Abundances of the specialist herbivore, Acalymma vittata (Fab.) (Coleoptera: Chrysomelidae), were assessed in small experimental plots with three levels of plant diversity (cucumber monoculture, cucumber/corn, and cucumber/tomato) and two levels of host plant growth form (horizontal on the ground and vertical, staked up or growing up other plant species). Host plant growth form more strongly affected beetle abundances than did plant diversity; greater numbers were found on vertically growing than on horizontally growing cucumber plants. The combination of cucumber monoculture and vertical growth form supported significantly greater herbivore abundances than did any other type of plot, emphasizing a strong interaction between diversity and growth form. Beetles were not more common in monocultures with horizontal growth forms than in mixed species plots, and beetles did not respond differently to plots with corn and plots with tomatoes.