laitue d'eau, pistie (French), Lechuguilla de agua, lechuguita de agua, repollo de agua (Spanish), apon-apon, tropical duckweed

Morphological variation is strongly influenced by environmental conditions and population density (Thompson 2007).

Nonindigenous Occurrences: Pistia stratiotes was first reported in Australia in 1887 (Parsons and Cuthbertson 2001). Pistia stratiotes had established in Volta Lake, Ghana before the 1960s (Hall and Okali 1974). It has established in the Erft River in Germany in 2008 (Hussner et al. 2014).

Pistia stratiotes has been reported in:

Alabama- Lower Conecuh, Lower Coosa, and Upper Alabama drainages (University of Alabama Biodiversity and Systematics 2007; Center for Invasive Species and Ecosystem Health 2015);

Arizona (Ramey 2001);

California- Laguna-San Diego Coastal, Salton Sea, Santa Ana (Regents of the University of California 2015), Lower Colorado (Potter and Dean 2015), Lower Sacramento (Fred Hrusa, CDFA, pers. comm.), Central California Coastal, Northern California Coastal, Ventura-San Gabriel Coastal (Calflora 2015), and San Joaquin (Consortium of California Herbaria 2014) drainages;

Colorado- Little Turkey Creek in the Upper Arkansas drainage (University of Colorado Museum of Natural History 2007);

Connecticut- Housatonic, Lower Connecticut, Shetucket (University of Connecticut 2011), and Quinnipiac (Yale University Peabody Museum 2009) drainages;

Delaware (Aquatic Resources Education Center 1995; Ramey 2001);

Florida- and occurs in the Econfina-Steinhatchee (University of Alabama Biodiversity and Systematics 2007), Ochlockonee (Anderson 2001; GISD 2005), and Florida Panhandle Coastal (Center for Invasive Species and Ecosystem Health 2015) drainages, and in Baker County (Aurand 1982);

Georgia (GISD 2005);

Hawaii- the islands of Kuaai, Molokai (Mehrhoff 1996), Maui (Wagner et al. 2005), and Oahu (Naturalis Biodiversity Center 2013);

Idaho- Bruneau River in Middle Snake-Boise drainage (Thomas Woolf, IDA, pers. comm.);

Illinois- Chicago (Center for Invasive Species and Ecosystem Health 2015), Des Plaines (Adam et al. 2001), Little Wabash (University of Connecticut 2011), and Upper Fox (Adam et al. 2004) drainages;

Kansas- Independence-Sugar (Jason Goeckler, KDWP, pers. comm.), Lower Cottonwood (Freeman 2000), and Middle Neosho (University of Kansas Biodiversity Institute 2008) drainages;

Louisiana- Lake Pontchartrain (Thomas and Allen 1993), Atchafalaya-Vermillion (University of Alabama Biodiversity and Systematics 2007), Big Cypress-Sulphur (Louisiana State University Herbarium 2010), Central Louisiana Coastal (Hodgson 2015), Calcasieu-Mermentau (Valentine 1976), and Lake Maurepas (Thomas M. Pullen Herbarium 2005) drainages, and Lincoln, Ouachita, and St. Tammany Parishes (Thomas and Allen 1993);

Michigan- Detroit, Huron, Lake Erie, Ottawa-Stony, and Upper Grand drainages (Michigan State University 2015; Adebayo et al. 2011;MacIsaac et al. 2016);

Minnesota - Buffalo-Whitewater, Twin Cities (Center for Invasive Species and Ecosystem Health 2015), Rush-Vermillion drainages (Balgie et al. 2010), and Lake Winona (Cochran et al. 2006);

Mississippi - Lower Big Black, Middle Pearl-Strong (Mississippi Museum of Natural Science 2016), Lower Yazoo (Sam Faulkner, Delta State Univ., pers. comm.), Mississippi Coastal (Center for Invasive Species and Ecosystem Health 2015), and Tibbee (Madsen 2010) drainages;

