Lemna perpusilla
(duckweed)

L. minor can be a commercial nuisance in waterbodies such as fish ponds in the USA and Europe. In countries of tropical Asia, L. perpusilla is a problem in the rice field agroecosystem. It is a nuisance in irrigation and drainage channels, reservoirs and recreational lakes. Several mosquito larvae, particularly those of Culex bitaeniorhynchus, C. tritaeniorhynchus and Ficalbia minima are closely associated with Lemna (Osman et al., 1994).

In enriched systems, the plant can be very productive, growing in thick mats that block light penetration to organisms living beneath the surface of the water (Ghanvzan et al., 2006).

L. perpusilla has been widely used in the treatment of wastewater and sewage effluent. It is highly adaptable to eutrophic conditions and has shown to very effectively absorb nutrients in the treatment of effluents (Ruenglertpanyakul et al., 2004) and municipal wastewater (Rattanaphan et al., 2007). This quality in turn lends it a certain suitability for use for fish food or other fodder (Hassan and Edwards, 1992).

L. perpusilla is pantropical in distribution, occurring in Africa, Asia and America, and is locally naturalized in some more temperate areas, including France and Italy in southern Europe (Tutin et al., 1980). Many other records from temperate areas are subject to confusion with the temperate species L. minor and no attempt is made here to distinguish between the two. Hence, the geographical distribution indicated is for the two species combined.

L. minor has been recorded in all European countries except the Faroes, Iceland and Spitsbergen (Tutin et al., 1980).

L. minor is generally found in the wet ricefield agroecosystem of some Asian countries. During heavy infestation, the entire area can be covered with the weed. This phenomenon, which is also observed in small ponds, puddles, ditches and lakes, can be mistaken for an algal bloom.

L. minor is a free-floating plant and, therefore, does not survive in fast-flowing water. It is rarely found in rivers and streams, but thrives along stagnant parts of these watercourses. Stagnant and slow moving water such as ponds, reservoirs, and lakes are the most suitable habitats for this species. Massive population growth of L. minor has also been recorded in drainage and irrigation channels. L. minor only grows well in freshwater ecosystems, but is occasionally found in brackish water. The species cannot survive in sea water.

L. perpusilla has a worldwide distribution and can be found across a broad range of environmental conditions. The plant generally prefers slow-moving water and protected or sheltered habitats (McLay, 1974) and it tends to grow favourably in enriched environments (Ghavzan et al., 2006). In a study of three habitats of L. perpusilla, water temperature ranged from 15-28°C, pH ranged from 6.9-7.8, conductivity spanned 200-890 muS/cm and alkalinity 0.84-7.42 meg/L (Khondker et al., 1994). Both species are found most commonly in mesotrohpic to eutrophic environments of temperate regions with relatively mild winters (eFloras.org, 2009) and can grow from sea level to nearly 10,000 m elevation (GBIF, 2009).

Most published information is on L. minor, unless otherwise indicated; it is assumed that the biology of L. perpusilla is similar.

L. perpusilla and L. minor are minute, stemless, free-floating, aquatic, perennial plants which thrive well in most wetland areas.

Plants overwintering in temperate regions by the production of buds, become water-logged and then sink below the surface into the substrate.

The plants reproduce asexually by buds, and are propagated by budding or bulbils. Daughter fronds are produced in pouches alternately on either side of the mother frond. The daughter grows out of the pouch when it reaches the size of a mature plant and can produce its own progeny while still attached. The fronds are usually separated by water movement, animal activity etc.

L. minor and L. perpusilla are monoecious, L. minor rarely and L. perpusilla frequently flowers. The flowers are unisexual, naked or initially enclosed by a membranous sheathing spathe. Flowers are borne in marginal clefts. The inflorescence emerges from a fissure in the stem, and may or may not be included by a spathe. It consists of one or two staminates and a single pistillate, neither has a perianth. The ovary is unicellular, with one to several ovules, erect or pendulous, and orthotropous. The fruit is a utricle, and the seed is without endosperm. Each plant produces 3-7 seeds. During the growing season, each seed produces a seed-bearing plant in 3 days. A water temperature of 21-23°C, and a light intensity of approximately 1600 lux at the water level are optimum conditions for reproduction of L. minor.

Nutrients, such as phosphate, can stimulate the growth of L. minor. Wind action can cause the duckweed to accumulate in mats, sometimes up to 1 m thick. Although the plant tends to prefer quiet, slow-moving, and sheltered waters (Ghavzan et al., 2006), shorelines and the shelter and structure of other plants (McLay, 1974), to open water where it is exposed to wind and wave action. The plants are not easily visible when scattered (Sainty and Jacobs, 1981), but dense growth produces a green mat which covers the surface of the water. Dense growth of L. minor is an indicator of eutrophic conditions. These plant masses may be dispersed by wave action, and new colonies of duckweed may be initiated in this way.

