Pistia stratiotes
(water lettuce)

P. stratiotes is a perennial monocotyledonous aquatic plant present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways. It floats on the water surface, with roots hanging below floating leaves. Its growth habit can make it a weed in waterways, where it can kill native submerged plants and reduce biodiversity. It is a common aquatic weed in the USA, and may clog waterways in warmer states such as Florida. It is listed as a noxious weed or invasive aquatic plant in some states of the USA (USDA-NRCS, 2012).

According to Holm et al. (1977), Pistia is one of the most widely distributed hydrophytes. It has a cosmopolitan distribution throughout tropical and sub-tropical regions. In South and Central America, Africa and South-East Asia it is considered an endogenous species. While Blake (1954) and others indicate that it was introduced into Australia some 50 years ago (it was first observed during a survey in 1946/47), Gillet et al. (1988) present evidence for it being indigenous in the Northern Territory. Parsons and Cuthbertson (2001) reference a record in the Northern Territory from 1887, where a complement of organisms naturally regulates its population. Since P. stratiotes has not been recorded at nuisance levels in this area, it is highly possible that northern Australia is part of the plant’s native range.

The widespread distribution in most countries with a tropical climate may be the result of its ancient use as medicine for humans, as well as its use as fodder for cattle and pigs (Sculthorpe, 1971). The growth area of Pistia seems to be limited by low temperatures (see Small, 1933; Muenscher, 1967; Wiggins, 1980). However, scattered ephemeral populations have been reported in cold climates in the Netherlands, the Erie Canal in upstate New York and in Lake Erie in northern Ohio (Dray and Center, 2002). In these colder areas, the plant can act like an annual, re-infesting from seed each spring. Other ephemeral populations may rely on actual re-introductions. P. stratiotes is a popular aquarium plant that is often discarded in waterbodies during the summer season. In the summer of 1976 there was an excessive growth of Pistia in an area southeast of the Hague, Netherlands (see Pieterse et al., 1981). The plants died during the following winter season. In northern Africa, Pistia occurs in the Nile Delta in Egypt, but in this area, in contrast to many tropical regions, it is not a major aquatic weed (Täckholm, 1974).

Pistia occurs in most tropical and sub-tropical regions. Therefore emphasis should be placed on preventing the development of dense vegetation. However, public awareness campaigns could play a role in preventing dispersion by man, for example via market gardens or the aquarium trade. It is conceivable that in certain regions this could prevent Pistia spreading from infested waterbodies to Pistia-free areas.

P. stratiotes is a monoecious perennial which reproduces by means of vegetative offshoots that are connected to the mother plant by stolons, which may be 60 cm in length, as well as by the production of seeds. Holm et al. (1977) stated that flowering and fruiting vary by regions and that in most areas, with the exception of Africa, vegetative reproduction is the most important. On the other hand, Dray and Center (1989) concluded that seed germination is an important factor in the dynamics of Pistia populations in the USA. Flowering of Pistia has been induced in vitro under short days, as well as under conditions of continuous light, when the plants were cultured on nutrient media (Pieterse, 1978). Also when plants were grown in a greenhouse they flowered under both long- and short-day conditions and as a consequence it has been concluded that Pistia is a day-neutral plant (Pieterse, 1985). Seed germination was described by Datta and Biswas (1970), Pieterse et al. (1981) and Harley (1990). Datta and Biswas (1970) did not observe germination of seeds when placed on or in mud at the bottom of a beaker of water. These authors ascribed this to a low oxygen or high carbon dioxide concentration, or both, in the water. On the other hand, Pieterse et al. (1981) and Harley (1990) reported germination of Pistia seed when submerged. Holm et al. (1977) describe how seeds germinate on the surface of bottom mud and float to the surface within 5 days. Germination also occurred in the dark but period for germination was longer than under light, and the percent seed germination was lower in the dark than under constant or intermittent light. As a tropical plant Pistia does not survive freezing conditions. Though the seeds are able to survive in ice at -5°C for a few weeks, germination does not occur below 20°C. Seed dispersal has not been studied in detail. However, after shedding the seeds float for 1-2 days and are presumably transported by currents and water fowl, before they sink to the bottom of a waterbody.

Very few detailed studies have been conducted on the autecology of Pistia. Chadwick and Obeid (1966) reported that optimal growth of Pistia was obtained in water cultures at a pH of approximately 4. Such a high acidity, however, was never found in heavily infested waterbodies. It was shown by Pieterse et al. (1981) that the plant performs best in water with a pH of 7. Pistia showed particularly vigorous growth, although with a relatively small root system, in polluted water in Nigeria (Sharma, 1984).

In Lake Volta in Ghana it was observed that Pistia biomass varies in the course of the year which could be related to nutrient availability (Hall and Okali, 1974). In Florida there is a marked decline in biomass during the cold season (Dewald and Lounibos, 1990).