Missouri - LaBarque Creek in Meramec drainage (Missouri Botanical Garden 2007);

New Jersey (Cochran et al. 2006);

New York - Bull and Ellicott Creeks in Niagara drainage (Mike Goehle, USFWS, pers. comm.), and Westbury Pond in Southern Long Island drainage (Conover 2007);

North Carolina - Burnt Mill Creek pond in the Ann McCrary Park section in Northeast Cape Fear drainage (Diana Rashash, NC Coop. Ext. Service, NCSU, pers. comm.);

Ohio - Metzger Marsh of western Lake Erie in Cedar-Portage drainage (Wilcox and Whillans 1999), and Olentangy River of Fawcett in Upper Scioto drainage (University of Connecticut 2011);

Ontario- Found in the Detroit River and Lake St. Clair (Adebayo et al. 2011; MacIsaac et al. 2016);

Rhode Island- James V. Turner Reservoir of East Providence in Narragansett drainage, and Chipuxet River at Taylors Landing of West Kingston in Pawcatuck-Wood drainage (Lisa Gould, RI Natural History Survey, pers. comm.);

South Carolina (Ramey 2001);

Texas (Ramey 2001);

Wisconsin- Buffalo-Whitewater (Center for Invasive Species and Ecosystem Health 2015), Castle Rock (WI DNR 2010), La Crosse-Pine (Roe 2015), and Upper Rock (Susan Graham, WI DNR, pers. comm.) drainages.

Pistia stratiotes reproduces rapidly by vegetative fragmentation from offshoots on short, brittle stolons. Seed production is now considered a major method of reproduction and dispersal (Dray and Center 1989). Seeds are wind pollinated and mature 30 days after fertilization (Parsons and Cuthbertson 2001). Seeds can disperse by water and sink into the mud or stream bed. Seeds germinate in light at temperatures above 20°C. Seedlings float to the surface once the primary leaf has developed. Each plant produces several stolons that are about 60 cm long that terminate in rosettes, and can become fragmented to produce new plants.

With appropriate substrate and hydrologic conditions, overwintering by seed could account for population reoccurrence in temperate regions.

Potential pathway(s) of introduction: Dispersal, hitchhiking/fouling, unauthorized release, stocking/planting/escape from recreational culture

Pistia stratiotes occurs in close proximity to the Great Lakes basin. It is found during spring through the fall in Lake St. Clair (Adebayo et al. 2011), Detroit River, and inland waters in Ontario, Ohio, New York, and Minnesota (Cochran et al. 2006).

Pistia stratiotes spreads via vegetative fragmentation and water dispersal. Fragments or seeds of P. stratiotes may potentially be introduced to the Great Lakes by dispersal. Pistia stratiotes can be unintentionally transported to the Great Lakes by hitchhiking on boats and recreational equipment.

This species is part of the aquarium trade (Parsons and Cuthbertson 2001). According to a study on aquarium and pet stores near Lakes Erie and Ontario, 20% of stores surveyed carried Pistia stratiotes (Rixon et al. 2005). Pistia stratiotes may be released into the Great Lakes when aquarists dispose this plant into waterways. This species is planted in water gardens and may be unintentionally introduced to the Great Lakes. It is unknown whether this species is commercially cultured in the Great Lakes region. Pistia stratiotes is not known to be taken up or transported by ballast water.

Identification as having probability of invasion if introduction to the Great Lakes is coupled with predicted increase in winter water temperature (Adebayo et al. 2011) and/or ability for seeds to overwinter (as an annual); listed as having extensive invasion history (Global Invasive Species Database).