The biomass of duckweed communities is much lower than the biomass of littoral, emergent macrophyte communities such as Phragmites or Typha; duckweed achieves at most 1/20 to 1/10 of the biomass of Phragmites. However, in enriched conditions, L. perpusilla can form a thick mat on the water’s surface and can grow to such density as to shade out the submersed vegetation, invertebrates and phytoplankton underneath (Ghavzan et al., 2006). The growth of L. minor fronds during summer is logarithmic with time and a growth rate of 0.066 per day, equivalent to a doubling time of 4.5 days, has been recorded. The life expectancy of the fronds is 34 days, but they show signs of ageing after 24 days (Harper, 1977). The plants appear to be pollinated by insects.

Climate change may have a significant effect on L. perpusilla. Under FACE ambient CO₂ enrichment in rice paddy fields L. perpusilla was present 3-fold in August as compared to the lack of effect of CO₂ enrichment on emergent and terrestrial weeds (Koizumi et al., 2004).

L. minor can be transported by adhesion. Birds carrying Lemna tend to rise from one sheet of water and come down on another which, however far away, is likely to be similar to that from which the bird came (Good, 1974).

In Europe and North America, L. minor is eaten by almost every species of duck, particularly migratory ducks. Epiphytic bacteria can also destroy the frond (Hossel and Baker, 1979). Insects associated with L. minor include Scirtes tibialis (duckweed beetle), Tanysphyrus lemnae (duckweed weevil), Rhopalosiphum nymphaeae, Aphis sembuci and Elophila (aquatic caterpillar). The aquatic bug, Trepobates pictus uses the undersurface of Lemna thalli for the attachment of its eggs (Scotland, 1940).

The diminutive spring-tail, Sminthurus aquaticus jumps about on floating mats of duckweed, which provides this species with a source of food and a place for depositing eggs. The spring-tails make many small holes in the thalli during prolific egg laying.

The bright green nymphs of the water strider, Mesovelia bisignata are common on duckweed. Adults of this species lay their eggs between the epidermal layers of the Lemna thallus (Buckingham, 1984). The pond-lily aphid, Rhopalosiphum nymphaeae is very prolific and feeds on the sap of Lemna thalli. Egg, larval and adult stages of the duckweed weevil, Tanysphyrus lemnae are commonly found on floating Lemna. The mining larvae devour the green interior of the thalli, leaving whitened plants, while the adults use their snouts to eat out sperical holes from the upper side of the Lemna. An aquatic case-making moth, Nymphula obliteralis also feeds on duckweed.

The Lemna fly, Lemnaphila scotlandae, is small (ca 1.2 mm long) and able to pass its entire life cycle on and within a single thallus. The larva bores into the centre of the thallus for food and shelter. Normally, there is only one pupa per plant (Mansor and Buckingham, 1989). Hymenopteran parasites (Opius lemnaphilae, Trichopria angustipennis and Trichopria paludis), emerge from fly puparia, leaving tell-tale holes in the empty shells.

Animal feed, fodder, forage

• Fodder/animal feed

Fuels

• Fuelwood

General

• Botanical garden/zoo
• Pet/aquarium trade

Materials

• Fertilizer

Medicinal, pharmaceutical

• Traditional/folklore

L. perpusilla and L. minor are closely related and widely confused, but may be distinguished by their root characteristics, the former having longer roots with a winged sheath and an acute cap, the latter having roots with a wingless sheath and an obtuse cap. The fronds of L. perpusilla are also more asymmetrical than those of L. minor. A number of related species can occur as floating weeds: Woolfia arriza has no roots; Spirodela polyrhiza has larger fronds, 4-10 mm across, and at least two roots per frond. Other Lemna species can also occur and expert help may be needed to confirm precise identity.

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.

Control

Chemical control

Chemical control of Lemna spp. is complicated by the large number of plants which accumulate in masses on the water surface due to wind action.

Contact herbicides are used for control of duckweed, and contact between the herbicide and the weed is necessary (Langeland and Demont, 1986). Herbicides used against Lemna include endothal and diquat; these are applied directly to the duckweed plant on the water surface as a fine spray. Sprayed plants turn white within a few days. The herbicide should then be re-applied, after 7-10 days, allowing time for affected plants to decay. The number of duckweed plants on the surface of the water can be substantially reduced using this procedure. Periodic applications of aquatic herbicides keeps the number of plants at a low level. However, if repeat applications are not made, the number of duckweed plants can increase and recover the entire surface of the waterbody, necessitating further control (Murphy and Barrett, 1990).

Biological control

Insects have been successully used for the control of several species of floating aquatic weeds. There are no reports of insects being used for the biological control of duckweeds, but common duckweed is attacked by several native insects, including a leaf-mining shore fly, Lemnaphila scotlandae. The larvae of the fly kill duckweed leaves by eating their entire contents, leaving behind a transparent, hollow shell.

Others insects which can be used include Lemnaphila neotropica, Hydrellia williamsi and Tanysphyrus lemnae (Pieterse, 1990). Grass carp (Ctenopharyngodon idella) also feed on duckweed.

16/12/2009 Updated by:

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