United States Department of Agriculture • Animal and Plant Health Inspection Service

PEST ALERT

APHIS 81–35–006 May 2000

Giant Salvinia

Giant salvinia, Salvinia molesta, is an invasive

aquatic weed from South America with the potential

to do serious harm to U.S. waterways. A free-floating

fern, S. molesta was first found in South Carolina in

1995. Although that outbreak was successfully

eradicated, giant salvinia has now been observed in

24 watersheds in Texas, California, Arizona, Louisiana, Mississippi, Alabama, Georgia, and Florida.

Giant salvinia can successfully winter over in some

parts of the United States. Due to their temperate

climate, Southern States are more susceptible to

salvinia infestation. Its estimated range extends

along the gulf and Atlantic coasts from Texas to

Virginia, and west to California as far north as the

Sacramento River delta.

The U.S. Department of Agriculture (USDA) listed

Salvinia molesta as a noxious weed in 1983. Animal

and Plant Health Inspection Service (APHIS) scientists are developing a strategy to control giant

salvinia using a biological control agent—a host -

specific weevil originally from southeastern Brazil.

Overseas releases of this weevil have successfully

suppressed salvinia in many countries.

Description

Salvinia molesta varies in color from green to gold

to brown. It is an aquatic fern with floating, oblong

leaves. And, while the leaves of young plants lie flat

on the water’s surface, the leaves of mature plants

grow to be between 1/2 and 1 1/2 inches long (1.27

and 3.8 cm) and are forced upright. A long chain

formation occurs as the plants grow together to form

mats. Giant salvinia is very prolific and under favorable conditions can double the size of its mats in 7 to

10 days. As the mats continue to grow, they form

thick layers of vegetation. Overseas, the layers have

grown more than 2 feet thick.

The surface of the salvinia leaf has rows of cylindrical “hairs” topped with four branches that are

joined at the tips to form a cage or eggbeater shape.

These hairs give a plush appearance and repel

water. In mature plants, underwater rootlike structures conceal stalks that can have egg-shaped spore

cases attached. In the United States, these spore

cases are not known to contain fertile spores. This

weed overwinters and reproduces vegetatively.

Figure 1—Giant salvinia has been known to cover entire lakes

and can double its size every week or so in warm weather.

Figure 2—In mature plants, underwater rootlike structures

conceal stalks with egg-shaped spore cases attached. These

spores are infertile; vegetative reproduction is the norm.
Spread

Giant salvinia can spread in many ways. Plants

can be carried overland by anything entering infested

waters. Boats and other recreational craft can

transport this invasive weed from one body of water

to another. Younger ferns can follow the natural

course of a moving body of water to establish

colonies anywhere downstream. And because the

public may not recognize the invasive nature of giant

salvinia, the plants are still found for sale at some

nurseries and in the catalogs of mail-order plant

suppliers. Giant salvinia is often found as a contaminant in other aquatic nursery stock. Same aquaticplant enthusiasts actually exchange it informally

amongst themselves for use as a decorative

specimen in outdoor ponds. As the salvinia ages,

however, it chokes out other plantings and becomes

a destructive plant pest.

Damage

Salvinia molesta can greatly alter aquatic ecosystems. As the mature plants weave themselves into a

thick, floating mat, oxygen and light are blocked from

the water. Native macrophytes and microscopic

algae that form the base of the food chain may die

off. The creatures that feed on these may die, too,

and so on up the food chain.

The rapid spread of this pest threatens cultivated

aquatic crops, and it can clog irrigation and drinking

water lines and foul hydroelectric plants. Salviniainfested waters cannot be used for boating or other

recreational purposes.

Control

The good news about managing giant salvinia is

that control has been demonstrated without the use

of toxicants. In tropical parts of Australia, India,

Namibia, Papua New Guinea, and Botswana,

introductions of the weevil Cyrtobagous salviniae

suppressed populations of this aquatic weed. Eradication of big infestations generally requires the use of

commercially available herbicides in addition to

biological control organisms.

APHIS scientists are currently studying the

success of C. salviniae and are developing plans for

its use. Also, APHIS is working with other Federal

and State agencies to prevent the spread of giant

salvinia by educating the public on how this invasive,

noxious weed fouls waterways and travels over land.

USDA and the U.S. Department of the Interior are

equal opportunity providers and employers.

Photo credits: Photographs were taken by Colette Jacono of the

U.S. Department of the Interior’s U.S. Geological Survey and are

reproduced by permission.

Figure 3—This invasive weed can travel over land attached to

gear and water craft. Boaters must make certain not to transport

giant salvinia.