Water chestnut (Trapa natans) is an annual floating aquatic plant native to Asia but invasive throughout northeastern North America. Information about the range of T. natans in Asia suggests that the plant has not reached its potential range in North America. We studied whether herbivory by the water lily leaf beetle, Galerucella nymphaeae, which has adopted T. natans as a new host in North America, can contribute to reductions in plant performance. We released 0, 10, 20, and 50 G. nymphaeae first-instar larvae on developing T. natans rosettes and measured larval survival, larval development time, plant growth, plant biomass, and reproductive output. The larval densities we chose in our experiment represent the range of densities we frequently observe in the field, with the highest density exceeding field attack rates. Larval mortality was particularly high for first instars (50% within 1 wk) and not associated with treatment or lack of food. Overall, larval survival was extremely low, with survival rates of 5.1% in the highest (50 larvae) to 11.3% in the lowest (10 larvae) herbivore treatment after 4 wk. We found no significant differences among plants exposed to different herbivore treatments for the number of leaves, rosette diameter, biomass, or reproductive output. In central New York, it seems that G. nymphaeae has no impact on water chestnut performance at the herbivore levels we see in the field and chose in our experiments. Water chestnut is a poor (but abundant) food plant for G. nymphaeae in the northeastern United States, but it is unlikely that the leaf beetle can contribute to local or regional control of water chestnut or reduce its spread.