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

## TL;DR

Insect manure (frass) can improve soil and plant health, offering a sustainable alternative to traditional fertilizers.

## Contribution

Frass enhances plant nutrient content and pest resistance while using less input than poultry litter.

## Key findings

- High frass rates increased phosphorus in soybean grains and potassium in switchgrass.
- Frass reduced soybean leaf damage by 35–48% compared to controls and poultry litter.
- Low frass rates achieved similar yields with 2–4 times higher nutrient use efficiency.

## Abstract

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.

Insect manure “frass” is a novel organic fertilizer that can close nutrient loops through waste nutrient recovery.Poultry litter had up to 12 times higher heavy metals and potentially toxic elements than frass.High frass (11.2 Mg ha−1) increased P in soybean grains and K in switchgrass biomass compared to the control.High frass reduced soybean leaf damage by 38%–45% relative to the control and poultry litter.Two to four times higher nutrient use efficiency and enhanced plant defense show frass as a multiuse organic amendment.

Insect manure “frass” is a novel organic fertilizer that can close nutrient loops through waste nutrient recovery.

Poultry litter had up to 12 times higher heavy metals and potentially toxic elements than frass.

High frass (11.2 Mg ha−1) increased P in soybean grains and K in switchgrass biomass compared to the control.

High frass reduced soybean leaf damage by 38%–45% relative to the control and poultry litter.

Two to four times higher nutrient use efficiency and enhanced plant defense show frass as a multiuse organic amendment.

As insect farming expands worldwide, insect manure—frass—production increases, and proper waste management is needed to support this growing industry and the upcycling of this enriched material. Scientists and industry stakeholders partnered in this research to test the frass of black soldier fly larvae, and understand how frass improves soils, plant growth, and grains and forage production compared to poultry litter, aiming to support frass utilization as an organic fertilizer. Frass not only increased essential nutrients in plants but also helped protect soybean leaves from pest insects, acting like a natural pesticide. Moreover, a lower frass application rate resulted in similar crop yields with much less nutrient input, showcasing frass efficiency as a nutrient source. These findings support frass use as a sustainable, eco‐friendly solution for farmers looking to improve their soils, crop production, and reduce reliance on traditional fertilizers.