Both the activity of phenoloxidases and the availability of phenoloxidase substrates are said to regulate melanotic encapsulation in invertebrates. The delayed induced resistance of mountain birches has been shown to increase the immunity of Epirrita autumnata (Geometridae: Lepidoptera), but the mechanism behind the increased immunity is unknown. The increase in the contents of aromatic amino acids as a response to herbivory in plants can in theory be one explanation for this carry-over effect. To test this hypothesis that an increase in the substrate availability increases also the melanotic encapsulation rate and the immunity of E. autumnata, we supplied excess levels of two aromatic amino acids, tyrosine (l-Tyr) and phenylalanine (l-Phe), to the diet of moth larvae. Surprisingly, the supply of both aromatic amino acids down-regulated PO activity, whereas the melanotic encapsulation rate was maintained at an unchanged level. PO activity was traded-off with body size and thus the cost of resistance is a probable explanation for the down-regulation of PO activity in the presence of surplus substrates. Our results suggest that both aromatic amino acids act as substrates of melanin, and an increase in substrate contents allows insects to decrease their PO activity without affecting the encapsulation rate.