water-lettuce, water-cabbage, river-lettuce, water-bonnet, shell-flower

Leaves: gray-green, about 10-20 cm long, widest at the apex, spongy near the base, with dense white hairs, 7-15 prominent parallel veins, and arranged in rosettes (i.e. circular leaf arrangement).
Stolons (i.e. stem or runner): arise from leaf axis, up to 70 cm long, and entangle to create floating mats.
Roots: covered with fine root hairs which give them a feather-like appearance, dense, and hang unbranched about 50 cm below the water.
Flowers: inconspicuous and clustered on small, fleshy stalks that are nearly hidden in the leaf axils. Flowers are single sexed with male: 3-9 flowers whorled above a single female flower.
Fruit: green berry with many seeds.

FOOD WEB: This species is an autotroph. It serves as a food source for many invertebrate species and is commonly associated with the salvinia stem-borer moth (Samea multiplicalis), the leafhopper (Draeculacephala inscripta), and the aphid (Rhopalosiphum nymphaeae) (Dray et al. 1988). The West Indian manatee (Trichechus manatus) is also known to consume P. stratiotes in the southern coastal United States (Allen and Keith 2015).

LIFE HISTORY: Pistia stratiotes reproduces asexually and sexually. It can spread rapidly by vegetative fragmentation from offshoots on short, brittle stolons. Also, seed production is now considered a major method of reproduction and dispersal (Dray and Center 1989a,b). P. stratiotes possess both male and female flowers. Seeds are wind and insect pollinated and mature 30 days after fertilization (Parsons and Cuthbertson 2001). Seeds can disperse by water and sink into the mud or stream bed, and 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.

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

Pistia stratiotes is reported in shallow waters of the Great Lakes basin. During spring through the fall, P. stratiotes is found in Lake St. Clair (Adebayo et al. 2011), Detroit River, and inland waters (Cochran et al. 2006). These populations are not believed to be self-sustaining but rather to stem from frequent re-introductions.

This species is common in 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). P. stratiotes may be released into the Great Lakes when aquarists dispose of this plant into waterways. This species is also planted in water gardens and may be unintentionally introduced to the Great Lakes. P. stratiotes spreads via vegetative fragmentation and water dispersal of seeds. Fragments or seeds of P. stratiotes may potentially be introduced to the Great Lakes by dispersal from cultured populations. P. stratiotes can also be unintentionally transported to the Great Lakes by hitchhiking on boats and recreational equipment between bodies of freshwater.

Predicted increase in winter water temperatures in the Great Lakes region due to climate change may increase the probability of this species becoming self-sustaining as an annual (Adebayo et al. 2011).

The recurrence of Pistia 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. P. stratiotes reproduces rapidly through seed production and vegetative fragmentation. The stolons and vegetative fragmentation has aided the establishment and rapid expansion of P. stratiotes and will likely facilitate its spread into the Great Lakes.

The Great Lakes region has similar abiotic conditions but slightly lower air and water temperature than the native and introduced ranges of Pistia stratiotes. Frost typically kills P. stratiotes, and so it exhibits optimal growth at water temperatures of 22-30 °C (Kasselmann 1995). Plants can tolerate temperatures as low as 15 °C and as high as 35 °C (Rivers 2002). 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), and so the seeds of P. stratiotes have the potential to overwinter in the Great Lakes. Seeds need temperatures above 20°C to germinate, leading to a growing season too brief for these plants to set seed on their own. In places where water temperatures warm earlier than usual for the region or where frost is significantly delayed, such as thermal outfalls, local populations might overwinter, but there is currently no direct evidence of this happening. The effects of climate change may also make the Great Lakes a more suitable environment for the establishment of P. stratiotes. Warmer water temperatures and shorter duration of ice cover would aid the establishment of P. stratiotes. Shallow lakes and slow-flowing streams in the Great Lakes may provide suitable habitats for P. stratiotes with even modest warming.

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) which may help to prevent establishment.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Pistia stratiotes can have detrimental effects on the environment by inhibiting the growth of plants and algae species through the production of allelochemicals, which can disrupt the food-webs and balance of aquatic ecosystems  (Aliotta et al. 1991,  Bich and Kato-Noguchi 2012). Additionally, it causes high evapotranspiration rates (Sharma 1984) and forms dense mats that reduce light availability for submerged macrophytes and planktonic algae (Attionu 1976). These factors, along with its impact on water temperature, pH, stratification, and oxygen levels, can lead to adverse consequences for native plants, fish, and wildlife (Attionu 1976; Šajna et al. 2007; Sridhar and Sharma 1986, FL DEP 2007). In Slovenia, the presence of P. stratiotes resulted in a decline in native freshwater plants three years after its introduction (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 mosquitoes, such as malaria vectors Anopheles and Mansonia (FL DEP 2007, Lounibos and Dewald 1989, Parsons and Cuthbertson 2001, Rejmankova et al. 1991).

Pistia stratiotes causes damage 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, damage flood control structures, and can create dams against bridges (FL DEP 2007). P. 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.

Pistia stratiotes is also used in traditional medicine for its therapeutic properties (Tripathi et al. 2010, Saddam et al. 2018, Abubakar et al. 2020).

Research has been conducted to utilize this species for biofuels (Lu et al. 2010, Mishima et al. 2008).

Pistia stratiotes is among one of the most common macrophyte species sold in aquarium stores around Lake Ontario and Erie (Rixon et al. 2005).

Pistia stratiotes is a potential candidate for the removal of heavy metals from contaminated water (Odjegba and Fasidi 2004, Ali et al. 2020, Leblebici et al. 2019, Vieira et al. 2019). Numerous experiments P. stratiotes could be used to remediate runoff from a variety of sources including nursery, greenhouse operations, domestic sewage, textile factories, rice mills, paper mills, and sugar mills  (Sridhar and Sharma 1980, Polomski et al. 2009,  Kumar et al. 2018, Ferreira et al. 2019, Schwantes et al. 2019, Kumar et al. 2020,Ekanayake et al. 2021). Several experimental trials indicate that P. stratiotes can dissipate herbicides, and so this plant could decontaminate waters near agricultural areas (Barchanska et al. 2019, Escoto et al. 2019, Alencar et al. 2020, Alonso et al. 2021).

This species is prohibited in Illinois. It is not prohibited in Indiana, Michigan, Minnesota, New York, Ohio, Ontario, Pennsylvania, Quebec, or Wisconsin (Great Lakes Panel on Aquatic Nuisance Species 2012).

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

Control

Biological
Research has been conducted to investigate biological control techniques such as the utilization of specialist herbivores (e.g. Spodoptera pectinicornis, Neohydronomus affinis) (Cilliers 1991, Wheeler et al. 1998). A South American beetle, Neohydronomus affinis was released in Australia and reduced P. stratiotes infestations by 40% or more within 12-18 months (Harley et al. 1990).

Physical
Mechanical harvestors and chopping machines can help remove water lettuce from the water and dispose after grinding the plant down to bits (Ramey 2001). The resulting slurry is then sprayed over the water.

Chemical
In general, the most common herbicides used to control water lettuce are herbicides with the active ingredient diquat or 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.