Hydrilla verticillata subsp. lithuanica: discovery and establishment outside of the Connecticut River

Hydrilla [Hydrilla verticillata (L.f.) Royle], an invasive aquatic weed, has had a rich introduction history into the United States with multiple subspecies being introduced since the 1960’s. The most recent occurred prior to 2016, when northern hydrilla (Hydrilla verticillata subsp. lithuanica) was discovered in the Connecticut River. By 2021, following a three-year survey from Agawam, MA to the Long Island Sound by the Connecticut Agricultural Experiment Station Office of Aquatic Invasive Species, H. verticillata subsp. lithuanica was found in over 113 km of the river, occupying 344 hectares. Since this survey, there has been concern that H. verticillata subsp. lithuanica would spread to nearby waterbodies and have a significant negative impact. Here we report the first documented spread and establishment of H. verticillata subsp. lithuanica from the Connecticut River to five waterbodies in Connecticut and one in Massachusetts. Of the total eight sites where H. verticillata observations were made, 75% (n=6) were confirmed to be H. verticillata subsp. lithuanica and 25% (n=2) to be wandering hydrilla (Hydrilla. verticillata subsp. peregrina). Except for one site, all six locations infested with H. verticillata subsp. lithuanica provide watercraft access through public or private boat ramps. The authors also postulate on the mechanisms facilitating the spread and establishment of this subspecies.