BIOCHEMICAL BASIS OF INSECT RESISTANCE IN WINGED BEAN (PSOPHOCARPUS-TETRAGONOLOBUS) SEEDS

Mature seeds of the winged bean (Psophocarpus tetragonolobus) are toxic to developing larvae of a range of cosmopolitan storage Bruchidae of economic importance, including the cowpea seed weevil, Callosobruchus maculatus. Insect feeding trails were carried out in which protein fractions from seeds of winged bean were incorporated at a range of concentrations into artificial seeds, and their effects upon development of C maculatus determined. Both albumin and globulin fractions were toxic to the developing larvae and their toxicity correlated with their haemagglutinating activity. Assay of Psophocarpin A, B and C fractions demonstrated Psophocarpin B to be the most insecticidal and to contain the highest haemagglutinating activity. Purified basic seed lectin was highly insecticidal to C maculatus larvae, with an LC50 value of c. 3.5 g kg-1. The physiological level of this protein in winged bean seeds is sufficient to account for their resistance to attack by C maculatus. Winged bean trypsin inhibitor was also purified and tested in artificial seeds against C maculatus. However, even at concentrations in excess of twice the physiological concentration it had no deleterious effects upon development.