Dearl Sanders, Seth Johnson and Bill Kelso

Over the past 120 years, many individuals have released aquatic plants into state and private waters in Louisiana with the best of intentions, only to find out that the seemingly innocuous and often very attractive plants have completely upset the ecology of the receiving water bodies. When released far from natural predators and diseases that regulate their numbers in their native lands, a number of these plants have displaced native aquatic vegetation in many water bodies throughout the state. Uncontrolled, these plants can eliminate access for anglers and other water users, degrade fisheries and waterfowl habitat, severely reduce water quality and cause the expenditure of millions of dollars annually in control costs.

The history of invasive aquatic plants in Louisiana can be traced to the World’s Industrial and Cotton Centennial Exposition of 1884 in New Orleans. Although difficult to document, numerous sources state water hyacinth was imported from Brazil and distributed at the exposition in decorative glass containers as mementos. Infestations of water hyacinth were reported across the southeast and as far west as California by 1890. Alligator weed was introduced into the Mobile, Ala., area in the late 1890s, probably in ship ballast water, and by 1920 was reported in Louisiana and other southern states. Hydrilla was introduced into Florida from Asia in the aquarium trade in the late 1950s. By 1969 hydrilla had infested several lakes in north Louisiana and by 1980 had spread throughout the state.

The most recent invader, giant salvinia, was reported in Toledo Bend Reservoir in 1999, in Cameron Parish in 2000 and in the Atchafalaya Basin in 2006. Investigation by the Louisiana Department of Agriculture and Forestry discovered that the plant was being sold for water gardens. Sales were immediately stopped, and all new plants were confiscated and destroyed. But by 2008, most lakes in northwest Louisiana had a severe infestation, and the freshwater marsh from Lafitte to Morgan City had unmanageable infestations.
 
Other invaders have made their way into the state over the past 100 years. Common salvinia, water lettuce, torpedo grass, Peruvian water grass, parrot feather, egeria and Eurasian water-milfoil have all established populations in the state but have not reached a level of economic importance.

Water hyacinth, alligator weed, hydrilla and giant salvinia all share the traits common to invasive plants. They are rapid colonizers and competitively superior to most native plants. Once established, they quickly dominate the aquatic plant community. These species can spread extremely quickly (giant salvinia can reach an 80 percent daily growth rate under ideal conditions), and all are capable of vegetative reproduction, either through fragmentation, budding or in the case of hydrilla, tubers and axilary buds. Because of their growth potential and their ability to adapt to wide-ranging water conditions, these plants are difficult and expensive to control.

Mechanical control

Early attempts to combat aquatic weeds centered on such equipment as mechanical harvesters, shredders, dredgers and cutters. By 1910, the U.S. Army Corps of Engineers was operating specially constructed, steam-powered saw boats in Louisiana’s navigable waters. The use of mechanical harvesters continues today. They also can be effective in small water bodies with little or no environmental concerns, assuming plants are removed from the water – if not, de composed plant material can reduce dissolved oxygen levels in the water.

For larger infestations on larger water bodies, mechanical control has proven ineffective mainly because limited areas can be harvested each day and target weeds grow rapidly. In addition, trees, stumps and other obstructions common in Louisiana make harvesters difficult to operate efficiently, and disposal of large amounts of harvested vegetation can be difficult and expensive.

Physical control

A common control method for aquatic plants is the use of water level manipulation, called a drawdown, to strand and expose both floating and submerged vegetation. Historically conduct ed in late summer or fall to expose plants to winter temperatures, this control method has been used effectively on a number of weeds. Drawbacks include the inability to use a drawdown on water bodies without dams or control structures, as well as the loss of commercial and recreational opportunities while a lake is drained. A less-common technique is the use of dyes, which cause the water to become opaque and limit sunlight to the submerged plants. Water dyes are not effective on floating or emergent plants and must be replenished as needed.

Another physical control technique is the use of screens placed on the water bottom to block sunlight to plant roots. Although they can be effective in small ponds, screening material can be expensive. If sediment is a problem, the screens will become covered, and new plants will simply root above the screen.

Chemical control  

Early attempts at chemical control involved the use of extremely toxic compounds such as sulfuric acid, sodium arsenate and peroxides. Used primarily on water hyacinth, they often were effective, but they were extremely hazardous to people, aquatic animals and the environment in general. Discoveries in the early 1950s showed copper-based compounds were effective on some aquatic plants and most algae, and copper-based herbicides are still common today.

About a dozen of the more-than-300 synthetic herbicides on the market in the United States are registered for use in water. The rest are either not effective in water, too toxic to non-target aquatic life or not economically practical to register as aquatic-use herbicides.

The contact herbicides diquat and endothall came into widespread use in the 1960s and are still widely used today, primarily for spot treatment of both emerged and submerged weeds. Herbicides such as 2,4-D that mimic the plant hormone auxin came into widespread use for controlling emerged and floating broadleaf weeds in 1959. Today, 2,4-D is used to treat between 75,000 and 150,000 acres of water hyacinth and alligator weed in Louisiana annually. The herbicide glyphosate received aquatic registration in 1977 and remains one of the few aquatic herbicides with both grass and broadleaf weed control capability.

The first herbicide dispersed in an entire water body for controlling submerged weeds like hydrilla was fluridone, which was registered in 1986. Since 2000, several new herbicides have been registered to control fluridone-resistant hydrilla. Repeated use of fluridone as the sole, effective, lakewide treatment for hydrilla in Florida has resulted in most Florida lakes being infested with fluridone-resistant hydrilla, rendering fluridone ineffective for its primary target plant.

