Recycling waste via insect agriculture: Frass impacts on soil and plant health

Frass-the main by-product of insect rearing for animal feed-is emerging as a promising soil amendment and plant growth promoter. However, basic agronomic information is lacking and prevents frass' widespread use as a biofertilizer. This study assessed impacts of black soldier fly (Hermetia illucens L. [Diptera: Stratiomyidae]) frass on soil fertility, crop growth, and quality compared to poultry litter (PL). Irrigated and non-irrigated soybean (Glycine max L. Merr.) and non-irrigated switchgrass (Panicum virgatum L.) plots received either PL (5.6 Mg ha^-1), low frass rate (LF; 5.6 Mg ha^-1), high frass rate (HF; 11.2 Mg ha^-1), besides the unamended control (CT). In general, soil nutrients and enzymes at the soil surface (0-15 cm) were unaffected by soil amendments. Irrigated-HF soybean had 7% higher grain P concentration than non-irrigated-HF, and 13% greater P concentration than the non-irrigated CT. Additionally, HF increased K concentration in switchgrass by 25% relative to the CT. HF reduced soybean leaf damage by 35% and 48% relative to the non-irrigated CT and PL-irrigated plots, illustrating for the first time frass' potential to enhance plant resistance to herbivory, likely owing to the presence of chitin. LF had 2-4 times greater nutrient use efficiency than HF and PL in organic soybean and switchgrass systems, reflective of similar yields despite lower nutrient inputs. These findings provide foundational knowledge for frass utilization as an organic fertilizer and biostimulant, closing nutrient loops through waste recovery during insect rearing, and supporting the development of an emerging sustainable industry.