Giant Salvinia

Domestic Programs Pest Evaluation. Arthur E. Miller, USDA-APHIS-PPQ, AERO, Raleigh, NC. November 16, 2001

Scientific name: Salvinia molesta (Salviniaceae)

Physical description: GS varies in color from green to gold to brown. It is an aquatic fern with floating, oblong leaves. While the leaves of young plants lie flat on the water surface, leaves of mature plants grow to be between 1/ 2 and 1 1/ 2 inches long and are forced upright. A long chain formation occurs as the plants grow together to form mats. GS is very prolific and under favorable conditions can double the size of its mats in 7 to 10 days. As mats continue to grow, they form thick layers of vegetation. Overseas the layers have grown more than 2 feet thick.

The surface of the GS leaf has rows of cylindrical "hairs" topped with four branches that are joined at the tips to form a cage or eggbeater shape.

The invasive aquatic fern Salvinia minima is a similar introduced species which can form mats several inches thick, but it was not listed as a FNW because it occurs in southern states. It has hairs with four branches that are not joined at the tips.

Origin and North American Distribution: GS is an invasive aquatic weed from South America. It has been found in AL, AZ, CA, FL, GA, LA, MS, NC, VA, and TX. It has been ranked #1 target weed under the APHIS Biological Control Canvassing process.

Quarantines: GS is a Federal Noxious Weed.

Dispersal: It has been sold at some nurseries and mail-order plant suppliers and exchanged by aquatic plant enthusiasts. GS may be found as a contaminant in other aquatic nursery stock. GS can be carried overland by boats and other recreational craft. Young ferns can float to establish new colonies downstream. There is no sexual reproduction thus no seed dispersal.

Control: Property managers and cooperators may use these strategies:

Cultural control. Part of a small infestation can be removed from water with a rake and plants left on dry land to dry. Young GS plants are difficult to find when mixed with other plants such as duckweed. Herbarium specimens should be collected at a new site.

Chemical Control. Usually diquat is used in areas with flowing water and Sonar (glyphosate) is used in enclosed bodies or ponds. When GS is treated with diquat it sinks and literally drowns. If other weeds prevent the submergence of GS kill will be incomplete. GS was eradicated from two ponds in Georgia after treatment with diquat late in 1999.

Biological Control. Introduction of the weevil Crytobagous salvinae has helped control the weed about 95% in tropical parts of Australia and other countries. CPHST in Mission, TX is rearing, monitoring and releasing the weevil in LA and TX. The weevil also has the advantage in ponds near residences and public areas that there is no large mass of rotting GS plants after herbicide treatment.

Economic impact: The southern states with temperate climates are most susceptible to salvinia infestation. Rice is an important crop in coastal and river delta areas, which requires flooding, and this provides ideal conditions for GS propagation. The catfish and crawfish industries in the central gulf area are also at risk. GS grows rapidly covering the surface of lakes and streams. It can clog irrigation and drinking water lines and foul hydroelectric plants. Salvinia infested waters cannot be used for boating or other recreational purposes.

Environmental impact: It can greatly alter aquatic ecosystems. As 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 too, and so up the food chain.

Benefits of control: If cooperators in the Eastern Region continue rapid detection and response, they may be able to avoid heavy infestations.