Frass fertilizers from mass-reared insects: species variation, heat treatment effects, and implications for soil application

Abstract

Insect farming has gained popularity as a resource-efficient and eco-friendly method of managing organic wastes by converting them into high-quality protein, fat, and frass. Insect frass is a powerful organic fertilizer, enriching the soil with essential plant nutrients and enhancing plant defense mechanisms through chitin stimulation. Given the importance of frass commercialization for many insect farmers and the use of increasingly diverse organic wastes as insect feedstock, the need for legal guidelines to enable clean production practices has emerged. The recent introduction of a legal definition for frass and heat treatment requirements by the EU commission marks a significant step towards standardizing its quality. However, frass composition is influenced by numerous factors, and little is known about the processes shaping its nutritional profiles and contributing to its maturation. Here, we analyzed the physicochemical, plant-nutritional, and microbiological properties of black soldier fly, yellow mealworm, and Jamaican field cricket frass from mass-rearing operations and assessed the impact of hygienizing heat treatment. Frass properties varied significantly across insect species, revealing concentrations of plant available nutrients reaching as high as 7000 μg NH4+, 150 μg NO2-NO3--N, and 20 mg P per g of total solids. Heat treatment affected microbial activity by reducing basal respiration and microbial biomass carbon, but also reducing viable counts of pathogenicE. coliandSalmonella sp.In terms of microbiome composition, alpha diversity showed no significant differences between fresh and heat-treated frass samples within each insect species, but significant distinctions were observed across the three insect species. The soil application of frass reactivated and boosted soil microbial activity, suggesting no long-term detrimental effects on microorganisms. These results further highlight the potential of insect frass as nutrient rich organic fertilizer, with promising benefits for soil health and nutrient cycling.