Impact of invertebrate herbivory on native aquatic macrophytes

PURPOSE: This technical note quantifies the impact that invertebrate herbivores have on native aquatic plants by comparing dry biomass of five macrophyte species between two treatments; an insecticide treatment to remove invertebrate herbivores, and a control where the herbivore complex was left to develop naturally. BACKGROUND: There is little information available that quantifies the impact of invertebrate herbivores on native macrophyte biomass in the United States. Early research indicated that while macrophytes were useful as substrates for invertebrates and epiphytic growth, they provided little if any nutritive value (Shelford 1918). However, additional studies have shown the importance of macrophytes as a nutritive source for invertebrates. Soszka (1975) found that Potamogeton species can lose 50 to 90 percent of their leaf area through insect herbivory and non-consumptive destruction mostly from lepidopterans, trichopterans, and dipterans. Sand-Jensen and Madsen (1989) found leaf area damage to be between 2 and 56 percent for Potamogeton species, depending on locality. This damage was primarily attributed to trichopterans and dipterans. Lodge (1991) stated that macrophytes are engaged in aquatic food webs, sometimes to the extent that biomass, productivity, and relative species abundance are dramatically changed by grazers. Finally, Cronin et al. (1998