Since 1999 more than 20 potential aquatic herbicides have been screened for efficacy at the AgCenter’s Bob R. Jones-Idlewild Research Station. Trials typically are conducted in 30-gallon containers stocked with water hyacinth, water lettuce, duckweed, common salvinia and hydrilla. Herbicide screening on giant salvinia has been conducted off-station to prevent unintentional infestation into area river systems.

The Bob R. Jones-Idlewild Research Station is one of only five or six facilities in the United States that routinely screen herbicides for aquatic use. Data obtained from trials conducted at the research station have provided support for the registration of imazapyr, imazamox and carfentrazone – all registered for use in Louisiana since 2002. Additional herbicide registrations are in the pipeline, with registration for flumioxazin (excellent on giant salvinia) and bispyrabac (excellent on water hyacinth) expected in late 2010.

Biological control

Biological control programs have been implemented against invasive aquatic weeds in the Southeast and Louisiana with mixed results. There is no evidence that biological control agents have had any effect on hydrilla. Partial success has been achieved with biological control agents against alligator weed and water hyacinth in Louisiana. Control of giant salvinia with the salvinia weevil appears possible.

The first use of an insect as an aquatic weed control agent was against alligator weed. The alligator weed flea beetle was introduced from 1964-1970 by the U.S. Department of Agriculture-Agricultural Research Service from waterways near Buenos Aires, Argentina, and established in the southeastern United States, including Louisiana. Feeding by larval and adult beetles destroys both leaves and stems, which become waterlogged and sink. This is still considered a successful example of biological control of an invasive weed, especially in Florida, Louisiana and Texas where it was most successful. However, control is sporadic and depends on maintaining a large population of the flea beetle in Louisiana. Unfortunately, thousands of acres of alligator weed are untouched by the flea beetle each year.

Two weevils and a moth from Argentina were introduced by the U.S. Army Corps of Engineers into the Gulf Coast states, including Louisiana, in the 1970s against water hyacinth. The weevil N. eichhorinae was probably the major contributor to the level of control that was reported in the 1980s when the area infested by water hyacinth was reduced to one-third of its former acreage in the Gulf States. Feeding by these insects destroys stem tissue, and plants loose buoyancy and sink. But often the plants only stop growing. Hundreds of thousands of acres of Louisiana lakes, bayous and canals continue to be infested with water hyacinth. The weevil life cycle takes 90 days, which limits its ability to respond to water hyacinth growth, especially if the winter is severe and insect populations are greatly reduced.

Relief may be on the way. In 2010, the LSU AgCenter, in cooperation with the U.S. Department of Agriculture-Agricultural Research Service and U.S. Army Corps of Engineers, began releasing a new agent for the biological control of water hyacinth. The plant hopper, which was collected in Argentina by the USDA, was cleared for release in spring 2010. The first Louisiana releases were in Gramercy in July and August. Additional releases are planned in other areas of the state. Time will tell what impact the plant hopper will have, but it is hoped that, combined with the weevils, it will help control water hyacinth in Louisiana.

Several biological control agents have been released against hydrilla in the Southeast. They include the Asian hydrilla leaf miners, the hydrilla tuber weevil and the stem-feeding weevil. The only one of these agents suspected of having any real impact is the Asian hydrilla leaf miner. Long-term monitoring for results has been limited, but direct effects have been observed in several locations in northern Alabama and Texas, Lake Seminole in the Florida panhandle and southern Georgia. In these areas, hydrilla populations declined and were replaced   by other submerged species that hydrilla normally outcompetes. The Asian hydrilla leaf miner was introduced in Louisiana and has been collected by the U.S. Army Corps of Engineers in Lake Henderson in south Louisiana and a lake in north central Louisiana north of Interstate 20.

The most successful use of an insect to control an aquatic weed is the salvinia weevil against giant salvinia. This aquatic weevil, which is native to Brazil, Bolivia and Paraguay, has been used successfully for the biological control of giant salvinia in a number of countries around the world. The weevil is a specialist whose larvae burrow into the rhizomes of the plant. Adults consume leaves and buds, inhibiting plant growth. Larval feeding causes the leaves to first darken and then drop off. The combined feeding of larvae and adults kills the plant.

The LSU AgCenter has had tremendous success establishing nurseries for the Brazilian salvinia weevil in Lafourche Parish. The initial weevil nursery on the Golden Ranch Plantation in Gheens was used to develop the protocol for salvinia and weevil production, the timeline for weevil population growth and the optimum times for weevil harvest and distribution. The south Louisiana nursery required 12-15 months from weevil seeding to weevil distribution in salvinia-infested water bodies. The weevil population builds rapidly in the spring, and harvest begins when the population reaches three weevils per pound of giant salvinia. The weevil population reaches a peak of 24 per pound in mid-June.

A highly successful partnership with the Louisiana Department of Wildlife & Fisheries facilitated collection and transport of more than 30 tons of weevil-infested salvinia with approximately 2.3 million weevils to Lake Bistineau near Bossier City and four other north Louisiana lakes in 2009. Several weevil nurseries now have been established in Lafourche and Terrebonne parishes for continued distribution of weevils to areas with giant salvinia problems.

In addition, a population of Brazilian weevils has adapted to feed on common salvinia and is capable of reaching populations of 80 weevils per pound. This is sufficient to control this smaller salvinia species. A nursery with this weevil strain has been established in St. Charles Parish, and weevils will be available in 2011 for distribution to areas where common salvinia is a problem.

Dearl Sanders, Floyd S. Edmiston Sr. Professor in Agriculture and Natural Resource Management, Bob R. Jones-Idlewild Research Station, Clinton, La.; Seth Johnson, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.; and Bill Kelso, F.O. Bateman Professor of Renewable Natural Resources, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

( This article was published in the fall 2010 issue of Louisiana Agriculture.)