Pistia stratiotes has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Pistia stratiotes inhabits tropical and subtropical lakes, reservoirs, and slow-flowing streams (Parsons and Cuthbertson 2001). Pistia stratiotes does best in warm waters, as it is killed by frost. Pistia stratiotes exhibits optimal growth at water temperatures of 22-30°C (Kasselmann 1995). It can tolerate temperatures as low as 15°C and as high as 35°C. This species has been observed to overwinter in the Erft River, Germany; the water temperature in that river is abnormally warm (>11°C) and only leaves that remained submerged survived (Hussner et al. 2014). Its seeds can survive for at least 2 months in water at 4°C and for a few weeks in ice at -5°C (Parsons and Cuthbertson 2001); its seeds have the potential to overwinter in the Great Lakes. Pistia stratiotes reproduces rapidly though seed production and vegetative fragmentation. Its short, brittle stolons that are involved in vegetative fragmentation may aid its establishment in the Great Lakes.

Pistia stratiotes has a widespread distribution encompassing 40 countries (Holm 1991), and occurs on every continent except Antarctica. This species is capable of expanding its distribution quickly. After 1 year, this species had rapidly spread and covered an entire lake (Sridhar and Sharma 1980, Venema 2001).

The native and introduced ranges of P. stratiotes have somewhat similar climatic and abiotic conditions as the Great Lakes; the Great Lakes may have lower air temperatures and lower water temperatures. The effects of climate change may make the Great Lakes a more suitable environment for the establishment of P. stratiotes. Warmer water temperatures and shorter duration of ice cover may aid the establishment of P. stratiotes. Freshwater lakes and slow-flowing streams in the Great Lakes may provide suitable habitats for P. stratiotes.

Pistia stratiotes has outcompeted other species where it has been introduced. Three years after P. stratiotes was first observed in Slovenia, it had covered the whole water surface and populations of native freshwater plants, Ceratophyllum demersum, Myriophyllum spicatum, Najas marina, and Trapa natans, had declined (Šajna et al. 2007).

Surveillance and management efforts are currently underway to detect, control, and/or eradicate this plant in Michigan (MI DEQ 2013) and Wisconsin (Falk et al. 2010). However, a basin-wide monitoring program is lacking (Dupre 2011). 

The recurrence of P. stratiotes on the southern shores of Lake St. Clair for three consecutive years raised suspicion that the macrophyte may have been overwintering in the basin. However, MacIsaac et al. (2016) did not find evidence of seed production—indicating that the persistence of P. stratiotes in the Great Lakes is likely dependent upon annual reintroduction by humans.

Pistia stratiotes may have detrimental impacts on other species and the environment. Pistia stratiotes produces α-asarone, a phenylpropanoid with antialgal activity (Aliotta et al. 1991), so it may interfere with the growth processes in algae. Pistia stratiotes causes high evapotranspiration rates where it occurs (Sharma 1984). By growing in dense mats, P. stratiotes can shade out and reduce the amount of light available to submerged macrophytes and planktonic algae (Attionu 1976). In addition, its dense cover may reduce water temperature, reduce pH, promote stratification, and inhibit mixing of the water by wind (Attionu 1976). As a result of its inhibition of hydrophyte and algal growth, the respiratory activity of its roots, decomposition when it dies, and the restriction of wind-generated mixing, P. stratiotes can reduce the amount of dissolved oxygen where it occurs (Attionu 1976, Šajna et al. 2007, Sridhar and Sharma 1986). It is suspected that the oxygen and light limitations caused by P. stratiotes may have killed native plants, fish, and wildlife (FL DEP 2007). Three years after P. stratiotes was first observed in Slovenia, there was a decline in native freshwater plants (Ceratophyllum demersum, Myriophyllum spicatum, Najas marina, and Trapa natans) (Šajna et al. 2007).

Pistia stratiotes has the potential for high socio-economic impact if introduced to the Great Lakes.

Pistia stratiotes is among the world’s worst weeds (Holm 1991) and has received significant media attention (e.g. de la Cruz 2014, Spear 2014).

