In ponds, lakes, and reservoirs from North Carolina to Hawaii, a stealthy invader called giant salvinia is making an unwanted appearance. This free-floating fern has earned a reputation as one of the world’s worst aquatic weeds—and with good reason. When conditions are right, salvinia’s small, oval leaves form dense mats—green, yellow-green, or brown—that can easily double in size in just a few days.
Sometimes 2 feet thick or more, the mats can cover the surface of an entire pond or small lake, blocking out sunlight that other plants need. And the mats use up oxygen that fish, insects, and other aquatic dwellers require.
Giant salvinia, or Salvinia molesta, is a bother to humans as well. It ruins conditions for fishing, boating, and waterskiing. The weed also clogs irrigation and electrical generating systems.
The dark-colored, one-tenth-inch-long weevil known as Cyrtobagous salviniae weevil has won kudos internationally for holding salvinia in check. The helpful insect has already been used—with great success—in more than 13 countries over 3 continents. This one-tenth-inch-long South American weevil, Cyrtobagous salviniae, is highly effective in reducing giant salvinia infestations to acceptable levels.
The adult female lays her eggs in a cavity that she creates by chewing into the leaf bud. The larvae that hatch feed on the base of the leaf bud. They eventually tunnel into the rhizomes or sometimes the petioles—the structures that attach the leaves to the stems.
The weevil larvae become adults in 17 to 28 days, depending on the weather. That means this species is capable of producing a new generation of hungry young in about a month during the warmer parts of the summer. The adults stop laying eggs in the cooler temperatures—the low 70s—in the spring and fall.
When salviniae weevils were used in Lake Moondarra in Australia, they destroyed more than 8,000 tons of giant salvinia in less than a year. Those are the sort of dramatic and rapid results that are hoped for from this biological control agent. The tests provided additional evidence that the weevils attack only salvinia and won’t pester other plants.
Weevils from Brazil were released in October 2001 at sites in Texas and Louisiana. Within two years, the salvinia mats almost completely collapsed, and water bodies formerly choked by the weed are now mostly open water. As the giant salvinia infestations have declined, so have the populations of the weevil, thereby striking a balance between the two. The end result is a permanent suppression of a fearsome weed into an almost unnoticeable background plant.
History – Texas
May – Salvinia molesta first collected in Texas at a schoolyard demonstration pond in Houston
June – First found naturalized in Texas at farm ponds near Tomball.
In November, heavy rains washed plants into a local creek. July – Farm pond near Garwood found blanketed with Salvinia molesta.
September – Floating giant salvinia was detected at Toledo Bend Reservoir, 186,000 acre impoundment of the Sabine River on the Texas-Louisiana border.
November – Reports were confirmed of Salvinia molesta in oxbows, canals and tributaries of the Lower Sabine River.
December – Initial discovery at Swinney Lake, a 13 acre dammed portion of the Swinney Marsh Complex on the Lower Trinity River.
March – Salvinia molesta recorded at Sheldon Lake State Park, in an old fish hatchery pond.
April – August The summer season found new occurrences at impounded creeks and private stock ponds in, including one reaching the north central border, near Flower Mound. New infestations were in the southeast near Houston, Lovelady, Franklin, Friendswood, Alvin and Mont Belvieu. In Mont Belvieu twin reservoirs encompassing close to 50 infested acres flooded into nearby rice irrigation canals, which themselves drain into Cedar Bayou.
Late August – Salvinia molesta was discovered at Lake Texana, a 11,000 acre impoundment of the Navidad River. The source was traced to a private pond two miles upstream, that had flooded into Sandy Creek, a tributary of the reservoir.
April – Thirty-five acres discovered in Lake Conroe, a 21,000 acre impoundment on the West Fork San Jacinto River, just north of Houston.
July – Fourth public reservoir impacted by giant salvinia in Texas. Twenty-five acres were identified in Sheldon Reservoir, Sheldon Lake State Park, near Houston.
October – Champion Lake, an 800 acre forested impoundment on the west side of the Lower Trinity River and a relatively new acquisition for the USFWS confirmed with Salvinia molesta.
June – A new Salvinia molesta occurrence at a private pond in Splendora fills a distribution gap in the salvinia hotspot of southeast Texas. A now contiguous block comprises eleven separate drainage units hosting Salvinia molesta populations.
