A preference experiment was set up with two planthopper species (Hemiptera: Delphacidae) to test the influence of competition on host plant choice. The delphacid Javesella pellucida was chosen as a generalist and the rarer Ribautodelphax imitans as a monophagous specialist, which feeds on the grass, tall fescue Schedonorus arundinaceus . In the absence of the specialist, the generalist showed a marked preference for tall fescue. In some experiments, however, the introduction of the specialist resulted in a shift of preference to an alternative plant if the specialist was established prior to the introduction of the generalist. This experiment supports the hypothesis that specialist herbivores can potentially alter the host plant choices of generalists, which may lead to differing host plant use patterns in insect communities.