Pistia stratiotes mats provide habitat for disease carrying mosquitos, such as malaria vectors Anopheles and Mansonia (FL DEP 2007, Lounibos and Dewald 1989, Parsons and Cuthbertson 2001, Rejmankova et al. 1991). Mansonia larvae perforate leaves and roots of P. stratiotes to reach air chambers (Lounibos and Dewald 1989). Taeniorhynchus (Mansonioides) africanus and Anopheles gambiae breed in ponds and streams that are clogged with P. stratiotes (Philip 1930).

Pistia stratiotes causes damages to infrastructure. Infestations of this species can block waterways, reducing the efficiency of irrigation and hydroelectric power (Howard and Harley 1998). Dense mats of P. straiotes reduce water flow, damages flood control structures, and can create dams against bridges (FL DEP 2007). Pistia stratiotes may impact recreation, as it interferes with navigation and fishing (Labrada and Fornasari 2002). Florida spent about $1.4 million dollars in 2005-2006 to treat P. stratiotes (FL DEP 2007).

Pistia stratiotes has the potential for moderate beneficial impact if introduced to the Great Lakes.

This plant has the fiber content, carbohydrate, and crude protein levels that are comparable with quality forages (Parsons and Cuthbertson 2001). This plant can be fed to pigs, but cows find it unpalatable. Pistia stratiotes is valued as an ornamental plant in water gardens. Research has been conducted to utilize this species for biofuels and water remediation (Lu et al. 2010, Mishima et al. 2008). Pistia stratiotes is used in Ayurvedic medicine for its diuretic, antidiabetic, and antidermatophytic, antifungal and antimicrobial properties.

Wisconsin prohibits the transport, possession, or introduction of Pistia stratiotes (Wisconsin Chapter NR 40). There are no regulations on P. stratiotes in Ontario, New York, Pennsylvania, Ohio, Michigan, Indiana, Illinois, or Minnesota.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological

Biological control techniques such as the utilization of specialist herbivores (e.g. Spodoptera pectinicornis, Neohydronomus affinis) have shown to reduce P. stratiotes infestations in several cases (Cilliers 1991; Wheeler et al. 1998; Harley et al. 1990; Diop and Hill 2009). In Australia, Neohydronomus affinis reduced P. stratiotes populations by 40% or more within 12-18 months (Harley et al. 1990). However, N. affinis has been reported to have limited impact in seasonally flooded areas infested with Pistia stratiotes (Cilliers et al. 1991).

Additionally, it is possible that Microcystis blooms in the Great Lakes may impact P. stratiotes populations if the species became established in the basin. Microcystin-LR is the predominant toxin produced by Microcystis in the lower Great Lakes (Dyble et al. 2009). P. stratiotes is capable of removing the cyanobacterial hepatotoxin [Dha⁷] microcystin-LR (MC-LR) by bioaccumulating the toxin in its roots and leaves. However, exposure to the toxin at concentrations of 0.5 and 1.0 mg/L resulted in a decreased root length by 9.15% and 18.59%, respectively. In addition to shortened and rotting roots, the leaves of P. stratiotes turned yellow and rotted off (Somdee et al. 2016).

Note: [Dha⁷] MC-LR is a congener of Microcystin that has the amino acids L-Leucine and L-Arginine in the X² and Z⁴ positions, respectively, and a dehydroalanine at position 7 of the amino acid ring (T. Davis, pers. comm.).

Physical

In Botswana, research has shown that manipulation of water levels paired with physical removal of P. stratiotes at regular intervals prior to anthesis reduced seed germination in surface sediment samples from 63.5%  to 31.7% in a one year span (Kurugundla 2014). Additionally, mechanical harvesters and chopping machines can help remove water lettuce from the water by grinding the plant down to bits (Ramey 2001).

Chemical

In general, the most common herbicides used to control floating aquatic weeds are 2,4-D, Diquat, and Glyphosphate (Howard and Harley 1998). Herbicides may cause weed die-off and subsequent decomposition that may remove dissolved oxygen from the water. Herbicides might not be able to kill the seeds of the floating aquatic weed.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

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