Papaya leaf insecticide contains papain, a proteolytic enzyme that cleaves proteins into smaller compounds such as amino acids. Papain, when sprayed on a plant, is a deterrent against caterpillar infestation and their consumption of leaves. However, scientists don"t yet understand the precise mechanism by which papain works. Research conducted in Japan by organizations such as the National Institute of Agrobiological Sciences suggests that papain is toxic to many herbivorous insects.