Duration of immune priming and changes in immune parameters following primary and secondary immune challenges in Tenebrio molitor

Abstract The yellow mealworm ( Tenebrio molitor ) is a key species in industrial insect farming due to its high nutritional value, low environmental footprint, and efficient feed conversion. However, intensive high-density rearing increases the risk of pathogen transmission, highlighting the need for sustainable disease management strategies. Enhancing natural immune defences, such as immune priming, may reduce reliance on antibiotics, but the duration, reactivation potential, and physiological costs of such responses remain poorly understood. This study investigated long-term immune priming in T. molitor females by assessing immune parameters 20 and 50 days after a primary challenge with heat-killed Bacillus cereus , with or without a secondary challenge. We measured changes in haemocyte composition, prophenoloxidase (proPO) system activity, and antibacterial activity in the haemolymph, while also evaluating female fecundity to assess potential trade-offs. Our results show that immune priming induced long-lasting changes in immune function, notably enhanced plasmatocyte counts and PO activity, but only in females that received a secondary challenge 50 days after priming. This suggests a memory-like recall response. Antibacterial activity was sustained for at least 50 days post-priming, independent of secondary challenge, reflecting durable inducible humoral immunity. However, this heightened immune activity was negatively correlated with fecundity, indicating a trade-off between immune investment and reproductive output. These findings demonstrate that T. molitor can mount robust, and long-lasting immune responses after bacterial challenges, but at a reproductive cost. While immune priming holds promise for improving disease resistance in mealworm farming, its application must consider impacts on productivity.