Duckweed Herbivores and Pathogens:
Animals and microorganisms that feed on duckweeds.
Insects Fungi
Biocontrol of Duckweeds
References

Ducks at the Phoenix ZooIn nature duckweeds serve as a food source for a wide variety of animals and microorganisms.  These range from familiar birds and fish that may consume duckweeds as part of their diet to little-known insects and microorganisms with specialized dependency on these plants.  Aquaculture systems have been designed in which domesticated fowl or fish are raised on duckweed grown in managed ponds.  In other cases, herbivorous fish or insects  are used to control unwanted growth of duckweeds.  This page provides information on just a few of the organisms that feed on duckweeds in nature.

Right:  Photo of ducks at the Phoenix Zoo, courtesy of Gayla Chandler.



Insects That Feed on Duckweeds

Two small insects are so commonly associated with Lemnaceae that their names reflect this fact:

Other insect pests feeding on duckweeds include:
Much of the information and the photos presented here are taken from a USDA technical bulletin.  Other references are given below.

Duckweed fly, Lemnaphila scotlandae Cresson

Duckweed flyThe duckweed fly grows primarily on duckweeds and is one of the smallest insects to attack an aquatic plant.

The adults are small (1.2 mm) black flies with yellow-tipped legs.  The adults feed on the plant by rasping the upper surface of the duckweed with spines on their mouthparts, making distinctive parallel gouges.

The eggs are usually yellowish (0.3 mm long by 0.08 mm wide), with parallel ridges running lengthwise and are usually laid singly on the edges of the fronds.  The incubation period is about 2 days, and the white larva digs down and feeds on the mesophyll tissue, tearing it apart with mouth hooks and then ingesting the macerated tissue.  After completely clearing out one frond, the larva transfers to an adjacent frond to continue feeding. The larvae can also swim to other duckweed plants separated by open water. The larval stage comprises three instars and requires
about 10 days.

Two black-tipped, cone-shaped structures on the posterior end of the abdomen are thrust into the lower epidermis of the frond prior to pupation. The pupa are amber in color and about 1.5 mm long.  The pupal stage lasts about 4 days.

The adult emerges by inflating a specialized bladder-like structure that then ruptures the anterior end of the puparium. The emerging adult crawls through this opening and forces apart the epidermal layers of the frond to exit the frond.  Feeding begins very soon after, but mating and egg laying are delayed until the second day. Although these insects can fly, their flights are usually low hops of a few inches.  Adults probably only live about 3 days.

This fly has since been reported in Illinois, Michigan, Ohio, and Florida, but likely occurs throughout the eastern US.

Duckweed weevil, Tanysphyrus lemnae Paykull

Duckweed weevilThe duckweed weevil Tanysphyrus lemnae is one of the most common and widespread duckweed herbivores.

The female lays her eggs one by one directly into the frond through a hole she chews into it.  The eggs are inserted through the top surface of the frond and generally fill the space between the upper and lower surfaces.  The female then plugs the hole, probably using feces.
Eggs hatch in about a week into nearly transparent larvae about 0.5 mm in length.  The newly hatched larvae immediately begin to feed.  Each larva eats most of the frond that contained the egg within the first 12 hours.  If other fronds are connected to the first, the larva will burrow directly from one to the next, and if not, will swim from one to the next.  The larvae consume the green contents of the fronds, leaving most of the epidermis intact. As the larva grows, it takes on a translucent beige color with a yellow-brown head and lengthens to about 3 mm.

Pupation occurs along the shoreline in the soil or under stranded duckweed.  The total generation time is about 16–20 days. Adults feed by chewing on the surfaces of the fronds, causing obvious round perforations.
Waterlily Aphid, Rhopalosiphum nymphaeae (Linnaeus)

Duckweed weevilIn addition to water lilies, this aphid feeds on many other species. It is also known as the “reddish-brown plum aphid”, a name that is derived from its association with fruit trees, particularly during winter.  This insect is widely distributed (cosmopolitan), and has long been known as a pest of cultivated aquatic plants.  Aphids suck sap from plant leaves, but can also cause damage by transmitting plant viruses.  The waterlily aphid is extremely destructive in aquatic gardens and nurseries and is known to transmit at least five plant viruses.

