Inhibits trypsin but not chymotrypsin. Inhibits the trypsin-like proteinase activity present in larvae of the crop pests Adoxophyes orana, Hyphantria cunea, Lobesia botrana and Ostrinia nubilalis. Forms a monomer at protein concentrations of below 1 mM. At concentrations of above 2 mM, self-associates. Belongs to the Bowman-Birk serine protease inhibitor family.