Biological Control of Invasive Salvinia sp.

in the United States.

    The floating fern, Salvinia molesta D. S. Mitchell, is one of the world’s most invasive aquatic weeds.  Native to south-east Brazil, it has spread around the world to Africa, India, South East Asia, and Australasia where it has formed thick mats in canals, lakes, rice paddies, and some rivers.  These mats impede water transport including larger vessels, clog irrigation and drainage canals, and reduce fisheries.  In some cases in Papua New Guinea, entire villages were abandoned because weed mats up to 1 meter thick eliminated the access of the local populations to food, schools, and medical care.  Its invasiveness is due primarily to its phenomenal growth rate: under optimum conditions it will double in size every 2-5 days and quickly cover the surface of slow moving bodies of water.  Management alternatives for salvinia include the three traditional weed management methods: mechanical, chemical, and biological control.  Mechanical shredders for control of salvinia are not effective because the plant reproduces vegetatively and any plant part with a bud can form new plants.  Physical removal has been effective in small-scale locations but is very expensive.  Giant salvinia can be controlled locally with herbicides although this is complicated by the hairy nature of the leaves, which hampers penetration of the herbicides.  Herbicide use has limitations because they impact non-target plants, some salvinia infestations are not easily located or even accessible, and herbicide treatments are costly and time consuming.  Ultimately, controlling these weeds with mechanical or chemical methods is unsustainable over a region of any size.  In the case of S. molesta, researchers quickly realized that biological control was the only practical regional approach to solving the problems posed by this weed. 

            The first attempts at classical biological control of this weed were unsuccessful in Africa, India, Fiji, and Sri Lanka.  The main reason was the misidentification of the plant as Salvinia auriculata Aublet.  Researchers surveyed S. auriculata in Guyana and Trinidad and found a small weevil identified as Cyrtobagous singularis Hustache (Coleoptera: Curculionidae).  Although C. singularis did establish in several areas, it had no effect on the infestations.  Salvinia molesta was separated from S. auriculata in 1972 and its native range in Brazil was discovered in 1978.  In 1980, what was thought to be a biotype of C. singularis was introduced at Lake Moondarra in Australia and proceeded to destroy more than 30,000 tons of S. molesta in less than one year.  Closer examination of the “biotype” resulted in its elevation to species status, namely Cyrtobagous salviniae Calder and Sands.  This new species proceeded to reverse earlier failures and successful programs were conducted in India, Botswana, Namibia, and Sri Lanka.  Control has been dramatic and rapid: in most cases S. molesta was reduced by more than 90% in less than a year following release of C. salviniae.

              Salvinia molesta was first reported in 1995 from southeastern South Carolina, where it was eradicated within the year.  In May 1998, plants were documented at a schoolyard pond in Houston, Texas.  Since then S. molesta has been found at more than 50 sites, correlating to 25 freshwater drainage basins in 10 states: Texas, Louisiana, Florida, Alabama, Mississippi, North Carolina, Hawaii, Arizona, California, and Georgia. Undoubtably this information is already out of date because of the rapid spread of the weed.  The majority and the most serious of infestations remain in eastern Texas where three public reservoirs, five streams and 20 ponds have been confirmed with S. molesta, totaling 28 sites within 14 drainage basins in that state.  At some locations in Texas, the mats are at least 30 cm thick.  The rate of spread appears to be increasing, especially with increasing contact of the weed with recreational fisherman.  Numerous boat trailers have been found covered with S. molesta and this is the suspected route of introduction for the most recent detection of the weed in Lake Conroe, Texas.  Additional locations of giant salvinia infestations continue to be reported, substantiating fears that it will become a major nuisance aquatic weed in the United States.  Preliminary studies suggest that giant salvinia is much more cold tolerant than expected which means that the area impacted in the U.S. could be substantial.

            The introduction of common salvinia, Salvinia minima Baker, to North America pre-dates that of S. molesta.  The smaller, yet, similarly invasive S. minima was first documented in the United States in the late 1920s.  The two

Boat Slip Clogged with Giant Salspecies can be separated

by the shape of the hairs on the adaxial surface of the leaves.  The tips of the hairs are joined together in a “eggbeater” shape with S. molesta and are splayed apart in S. minima.  Since then its known range has expanded substantially to over 640 sites in 70 freshwater drainage basins of seven southern states: Florida, Georgia, Louisiana, Texas, South Carolina, Alabama, and Mississippi.

             The same biological control agent that has been used so successfully in 12 countries on 3 continents, Cyrtobagous salviniae, was detected in Florida in 1960 where it attacks S. minima.  Surveys have found the weevil in most of the drainages in the state where S. minima is present.  Interestingly, S. minima is usually found only in scattered populations in Florida, unlike the situation in Louisiana where C. salviniae is not present.  In these areas like southeastern Louisiana, S. minima is a bigger problem than S. molesta.  Long-term studies at Jean La Fette National Historical Monument and Preserve in Louisiana have documented the complete replacement of native species like duckweed (Lemna sp.) by S. minima.  The less aggressive nature of S. minima in Florida may indicate that it is under significant biological control by C. salviniae.  Studies are currently under way to determine the population dynamics of the weevil in Florida and any suppressive role it may have on S. minima. 

The ultimate goal of this project is to reduce the environmental, social, and economic impacts of both S. molesta and S. minima in the areas where it has established and to prevent future problems in the potential range of both species.  The chances of success are good because biological control agents that have worked previously in similar climates have a higher probability of success when tried in a new area.