF frass at 70 °C substantially improves vegetative growth in B. rapa without affecting seed germination, reinforcing its value as a sustainable and effective soil amendment. Integrating heat-treated frass into soil management aligns with European standards and global sustainability goals, offering a validated approach to boost crop productivity without synthetic inputs. Thus, this practice adheres to regulatory standards contributing to sustainable agriculture by linking insect farming with the production of crops.