Evaluation of the Effect of Tenebrio molitor Frass on the Growth Parameters of Canasta Lettuce ( Lactuca sativa var. capitata ) as a Model Plant

The European Commission approval of some insect species for human consumption, starting with Tenebrio molitor (TM) in 2021, has drawn attention to the production of insect-derived protein flours and the sustainability of insect-rearing systems, particularly on a large scale. This has also highlighted the importance of utilizing byproducts, such as frass, and obtaining high-value-added products, such as biofertilizers. This study explored the potential for TM frass (TMF) to serve as a natural fertilizer for the cultivation of Canasta lettuce ( Lactuca sativa var. capitata ). Specifically, a series of tests was carried out to assess the efficacy of thermal treatment and to verify the trend of certain chemical and growth parameters as a function of the TMF percentage to be added to the potting soil. For this purpose, different percentages of both thermal-treated and untreated TMF and their effects on various growth parameters of Canasta lettuce were evaluated through pot trials. Furthermore, TMF was characterized by using scanning electron microscopy (SEM) to gain insights into its structural features and potential influence on soil–plant interactions. Our results show that heat treatment of TMF is essential to ensure plant survival, and at least in pots, TMF percentages above 5% of soil dry weight are not recommended. In our tests, the most suitable percentage was 4%.