# Could Insect Frass Be Used as a New Organic Fertilizer in Agriculture? Nutritional Composition, Nature of Organic Matter, Ecotoxicity, and Phytotoxicity of Insect Excrement Compared to Eisenia fetida Vermicompost

## TL;DR

Insect frass can be a valuable organic fertilizer, but its effectiveness and safety depend on the insect species and proper blending with vermicompost.

## Contribution

This study is the first to systematically compare frass from four insect species and evaluate its potential as a fertilizer, highlighting species-specific differences and optimal usage strategies.

## Key findings

- Frass from mealworms and crickets is highly concentrated and can be phytotoxic unless diluted.
- Frass provides rapid nutrient availability, while vermicompost improves long-term soil health.
- Combining frass with vermicompost mitigates phytotoxicity and maximizes fertilizer benefits.

## Abstract

Large amounts of “frass” (insect excreta mixed with shed skins and feed residues) are generated by insect farming and this might be used as an amendment. We compared frass from four insects (mealworms, waxworms, black soldier flies, and crickets) and compared them with worm compost (vermicompost). Using a combination of chemical and spectroscopic analyses together with thermal tests and seed-based bioassays, we identified clear, species-specific differences that are important for field use. Frass from mealworms and crickets was so concentrated that it could harm plants unless it was diluted or blended, whereas frass from the other species was much gentler. Our research shows that frass acts as a rapid-action fertilizer, providing a quick nutrient boost, whereas vermicompost is better for building long-term soil health. The most effective approach is to combine the two; a small amount of frass provides plants with an immediate nutrient boost, while vermicompost creates healthy soil and protects the plants, effectively transforming insect waste into a safe and valuable organic fertilizer.

The expanding insect farming industry generates up to 67,000 tons of frass per year. Its potential use as fertilizer is promising, but has not yet been widely studied. This study aimed to characterize the chemical composition, organic matter structure, ecotoxicity, and phytotoxicity of frass from four insect species in order to evaluate its potential as a fertilizer. We compared four types of insect frass (IF) (Tenebrio molitor, Galleria mellonella, Hermetia illucens, and Acheta domesticus) to Eisenia fetida vermicompost (EFV). We used physicochemical analyses (pH, electrical conductivity (EC), macro-micronutrients and dissolved organic carbon (DOC), spectroscopy (solid-state 13C nuclear magnetic resonance (NMR), and Fourier-transform infrared spectroscopy (FTIR)) and thermogravimetry/differential scanning calorimetry (TGA/DSC: R1, R2, Tmax), together with phytotoxicity (germination index, %GI) and ecotoxicity (toxicity units, TU) bioassays. Composition was species-dependent: A. domesticus showed the highest levels of nitrogen (N), phosphorus (P), and potassium (K); the concentration of DOC was higher in insect frass (IF) than in EFV, with the highest concentration found in IF of T. molitor. 13C NMR/FTIR profiles distinguished between frass (carbohydrates/proteins and chitin signals) and EFV (humified, oxidized matrix). Thermal stability followed: G. mellonella (R1 ≈ 0.88) ≥ A. domesticus (0.79) > H. illucens (0.73) > EFV (0.67) > T. molitor (0.50). In bioassays, T. molitor and A. domesticus exhibited phytotoxicity (%GI < 30), whereas G. mellonella and H. illucens did not. EFV exhibited the highest %GI. Dilution increased %GI in all materials, especially in T. molitor and A. domesticus, and reduced acute risk (TU). Frass is not a uniform input: its agronomic performance emerges from the interaction between EC (ionic stress), the availability of labile C (DOC, C/N and low-temperature exotherms), and structural stability (R1/R2 and aromaticity). In terms of formulation, IF can provide nutrients that mineralize rapidly, whereas EFV contributes stability. Controlling the inclusion and dilution of materials (e.g., limiting the amount of T. molitor in blends) and considering the mixing matrix helps to manage phytotoxicity and ecotoxicity, and realize the fertilizer value of the product.