Winged adult females migrate from aquatic habitats to fruit trees in late fall and lay their eggs on the trees.  There they overwinter, and, after hatching, subsequent generations spend the spring and early summer on the fruit trees.
In mid- to late summer, the aphids migrate to aquatic plants. While on fruit trees, they are oviparous (egg laying) but after migrating to hydrophytes, they become ovoviviparous (retaining their eggs in their oviducts until the eggs hatch and then giving birth to living young).
The ovoviviparous females are wingless, but those that migrate to and from fruit trees are winged.  The waterlily aphid can walk on the water surface and can crawl down under fronds to feed underwater.  Specialized “hairs” on their bodies trap and hold air while the aphids are underwater.

After colonizing aquatic sites, the aphids reproduce quickly.  The developmental period from the birth of the first instar to the adult stage ranges from 7 to 10 days, depending upon temperature (optimally 21-27°C). Each female produces up to 50 nymphs at an average rate of two to four nymphs per day. The nymphs normally progress through five instars during the course of their development, although they will occasionally produce a sixth instar.

Photos above:  A, Adult female and two nymphs of the waterlily aphid.  B, Relatively dense waterlily aphid colony on the underside of a waterlettuce leaf.



Fungi that Colonize Duckweeds

There have been several reports of fungi infecting duckweeds.  The most comprehensive study of a duckweed pathogen was published by Rejmankova et al. (1986).  This group identified the fungus Pythium myriotylum (see below) as the cause of duckweed kills occurring in Louisiana lagoons.  Another example of a fungus infecting duckweeds was presented by Wagner (1969), who described a monocentric holocarpic fungus infecting Lemna minor.  The term monocentric refers to a fungus having a single center of growth, and a holocarpic fungus is one where the entire body of the fungus functions as the fruiting body.

Pythium myriotylum, a root and stem rot fungus.

Rejmankova et al. isolated Pythium myriotylum from dying duckweeds growing in the affected Louisiana lagoons.  They demonstrated its pathogenicity towards duckweeds in outdoor cultivation tanks and under natural conditions in lagoons.  The quantity of duckweeds killed by the fungus increased exponentially and a whole stand would die in several days.  Of six duckweed species tested in laboratory, Lemna gibba, L. minor, and Spirodela polyrrhiza were the most susceptible to the fungal infection.  Lemna valdiviana was more resistant, while L. aequinoctialis and Spirodela punctata never showed symptoms of disease .  The optimum temperature for infection was at about 32°C.

Phythium
                  thorugh the microscopePythium myriotylum is one of the most common species of Pythium found in the soil in damp climates, often causing damping-off of seedlings and root-rot, so this species cannot be said have any special affinity for duckweeds.  These fungi produce masses of microscopic, motile zoospores (see drawing) that can swim short distances to attack wet surfaces of plants.  The fungi produce enzymes that break down the pectin in plant cell walls.  Pectin breakdown results in a soft, watery rot.  Pythium can survive indefinitely in the soil as a saprophyte, feeding on soil organic matter.  In the soil it can also form thick-walled sexual oospores (see drawing).  Oospores  are the primary overwintering form.  Pythium species are not vigorous competitors with other microorganisms in the soil.  The fungi are disseminated in surface-drainage water and in infested soil on farm equipment, tools, and the feet of humans and animals.

Above:  Drawings of Pythium
, (a) oogonia fertilized with monoclinous antheridia; (b) inflated sporangium (vesicle) containing immature zoospores; (c) typical sporangium; (d) two zoospores.  Drawing by L. Gray.

Reference, see Univ. of Ill. RPD No. 922, 1989.




References:

USDA/ARS Insects and Other Arthropods That Feed on Aquatic and Wetland Plants. Technical Bulletin 1870 October 2002 http://www.ars.usda.gov/is/np/aquaticweeds/aquaticweeds.pdf

Mansor, M; Buckingham, GR (1989) Laboratory host range studies with a leaf-mining duckweed shore fly. Journal of Aquatic Plant Management 27: 115-118.

Rejmankova E, Blackwell M, Culley DD. (1986) Dynamics of fungal infection in duckweeds (Lemnaceae). Veroeffentlichungen des Geobotanischen Institutes der Eidgenoessische Technische Hochschule Stiftung Ruebel in Zuerich 0(87): 178-189.

Scotland, M.B. 1940. Review and summary of insects associated with Lemna minor. Journal of the New York Entomological Society 48:319–333.

Univ. of Illinois, Dept. of Crop Sci., "Root and stem rots of garden beans." Report on Plant Disease, RPD No. 922, May 1989. http://web.aces.uiuc.edu/vista/pdf_pubs/922.PDF

Wagner, D.T. (1969) Monocentric holocarpic fungus in Lemna minor L. [Ressia amoeboides] Nova Hedwigia  8 (1): 203-208.





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Revised:  December 10, 2005