The Next Serious Threat
In ponds, lakes, and reservoirs from North Carolina to Hawaii, a stealthy invader called giant salvinia (Salvania molesta ) is making an unwanted appearance. This free-floating fern that originated in Brazil has earned a reputation as one of the world’s worst aquatic weeds—and with good reason. The United States Geological Survey calls this weed one of the world’s most noxious aquatic plants. Giant salvinia grows very fast, doubling itself every eight days, and under ideal conditions in the late summer it may in fact double itself in size every five days.
Most anglers in our area relate aquatic vegetation to good fishing. But that is not the case with 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. Light cannot penetrate the dense mats of vegetation and it basically turns all the water under it sterile. Giant salvinia has the unique capability of growing on top of itself and can sometimes be 3 feet thick or more. That means no plankton, no baitfish and no bass.
The threat however doesn’t stop there. It is also believed that the plant interferes with bass spawns in the spring. Of course many bass will go to other spots to spawn, but the ideal spots are covered. Giant salvinia is a bother to recreation as well since it ruins conditions for swimming, boating, jet skiing and waterskiing. The weed is also capable of clogging irrigation and electrical generating systems.
Brief History of Giant Salvinia in Texas/Louisiana
May – Giant salvinia (GS) collected in Texas at a schoolyard pond in Houston
June – First found naturalized in Texas at farm ponds near Tomball.
September – Floating GS was detected at Toledo Bend Reservoir, the 186,000 acre impoundment of the Sabine River on the Texas-Louisiana border.
November – Reports were confirmed of GS in oxbows, canals and tributaries of the Lower Sabine River.
December – Initial discovery at Swinney Lake on the Lower Trinity River.
March – GS 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 Texas 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.
Late August – GS was discovered at Lake Texana, an 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 GS in Texas. Twenty-five acres were identified in Sheldon Reservoir at 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 GS.
June – A new GS occurrence at a private pond in Splendora. A now contiguous block comprises eleven separate drainage units hosting GS populations.
December – Four public reservoirs, six rivers or streams and twenty-five ponds have been confirmed with GS 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 exists in the Lower Trinity River bottom land.
September – GS appears at an 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 – GS 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 the pond.
October – GS confirmed at a private, 6 acre pond in Center, Texas, west of Toledo Bend Reservoir. Texas Parks and Wildlife chemically treated the infestation in late October and monitored the pond over the winter months.
July – GS appears for a second time in Fort Bend County and in the Lower Brazos drainage. The new infestation in Smithers Lake, a cooling reservoir for a local power plant, will undergo herbicide treatments. In Lake Conroe there were 25 acres of GS reported as compared to 40 acres in 2003. In 2004 TPWD treated 228 acres on Toledo Bend at a cost of $100 and $112 an acre. That translates into between $22,800 and $26,000 to control less than 10 percent of the total estimated 3,000 acres of the reservoir infested with GS. A Brazilian weevil is introduced to Texas/Louisiana waters to combat the GS.
A total of 37.2 acres of GS in privately owned ponds were treated. An aggressive herbicide treatment of boat ramps became a priority at Toledo Bend in an effort to contain the plant and prevent its transport to other water bodies.
Biological Control Agent – Supplemental releases of the Cyrtobagous salviniae weevil from Brazil were conducted at all major reservoirs containing giant salvinia (Toledo Bend, Conroe, Shelton and Texana). The dark-colored, one-tenth-inch-long weevil has already been used—with great success—in more than 13 countries over 3 continents. The adult female weevil lays her eggs in a cavity that she creates by chewing into the leaf bud of GS. The larvae that hatch then 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.
Biologists report that the weevil has won kudos internationally for holding giant salvinia in check. When salvinia weevils were used in Lake Moondarra, Australia, they destroyed more than 8,000 tons of GS 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.
The weevils have been very effective in Texas and Louisiana and have over-wintered and reproduced successfully on Toledo Bend. Since their large-scale introduction, biologists have observed localized, but substantial impacts on GS near release sites. These results have spurred a commitment to continue the program (along with aggressive herbicide treatments) for as long as it takes to bring the vegetation under control. The Texas Parks and Wildlife, Sabine River Authority and the Louisiana Department of Wildlife and Fisheries want to keep the pressure on. The only question remaining is how long it will take to reach equilibrium and effectively control GS infestation.
State officials have released more than 600,000 weevils into four lakes. Since 2004, biologists have placed 300,000 GS weevils into Toledo Bend Reservoir, 200,000 weevils into Lake Texana, 87,000 into Lake Sheldon and 60,000 into Lake Conroe. The weevils are provided by the Lewisville Aquatic Ecology Research Facility. Costs are minimal, with expenses limited only to manpower, fuel, and related per diem.
It is estimated that it could take three years (or more) to bring giant salvinia under control in the hardest hit areas. It was also suggested that giant salvinia may never be completely eradicated since the plant is extremely durable and always seems to return. 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 potentially the plant could 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.
Common salvinia, a close cousin of giant salvinia, is on Lake Sam Rayburn, and with the volume of boats fishing Toledo Bend and Sam Rayburn the potential threat of giant salvinia showing up at Sam Rayburn and many other popular reservoirs in Texas and Louisiana continues to grow. At Caddo Lake, surveys in September indicated that water hyacinth cover 1,700 acres on the Texas side compared to 1,000 estimated acres in 2004. To complicate matters even further, giant salvinia has been found growing in the base of the water hyacinth. Jim Pratt who heads up the Sabine River Authority in Louisiana recently told me that the hyacinths have become so thick in one area in the northern portion of Toledo Bend that the plants are literally moving the navigation buoys from their locations.
The magnitude of the invasive aquatic vegetation problem is huge and if not brought under control in a timely manner it could have a significant economic and recreational impact on Texas and Louisiana. It’s a fight that will obviously need the coordinated support of our state legislators in providing the proper funding for adequate manpower and treatment methods.
So where do we go from here? Well I suggest that you mark your calendar to attend a Giant Salvinia Seminar hosted by the Bi-State Alliance on December 10th (6:30pm at the Hemphill VFW Hall). Howard Elder (Aquatic Habitat Biologist for the Texas Parks and Wildlife) and Jack Canson (Marshall, Texas – Organizer for the Caddo Lake Institute) will be giving their thoughts and recommendations for addressing this invasive and destructive plant. This will be a great time to listen, learn and ask questions about an enemy that if not controlled could have a devastating effect on how you, your children and your grandkids enjoy our future lake resources. Giant salvinia is an enemy that cannot be ignored.
I look forward to seeing you at the seminar – JB