Food-induced changes in the phenoloxidase activity and development of Agelastica alni (L.,1758) (Coleoptera: Chrysomelidae) larvae reared on unbalanced artificial diets were examined. The study was conducted between 2015-2016. Food quality had an impact on the phenoloxidase activity and growth performance of alder leaf beetle. The maximum pupal mass was recorded in the 0.5:1 P:C (protein:carbohydrate) diet and the minimum pupal mass was recorded from the larvae fed on the 3:1 P:C diet. The amount of carbohydrate consumed affected the pupal mass positively, whereas the amount of protein consumed negatively affected the pupal mass. The highest amount of pupal lipid was found in the 0.5:1 P:C diet and the lowest pupal lipid amount in the 1:3 P:C diet. Also, imbalance diets affected phenoloxidase activity. There was a positive relationship between P ratio of the diet and phenoloxidase activity. Phenoloxidase activity decreases as the amount of carbohydrate consumed by larvae increases. As a result, unbalanced diet affects the immune system of larvae. Carbohydrate also has a significant effect on immune defenses as much as protein. In addition, larvae increase their body size with excessive consumption of carbohydrate.