Deterrency of Mexican Bean Beetle (Coleoptera: Coccinellidae) Feeding by Free Phenolic Acids

We investigated the role of simple phenolic acid precursors of isoflavonoid phytoalexins, as free compounds, in food selection by the Mexican bean beetle, Epilachna varivestis Mulsant, a pest of soybean, Glycine max Merrill, and other legumes. An aerosol spray method was developed to apply compounds in a uniform and reproducible manner to leaf surfaces of common bean, Phaseolus vulgaris L., a preferred host. Twenty-three compounds were tested in dual choice preference tests. Included were benzoic acid, cinnamic acid, simple phenolic acids (hydroxylated benzoic and cinnamic acids), and their methoxylated analogs. Six compounds were found to cause rejection of leaves. All of these were hydroxylated acids with the hydroxyl group adjacent to the carboxylic acid group on the benzene ring. Two of these six (gentisic acid and salicylic acid) are reportedly produced in soybean leaves.