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

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.

Automated summary

This study investigates the potential application of frass, the residual excrement of mealworm larvae, as an organic fertilizer for crops. When combined with a biostimulant based on efficient microorganisms, frass can be mineralized effectively due to its high organic matter content. The experiment conducted using lettuce plants under greenhouse conditions shows that frass can be a beneficial organic fertilizer, providing macronutrients and promoting lettuce aerial biomass. However, the application rate of frass needs to be carefully controlled to avoid potential negative effects on plant growth, especially root development, which may be caused by an increase in potentially pathogenic fungi. The addition of a PGP-based biostimulant counteracts the negative effects and enhances nutrient uptake, biomass, and quality of lettuce, while also promoting beneficial microbial diversity and reducing potentially pathogenic fungi. The positive synergy between frass and the PGP-based biostimulant has significant implications for the development of new organic fertilization strategies.