Host plant variation plastically impacts different traits of the immune system of a phytophagous insect

1. Host plant quality affects herbivorous insect performance and consequently their susceptibility to natural enemies. Recently, it has been hypothesized that the immune function of herbivorous insects can be altered by their host plant, thus generating variation in their susceptibility to entomopathogens. Previous studies testing this hypothesis provided contradictory outcomes, mainly as a result of the differences in methodology such as measuring a single‐immune parameter rather than considering trade‐off‐mediated interactions between immune defence systems of the insect. Here, we hypothesized that plant‐mediated changes in insect immunity could result from the alteration of physiological immune effectors of the herbivore evident as trade‐offs. 2. Larvae of an inbred strain of the European grape berry moth Eupoecilia ambiguella were reared on five artificial diets each based on a different grape variety (Chardonnay, Chasselas, Gewurztraminer, Merlot, Riesling) and tested for changes in the baseline concentration of haemocytes, activities of the prophenoloxidase (PPO) system and of antimicrobial peptides of their haemolymph. Immune responsiveness of larvae across diets was also assessed by measuring changes in haemocyte concentration and activity of the PPO system after a bacterial immune challenge. 3. We found that variation among diets significantly affected immune defences of larvae. The alteration of the E. ambiguella immune system appears plastic and partly mediated by existing physiological trade‐offs between immune pathways, at least between induced antibacterial defences and the PPO system. 4. These results clearly show that host plant quality can affect immune defences and potentially disease resistance of E. ambiguella and that these changes in immunity may also result from intrinsic trade‐offs between immune defence systems in insects.