Effect of Substituent and Ring Changes in Naturally Occurring Naphthoquinones on the Feeding Response of Larvae of the Mexican Bean Beetle, Epilachna varivestis

Behavioral evaluation of the antifeedant effect of 10 naturally occurring 1,4-naphthoquinones on larvae of the Mexican bean beetle, Epilachna varivestis Mulsant, was undertaken concurrently with that of a series of synthetic analogs and model compounds in order to assess structure–activity relationships. Plumbagin, 1,4-naphthoquinone, juglone, menadione, and naphthazarin, which were found to be active at 0.3% concentrations, were also bioassayed at 0.1, 0.05, and 0.01% at which concentration 1,4-naphthoquinone still retained some activity. The model studies suggest that two structural features might be operative independently against E. varivestis: one consisting of a properly substituted naphthoquinone moiety and the other requiring a benzo- or naphthohydroquinone. Within the naphthoquinone group, the relative activity is determined by a substituent effect which is the outcome of a complex interplay of electronic, steric, electrochemical, and positional requirements. Among the model compounds, 2-chloro-3-amino-1,4-naphthoquinone and α-naphthylamine displayed appreciable activity even at 0.01% The results should enable selection of plant sources for naphthoquinones possessing larval inhibition properties.