The Neotropics support 21 herbivorous species that exploit P. stratiotes; 5 are reported from Africa, 11 occur in Asia, and 9 insects feed on waterlettuce in Florida (Dray and Center, 2002). One of the most well studied enemies is the weevil Neohydronomus affinis. The original habitat of this promising biological control agent is South America. Neohydronomus spp. are specialist feeders on P. stratiotes throughout their natural distribution in Central and South America. Confusion over the identity of the Neohydronomus sp. introduced into Australia for biological control was clarified by O'Brien and Wibmer (1989) who recognised three species. There are a number of other P. stratiotes specialists that hold promise for use as agents of biological control. Larvae of the moth Argyractis drumalis have been found exclusively on P. stratiotes roots. The weevil Argentinorhynchus bruchi has been reported to feed, oviposit and complete larval development only on P. stratiotes. The noctuid moth Spodoptera pectinicornis was tested with 125 different plant species, but was found to complete development only on P. stratiotes and the weevil Bagous pistiae is known exclusively from P. stratiotes (see Dray and Center (2002) for detailed biocontrol information). Several other organisms cause damage to Pistia, but these generally are polyphagous.

A further range of organisms attacking P. stratiotes has been recorded from Northern Territory, Australia. These include Nymphula spp., Hydrozetes tobaicus, Nisia atrovenosa and Cercospora canescens (Gillett et al., 1988). See also Waterhouse (1993) for a listing of natural enemies of P. stratiotes.

Animal feed, fodder, forage

• Fodder/animal feed

Environmental

• Biological control
• Revegetation
• Wildlife habitat

Fuels

• Biofuels
• Miscellaneous fuels

General

• Botanical garden/zoo
• Ornamental
• Pet/aquarium trade
• Sociocultural value

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Management of Pistia can be carried out by physical, chemical or biological control or by a combination of these methods (integrated control).

Physical/Mechanical Control

Physical removal of the plants can be done manually or by means of machines. Depending on the size and type of the infested waterbody different machines can be used. Weeds may be removed from irrigation and drainage canals using standard equipment such as mowing buckets attached to a tractor or a hydraulic excavator, as well as mowing launches. In general Pistia plants will grow in a mixed vegetation with rooted plants and by using these machines the total aquatic vegetation will be removed (see Wade, 1990). Special floating harvesters, which first collect the material and subsequently dump it on the shore, may be used in lakes and rivers. However, it should be taken into consideration that, in general, re-colonization of Pistia will occur. This implies that control measures should be part of a long-term maintenance programme. Cost effectiveness of the control measures will depend on the losses brought about by Pistia infestation. This will be connected with the economic importance of the waterbody as well as the indirect effect on health of the local population via water-borne diseases.

Biological Control

Neohydronomus spp. are specialist feeders on P. stratiotes throughout its natural distribution in Central and South America. Confusion over the identity of the Neohydronomus sp. introduced into Australia for biological control was clarified by O'Brien and Wibmer (1989) who recognised three species. Very promising results have been obtained with the weevil Neohydronomus affinis (Hustache) which originated in South America. DeLoach et al. (1976) reported its specificity, and in 1982 it was released in Australia and subsequently in Botswana, Zimbabwe, Benin, Senegal, South Africa, Papua New Guinea and USA (see Harley et al., 1990; ECOWAS, 1995). The insect is well established in these countries, producing relatively high levels of control.

Spodoptera pectinicornis is reported to control P. stratiotes in Thailand and has been screened and recommended for introduction into Florida, USA (Habeck and Thompson, 1994). A three-pronged attack using the related weevils Argentinorhynchus bruchi, Argentinorhynchus breyeri and Argentinorhynchus squamosus have been successful in controlling the weed’s spread in lab tests in Argentina (Anonymous, 2001). Several species of fungus have also been evaluated; Ramularia spp. (Fernandes and Barreto, 2005) and Sclerotinia sclerotiorum (Waipara et al., 2006) have shown potential for controlling P. stratiotes populations. See Waterhouse (1994) and Dray and Center (2002) for further information on biocontrol of P. stratiotes.

Chemical Control

Herbicides effective against Pistia include diquat (Thayer and Haller, 1985), a combination of diquat and triclopyr (Langeland and Smith, 1993), glyphosate (Thayer and Haller, 1985; Van et al., 1986), chlorsulfuron (Madin, 1984), terbutryn (Vermeulen et al., 1996), 2,4-D (Langeland and Smith, 1993) and endothall (Rivers, 2002).

The application of herbicides in or near waterbodies may have serious consequences for the environment. It may also endanger the health of local people if the water is used for drinking, bathing, swimming or washing. These possibilities have to be carefully assessed before the use of herbicides on any but a limited experimental basis. A special formulation of diuron known as AF101 has been recommended in Australia, as well as diquat (Parsons and Cuthbertson, 1992).

30/08/13 Updated by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Washington DC, USA

Pedro Acevedo-Rodríguez, Department of Botany-Smithsonian NMNH, Washington DC, USA

06/05/2009 Updated by:

Alison Mikulyuk, Wisconsin Dept of Natural Resources, Science Operations Center, 2801 Progress Rd, Madison, WI 53716, USA