Insecticidal action of the phytohemagglutinin in black beans on a bruchid beetle.

Black bean (Phaseolus vulgaris) phytohemagglutinin added to the normal diet of a bruchid beetle (Callosobruchus maculatus) that can eat phytohemagglutinin-free cowpeas (Vigna unguiculata) but not P. vulgaris seeds kills the bruchid larvae. Trypsin inhibitors, found in both P. vulgaris and V. unguiculata seeds, have virtually no effect on the bruchid larvae. The conclusion is that a major part of the adaptive significance of phytohemagglutinins in black bean and other legume seeds is to protect them from attack by insect seed predators.