Hydrilla verticillata (L. f.) Royle

Major Threats

Freshwater anglers recognize the role of hydrilla beds as fish attractors and regularly fish over beds and at their margins. In particular, dense hydrilla beds are common aggregation sites for chain pickerel (Esox niger), whose ambush style of predation benefits from the presence of vegetative cover. Experimental research has indicated that the presence of dense vegetation in general may result in a shift in foraging strategy in fish species, away from a pursuit predator strategy in favor of ambush predation that takes advantage of vegetative cover (Crowder and Cooper 1982, Savino and Stein 1982).In Florida, Hydrilla verticillata often occurs in mixed beds with another non-native aquatic weed Egeria densa. The two plants are very similar in appearance.Invasion History: The broad native range of Hydrilla verticillata is believed to include parts of Asia and India, Australia, and possibly Africa, and the plant has been subsequently introduced occurs on every continent except Antarctica (Pieterse 1981, Cook and Luond 1982, ISSG).The first introduction of the species in North America was through a Florida west coast aquarium dealer in the early 1950s who shipped live H. verticillata from Sri Lanka (dioecious, exclusively female strain) for the aquarium trade under the common name Indian star-vine (Note that more recent genetic studies, e.g., Madeira et al. 2004, suggest the original material may have come from Bangalore, India.). The plants were deemed unsatisfactory and were disposed of into a canal near Tampa Bay where they survived and thrived (McCann et al. 1996). By 1955, samples from this introduced Tampa population had been transported to Miami for cultivation and pet trade sale. Subsequent undocumented accidental/careless releases no doubt followed, as evidenced by the extensive spread of the Sri Lanka biotype throughout Florida and elsewhere in the southeastern U.S.Introduction of the monoecious (Korean) strain of H. verticillata to the eastern seaboard occurred perhaps two decades after the initial Florida introduction. This hydrilla biotype was first reported from Delaware in 1976, and from the Potomac River around 1980 (Madeira et al. 2000).Considering both biotypes together, Hydrilla is now present throughout the southeast, on the east coast from Florida north to Massachusetts, west into Texas, and in Arizona and California as well (Pieterse 1981, Cook and Luond 1982, Langeland 1996). Potential to Compete With Natives: Hydrilla verticillata is an aggressive invader that has been shown capable of displacing native submersed plant communities (Haller and Sutton 1975, Bowes et al. 1977). Dense beds of hydrilla alter the community structure at multiple levels. Water chemistry is altered, zooplankton populations decline, and fish population and community structures are altered as well (Colle and Shireman 1980, Canfield et al. 1983, Schmitz and Osborne 1984). Possible Economic Consequences of Invasion: Hydrilla has been recognized as one of the most invasive weeds in the world and infestations are capable of choking waterways and public water supplies (Illinois-Indiana Sea Grant undated). It is listed as a Category I invasive exotic plant in Florida, indicating that the species is currently altering native plant communities by displacing native species and changing community structures or ecological functions.Worldwide economic impacts of Hydrilla verticillata include impacts relating to infestation of rice fields, irrigation canals, fishponds and public waterways (Cook and Luond 1982). Oxygen depletion is a potentially serious consequence of decomposition of large amounts of hydrilla plant biomass in infested lakes (Engel 1995).Hydrilla control and management is expensive. The state of Florida spent approximately 14.5 million dollars on H. verticillata control in 1994-1995. The economic cost of lost recreational dollars is also considerable. Recreational activies worth $11 million were lost just in Orange Lake (Marion County) in those years when hydrilla infestations entirely choked the lake. (Langeland 1996).

South & East Europe, Africa, South & East Asia to Australia.

Conservation Actions