Mealworm Frass as a Potential Organic Fertilizer in Synergy with PGP-Based Biostimulant for Lettuce Plants

Abstract

This study explores the potential use of frass, the larval excrement residue obtained from mealworm rearing, as organic fertilizer for crops. Its high organic matter content means that its joint application with a biostimulant based on efficient microorganisms, favoring its mineralization, is of interest. An experiment with lettuce plants (Lactuca sativa L.) was conducted with two factors and six replicates under greenhouse conditions. The first factor was frass amendment at 0%, 1%, 2.5%, and 5% of the peat substrate, and the second factor was a Bacillus-based BS at two levels, with and without efficient microorganism application. The results reveal that frass shows great potential as an organic fertilizer, providing macronutrients and increasing lettuce aerial biomass, although its effect is mediated by the application rate. Rates of 2.5% or higher proved negative for lettuce plant growth, especially root development, probably due to an increased incidence of potentially pathogenic fungi. The negative effect of medium–high frass rates was counteracted by the addition of a PGP-based biostimulant, enhancing lettuce plant nutrient uptake, aerial biomass, and quality in terms of succulence, but also favoring microbial diversity in the rhizosphere, increasing the incidence of beneficial microorganisms, and decreasing potentially pathogenic fungi. This positive synergy observed between frass and the PGP-based biostimulant is of interest for the design of new organic fertilization strategies.