LdCD is a digestive cathepsin D-type protease of the Colorado potato beetle, Leptinotarsa decemlineata. Using two approaches, protease activity measurements and microscale thermophoresis binding analysis, we demonstrated that LdCD is inhibited by the interaction with PCDI (Potato Cathepsin D Inhibitor), a Kunitz-family wound-inducible protein from potato leaves. This suggests that LdCD is a target for PCDI acting as an antifeedant in plant defense against insect herbivory. Recombinant LdCD was produced in Pichia pastoris and its crystal structure was determined at 1.95 Å resolution. PCDI was purified from potato and its crystal structure was determined at 2.1 Å resolution. Based on a docking model between LdCD and PCDI, we describe a new structural mechanism for natural inhibition of aspartic proteases.