The influence of Ischnodemus variegatus feeding on photosynthesis and growth of the invasive semi-aquatic grass, Hymenachne amplexicaulis, was investigated in field and greenhouse environments. In the field, carbon dioxide assimilation of infested plants was approximately 35% less than that of non-infested plants, and the rate of assimilation was related to I. variegatus density. The relative growth rate of infested plants in the greenhouse was 77% of that of non-infested plants, and biomass of infested plants was significantly less than for non-infested plants 79 days after infestation. The value of I. variegatus as a fortuitous biological control agent of H. amplexicaulis is discussed.