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 PDI (Potato Cathepsin D Inhibitor), a Kunitz-type wound-inducible protein from potato leaves. This suggests that LdCD is a target for PDI 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. PDI was purified from potato and its crystal structure was determined at 2.1 Å resolution. We propose the interaction mechanism of LdCD with PDI based on a docking model. Crystallization of LdCD-PDI complex is currently in progress aimed to describe a new mechanism for natural inhibition of aspartic proteases.