Establishment of barley (Hordeum vulgare L.) seedlings is affected by application of frass from Hermetia illucens

Climate change poses a threat to the establishment of winter cereals due to warmer temperatures and reduced precipitation in fall and winter. This can hinder germination by lowering soil water content or lead to faster initial growth that depletes residual water, causing subsequent water stress. Insect frass application has been proposed as a soil amendment with various effects on plant growth. This study tested the hypothesis that larval frass from Hermetia illucens, influenced by different diets and thermal treatments, affects barley (Hordeum vulgare L.) cv. Laureate establishment at 21 °C. A pot experiment compared a control silty loam soil with treatments amended by frass based on three factors: diet (Gainesville or whey-seed), thermal treatment (untreated or 70 °C for 1 h), and frass dose (5, 12, 19, 26 g/pot). Measurements included shoot biometry, chlorophyll content, root morphology, root/shoot ratios, and water use efficiency. Seedling establishment varied across treatments, with the Gainesville diet enhancing shoot and root growth more effectively. High frass doses negatively impacted growth. Specifically, total dry matter content decreased by 50% with 26 g of Gainesville diet frass compared to 5 g, and by 75% with whey-seed diet frass at the same doses. These results suggest that frass application may limit establishment at high temperatures and indicate that both chemical and physical properties of frass should be further investigated for their effects on plant growth.