Our field observations on submersed macrophytes in the littoral zone of Cayuga Lake, N.Y., USA indicate that the shift in dominance from Myriophyllum spicatum L. to Elodea canadensis Michx. may be explained by the high abundance of an aquatic lepidopteran larva, Acentria ephemerella Denis & Schiffermüller. Experimental evidence for the preference of Acentria for Myriophyllum over Elodea was obtained from small-scale, short-term laboratory experiments and from a tank experiment that represents a spatial and temporal scale intermediate between that of the small-container laboratory study and whole-lake observations. In laboratory no-choice feeding assays, Acentria exhibited higher feeding rates on Myriophyllum than on Elodea. In choice experiments, the larvae clearly preferred Myriophyllum over Elodea. Mesocosm tanks were set up with both Myriophyllum and Elodea planted in patches, and larval densities of 0, 75, 200 and 400 larvae m–2. After 3 weeks, biomass and shoot length of Myriophyllum was inversely correlated with larval density, but biomass of Elodea was unaffected. In this study, we explore whether a generalist macroinvertebrate herbivore such as Acentria, by preference for one macrophyte species over others, may affect the competitive interaction between two rooted plant species and subsequently may change the community composition within submersed macrophyte beds.