December – Four public reservoirs, six rivers or streams and twenty-five ponds have been confirmed with Salvinia molesta in Texas. This year populations were significantly smaller on Toledo Bend and the other public reservoirs, however herbicide treatment was still necessary. The most concentrated infestation still exist in the Lower Trinity River bottom land, Libery County, Texas, just north of Interstate 10, at the Swinney Marsh area.
September – Salvinia molesta appears at a outdoor learning center at League City Intermediate School, Galveston Co. Apparently not introduced with other plants, its origin at the 9 month old pond is unknown.
March – Salvinia molesta discovered in a private pond in Channelview, TX just east of Houston in the Buffalo-San Jacinto drainage. The landowner reports that the plant has been present in her private pond for almost a year, and became suspicious about the identity of the aquatic plant when it began to spread rapidly throughout her pond. S. molesta is presently established and abundant in this new location.
October – Salvinia molesta was confirmed at a private, 6 acre pond in Center, Texas, approximately 28 km west of Toledo Bend Reservoir. Texas Parks and Wildlife chemically treated the infestation in late October and will monitor the pond over the winter months. This is the second occurrence of giant salvinia in Shelby County, Texas.
July – Giant salvinia appears for a second time in Fort Bend County and in the Lower Brazos drainage. The new infestation in Smithers Lake is approximately 25 miles southeast from the first county occurrence. Smithers Lake, a cooling reservoir for a local power plant, will undergo herbicide treatments.
Late Update – State Officials have released more than 600, 000 weevils into four Texas lakes in an effort to control the spread of Giant Salvinia, a fast growing exotic plant believed imported in the aquarium trade. The plant grows very fast, doubling itself every eight days. Under ideal conditions in the late summer it may in fact double itself every five days. Since 2004, biologists have placed 300,000 giant salvinia weevils into Toledo Bend Reservoir, 200,000 weevils into Lake Texana, 87,000 into Lake Sheldon and 50,000 into Lake Conroe. These releases replaced chemical treatment which can be effective but extremely expensive.
In 2004 TPWD treated 228 acres on Toledo Bend and it cost between $100 and $112 an acre. That translates into between $22,800 and $26,000 to control less than 10 percent of the total 3,000 acres of the reservoir infested with giant salvinia. The giant salvinia weevils on the other hand are provided free of charge by the U.S. Department of Agriculture and the U.S Army Corps of Engineers. The weevils feast on the vegetations terminal buds, preventing continued growth and reproduction. The weevils are native to Brazil and they feed exclusively on giant salvinia. They spend their entire life feeding on giant salvinia and they will die rather than eat anything else. This makes them the perfect offensive weapon, so biologists have dumped huge quantities into the lakes to try and bring the infestation under control.
Biologists report that the weevils have been very effective in Texas. They have over-wintered and reproduced successfully on Toledo Bend. Since the large-scale introductions in 2004, biologists have observed localized, but substantial impacts on giant salvinia near release sites. These results have spurred a commitment to continue the program for as long as it takes to bring the vegetation under control. TPWD would prefer not to spray herbicides if they can help it, so will continue stocking large numbers of the weevils. They want to keep the pressure on. The only question remaining is how long it will take to reach equilibrium and effectively control giant salvinia infestation.
It is estimated that it could take three more years to bring the giant salvinia under control in the hardest hit areas.
It was also stated that the plant may never be completely eradicated. The plant is extremely durable and it always seems to come back. In areas that have dried up during low water periods, it has been found growing under layers of (dead) vegetation, just waiting to come back. That means that the plant will probably be present from now on, but the state’s goal is to control the plant’s spread. It will hopefully be reduced to just a fringe around the lakes.
Most anglers relate aquatic vegetation to good fishing. But that is not the case with the exotic nuisance plant called giant salvinia. When you have a shallow area covered by giant salvinia, it interferes with phytoplankton and zooplankton production. It lowers the ph level and affects water quality. It basically turns all the water under it sterile. Light cannot penetrate the dense mats of vegetation. Giant salvinia has the unique capability of growing on top of itself. It can actually grow up to a foot thick. That means no plankton, no baitfish and no bass.
The threat however doesn’t stop there. It is also believed that the plane interferes with bass spawns in the spring. Of course many bass will go to other spots to spawn, but the ideal spots are covered.
Credits: Bass Times