ROLE OF THE ISOFLAVONOID COUMESTROL IN THE CONSTITUTIVE ANTIXENOSIC PROPERTIES OF DAVIS SOYBEANS AGAINST AN OLIGOPHAGOUS INSECT, THE MEXICAN BEAN BEETLE

The antixenosic properties of the isoflavonoid, coumestrol, were tested in dual-choice leaf disk bioassays with the Mexican bean beetle (Epilachna varivestis Mulsant). E. varivestis preferred the methanol-treated (solvent-control) disk when the coumestrol concentration was 1.8 or 0.9-mu-g/leaf disk. No preference was observed between the coumestrol-treated and the solvent-control disks when the coumestrol concentration was higher, at 3.6, or lower, at 0.45-mu-g/leaf disk. Coumestrol alone clearly is not responsible for the significant constitutive antixenosic properties of "Davis" soybeans, Glycine max (L.) Merrill, because the amount of coumestrol in these plants is significantly less than the minimum concentration which was antixenosic in this study. However, it might contribute to a constitutive antixenosis in "Davis" involving a profile of allelochemicals. A computer-aided densitometer, adapted to measure the leaf disk area, increased the resolution of the leaf area 250 (X)-fold as compared to the standard LI-COR leaf area meter.