Taxonomic name: Pistia stratiotes L.
Common names: laitue d'eau (French), Lechuguilla de agua, lechuguita de agua (Spanish), pistie (French), repollo de agua (Spanish), salade d’eau (French-Burkina Faso), tropical duckweed (English), water lettuce (English)
Organism type: aquatic plant
Pistia stratiotes is a freshwater invasive weed that is found throughout the tropics and subtropics. It is a free-floating plant that is capable of forming dense mats on the surfaces of lakes, ponds, rivers and other bodies of water. Pistia stratiotes is a popular garden pond plant and is often spread by the dumping of aquarium or ornamental pond plants. Fragments, or whole plants, can be spread via boats or fishing equipment from an infested area to a clean body of water.
Glazier (1996) describes P. stratiotes as a free-floating perennial of quiet ponds. It is stoloniferous, forms colonies, and has rosettes up to 15cms across. It has long, feathery, hanging roots. Its leaves are obovate to spathulate-oblong, truncate to emarginate at the apex, and long-cuneate at the base. Leaves are light green and velvety-hairy with many prominent longitudinal veins. Inflorescences are inconspicuous and up to 1.5cms long. Flowers are few, unisexual, and enclosed in a leaflike spathe.
lakes, water courses, wetlands
Rivers (2002) notes that for P. stratiotes to survive, it requires a wet, temperate habitat. It is usually found in lakes and rivers, however, it can survive in mud. P. stratiotes can endure temperature extremes of 15° C (59° F) and 35° C (95°). The optimal growth temperature range for the plant is 22-30° C (72-86° F). P. stratiotes prefers slightly acidic waters (6.5 - 7.2 pH) and moderate hardness (5 - 20 KH).
According to Rivers (2002), P. stratiotes can inflict a severe impact on the environment and economy of infested areas. The dense mats created by connected rosettes of the plant lead to the majority of problems encountered with water lettuce. These mats can have a negative economic effect by blocking waterways, thus increasing the difficulty of navigation and hindering flood control efforts. Mats of P. stratiotes can also disrupt natural ecosystems. They can lead to a lower concentration of oxygen in covered waters and sediments by blocking air-water interface and root respiration. Extremely thick mats of P. stratiotes can prevent sunlight from reaching underlying water. The cumulative effect of these negative characteristics of the plant is a loss of biodiversity in invaded habitats. P. stratiotes mats can also serve as a breeding place for mosquitoes.
According to Rivers (2002), P. stratiotes is a popular ornamental plant, used in ponds and aquariums.
Native range: Pistia stratiotes is probably native to South America (Rivers, 2002).
Known introduced range: It is now found throughout the tropics and subtropics (Glazier, 1996).
Introduction pathways to new locations
Floating vegetation/debris: P. stratiotes is a free floating plant that is capable of forming dense mats on the surfaces of lakes, ponds, rivers and other bodies of water (Rivers, 2002).
Internet sales/postal services: According to Ramey (2001), P. Stratiotes continues to be sold through aquarium supply dealers and through the internet.
Pet/aquarium trade: According to Ramey (2001), P. Stratiotes continues to be sold through aquarium supply dealers and through the internet. Rivers (2002) cites that dumping of aquarium or ornamental pond plants is often the means of spread for P. stratiotes.
Ship/boat hull fouling: P. stratiotes can spread from broken-off pieces or whole plants being moved on boats or fishing equipment from an infested to a clean body of water (Rivers, 2002).
Local dispersal methods
Boat: P. stratiotes can spread from broken-off pieces or whole plants being moved on boats or fishing equipment from an infested to a clean body of water (Rivers, 2002).
Garden escape/garden waste: The popularity of P. stratiotes as a garden plant has also led to its spread (Rivers, 2002).
Water currents: Once establihed within a waterbody P. stratiotes can be spread by watercurrents and floods
Preventative measures: A Risk assessment of
Pistia stratiotes for Australia was prepared by Pacific Island Ecosystems at Risk (PIER)
using the Australian risk assessment system (Pheloung, 1995). The result is a score of 18 and a recommendation of: reject the plant for import (Australia) or
species likely to be a pest (Pacific).
Physical: The most common physical control method is raking or seining it (using a large fishing net) from the pond’s surface. In the United States, raking is done by mechanical harvesters. The plant is then removed from waterways to the shore where it is cut up by chopping machines and disposed of by spraying across the water (Ramey, 2001).
Chemical: Chemical control methods that have been successful in treating P. stratiotes include the herbicide endothall, which can act quickly and kill all plant cells that it contacts.
Biological: According to Rivers (2002), water lettuce leaf weevil (Neohydronomus affinis) is a native species of South America and was first introduced into Australia in the early 1980's for biocontrol of P. stratiotes. Additional releases of this weevil for research are currently being conducted. These weevils have a very short life cycle (approximately 30 days), which allows for quick establishment of populations. Adult weevils feed on the leaf, while the larvae attack the inside of the leaf. The other effective method of controlling P. stratiotes is the introduction of the water lettuce leaf moth (Spodoptera pectinicornis). The moth is native to Thailand and was imported into Florida for the biological control of water lettuce. The moth has a very short life cycle (approximately 35 days), with the larval stage lasting 17-20 days. The adult moth does not feed on water lettuce, however, moth larvae are capable of inflicting significant damage to P. stratiotes. The larvae are fairly large, which means that fewer larvae can provide a greater effect.
Rivers (2002) states that P. stratiotes reproduces vegetatively and by seed. Vegetative reproduction involves daughter vegetative offshoots of mother plants on short, brittle stolons. Rapid vegetative reproduction allows water lettuce to cover an entire lake, from shore to shore, with a dense mat of connected rosettes in a short period of time.
Reviewed by: Dr John Clayton NIWA, National Institute of Water and Atmospheric Research. Hamilton, New Zealand
Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Last Modified: Friday, 30 December 2005