Plants recognize herbivore-associated molecular patterns (HAMPs) during herbivory that activates signaling to induce immune defenses. Caterpillar oral secretions contain Inceptin 11 (In11) which is a HAMP recognized by legumes such cowpea via Inceptin Receptor (INR). Thus, In11 and INR are a model system to study proteins involved in HAMP induced defenses, including Kunitz trypsin inhibitors (KTIs). It is known that KTIs are serine protease inhibitors with anti-herbivore activity; however, the precise role of In11 induced KTIs and the effect of cysteine content variation in cowpea KTIs remains unknown. Here, we show that selective removal of cysteines has a negative effect on KTI function in cowpea experiencing herbivory from the fall armyworm (Spodoptera frugiperda). We found that cowpea KTIs act as antiherbivore proteins against the fall armyworm when expressed in Nicotiana benthamiana, as we saw reduced weight gain on larvae feeding on leaves expressing wildtype KTI. Furthermore, we found that KTI function was negatively affected by the removal of cysteines, and larvae fed leaves expressing any of the mutant gained more weight than those feeding on wildtype. We hypothesize that these findings are due to reduced protein stability because we did not detect mutant KTIs in frass samples by westernblot. Understanding KTI protein structure and how it influences protein function is important for designing and selecting antiherbivore proteins to be used for plant defense in agriculture.