Hermetia illucens Frass Fertilization: A Novel Approach for Enhancing Lettuce Resilience and Photosynthetic Efficiency under Drought Stress Conditions

Agriculture is faced with the need to reduce mineral fertilizers in order to reduce costs but also to meet political goals. Resilience-enhancing climate change, especially in the face of increasingly frequent and prolonged droughts, has become another issue. The dynamically increasing production of insects for feed and food purposes has become one of the answers to this challenge. This study assesses the fertilizing efficacy effect of frass derived from Black Soldier Fly (Hermetia illucens) production on lettuce (Lactuca L.) growth, including aspects such as yield, photosynthesis activity, photosystem II performance (chlorophyll fluorescence), mineral profile, and antioxidant properties. Additionally, the properties of the soil were assessed by measuring the gas exchange between the soil and the atmosphere. The lettuce plants grew under two water regimes—optimal irrigation and induced drought. The efficiency of frass fertilization was compared with the control and traditional cattle manure. The results indicate that H. illucens frass (HI frass) used as a fertilizer increased the content of essential nutrients in plants—such as potassium and iron. As the dosage of frass increased, the content of vitamin B2 (riboflavin) doubled. The plants that were subjected to drought and properly fertilized showed greater resistance; therefore, a reduction in the synthesis of polyphenolic compounds was observed. Fertilizer had a positive effect on the efficiency of photosynthesis. This study underscores the promising impact of unconventional organic fertilizers, such as H. illucens frass, on enhancing plant performance, especially in challenging environmental conditions. Fertilizers obtained from insect production can be green chemicals in a sustainable food production model.