Effect of Glyceollin, a Soybean Phytoalexin, on Feeding by Three Phytophagous Beetles (Coleoptera: Coccinellidae and Chrysomelidae): Dose versus Response

Abstract

Laboratory studies showed that the soybean phytoalexin glyceollin, an isoflavonoid previously investigated as an inhibitor of fungal pathogens, is an effective antifeedant for some insect species. Glyceollins extracted from soybean, Glycine max Merrill, cotyledons were applied to the surface of common bean leaves, Phaseolus vulgaris L., in five concentrations, including physiological concentration, for feeding preference tests. Leaves treated with glyceollin at concentrations below physiological levels were less acceptable to the southern corn root worm, Diabrotica undecimpunctata howardi Barber, and the Mexican bean beetle, Epilachna varivestis Mulsant, than untreated leaves. Feeding deterrence was positively correlated with increasing concentrations. The bean leaf beetle, Cerotoma trifurcata Forster, was not affected even by very high doses. An ethological concentration (EC 50) was computed based on the log dose–reduction of acceptance of treated disks. The EC 50 for the Mexican bean beetle was 6.1 μg/mg leaf dry weight and 3.5 μg/mg for the southern corn rootworm. Results indicate that soybean phytoalexins may represent a common defense against microorganisms and insect herbivores.