Impact of the Native Water Lily Leaf Beetle,Galerucella nymphaeae(Coleoptera: Chrysomelidae), Attacking Introduced Water Chestnut,Trapa natans, in the Northeastern United States

Abstract

Water chestnut (Trapa natans) is an annual soating 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 50G. nymphaeae Þrst-instar larvae on developingT.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 Þeld, with the highest density exceeding Þeld attack rates. Larval mortality was particularly high for Þrst 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 signiÞcant 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 thatG. nymphaeae has no impact on water chestnut performance at the herbivore levels we see in the Þeld and chose in our experiments. Water chestnut is a poor (but abundant) food plant forG.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.