Effects of Thermal Sanitization on Insect Frass Nutrient Composition and Biofertilizer Performance in a Lettuce Pot Trial

Insect farming has rapidly expanded in Europe following regulatory approval of insect-derived proteins in aquaculture feed and increasing interest in the valorization of insect by-products. Insect frass, consisting of excreta and exuviae, is a nutrient-rich material with beneficial microorganisms and potential as a sustainable alternative to conventional fertilizers, although its composition varies with insect species and feedstock. EU legislation requires thermal sanitization prior to market release, yet the effects of the thermal treatment on frass nutrient composition and biofertilizer performance remain poorly understood. Insect frass from black soldier flies (BSFFs) fed on a diet based on dairy industry byproducts was sanitized and mixed with sandy soil and used in two lettuce pot trials under greenhouse conditions. The aim of our study was to determine the effects of thermal sanitization on (1) macro- and micronutrient contents and dynamics (plant N and P uptake); and (2) biofertilizer potential, including plant physiology (chlorophyll, anthocyanins, flavonols, Fv/Fm), plant growth (biomass), and soil microbial activity (dehydrogenase and β-glucosaminidase). BSFF showed a clear potential to induce growth of lettuce plants by increasing chlorophyll content, biomass and microbial activity. Furthermore, the sanitization process did not significantly alter the measured agronomic performance of frass under the tested conditions or reduce its benefits on biomass growth, chlorophyll content, microbial enzyme activity and on nutrient uptake by the lettuce plants. These findings suggest that the mandatory sanitization does not compromise its agronomic functionality, supporting its strong potential within circular agricultural systems under the tested conditions. However, the results are valid under greenhouse conditions and for the specific frass, soil and crop combinations; field validation is needed to confirm these results under large-scale high-value crop production conditions.