Biological Control of Giant Salvinia on Caddo Lake
By Lee Eisenberg
Giant salvinia is an invasive aquatic weed with an explosive growth rate that enables it to out compete native plants and form large floating mats. Under these mats, the lack of sunlight and oxygen create conditions that can kill phytoplankton (the base of the food chain), as well as most fish.
First found at Caddo Lake in 2006, salvinia has proven to be the most destructive threat that Caddo has ever faced. There are currently over 3,000 acres of salvinia on the lake, and the acreage will only increase throughout its growing season. Salvinia originated in South America, and has been spread around the world (to over 20 countries) by the aquarium and water gardening industries. The first large infestation in the U.S. (1200 acres) was found at Toledo Bend in 1998, and the number of infested water bodies throughout the south has increased dramatically since then. Where it has been introduced, salvinia can restrict any and all activities related to water. Salvinia can clog water intakes and irrigation ditches, degrade water quality, destroy habitat for native wildlife and greatly reduce opportunities for boating, fishing and hunting.
It is difficult to imagine how serious a salvinia infestation can be without seeing it for yourself. This was demonstrated in Australia in 1954, where it first began to appear on local water bodies. Despite an aggressive herbicide program, by 1978 salvinia had spread to “vast areas” of the tropical and subtropical regions of Australia. Mechanical removal and habitat manipulation (such as drawdowns) were also employed, but were not effective. Australian scientists turned to biological control. They went to the home range of salvinia, to see what kept it under control there. Invasive weeds are not usually a problem in their home range, because other species that co-evolved with them keep them in check. After it was tested for specificity, the salvinia weevil was imported to Australia and released on a 200,000 acre lake, about half of which was infested with salvinia. Within a year, the infestation had been reduced to less than 3 % of its former area, and a population of weevils was present on the remaining plants. The salvinia population had gone into equilibrium with the weevil population, which will only feed on salvinia. This is the most spectacular success in the history of weed biocontrol.
Since then, the salvinia weevil has been used against salvinia infestations around the world, and is responsible for controlling the weed in 12 of those countries, and for less than complete control in 4 others. It is most effective in tropical and sub-tropical climates. Unfortunately, the weevil is less effective in temperate climates, such as that of the U.S. Control efforts have been mostly devoted to chemical control, i.e., herbicides. This works fairly well in open water, but there are hundreds of acres of forested wetlands at Caddo Lake and Toledo Bend that are inaccessible to spray boats. These can serve as a refuge for healthy salvinia, which will grow rapidly in water enriched by decaying vegetation from the spray operation. To further complicate things, a tiny piece of salvinia can start a new infestation. A single ramet of salvinia, in nitrogen-rich water, can grow into a 40 square mile mat in 3 months. A ramet is the structural unit of salvinia; it consists of two leaves and a root.
Eventually, weevils were introduced to Toledo Bend. Despite floods, drought, and vandalism, a small population did become established. As it grew, this population was transferred to other infested areas that were inaccessible to spray crews, with some good results. Unfortunately, no one has calculated the actual area treated with weevils, but herbicides were clearly the primary means of control. This population of weevils has also served as foundation stock for a number of colonies in Louisiana and Texas, including (indirectly) the colony at Caddo Lake National Wildlife Refuge. Louisiana has been successfully using pond-reared weevils (initiated from the Toledo Bend release) to treat salvinia infestations throughout the southern half of the state. The water bodies in northern Louisiana and Texas have proven to be more difficult to address, due to colder weather. Although herbicide treatments can be effective, at least in open water, the cost of doing so is becoming increasingly prohibitive.
Salvinia has continued to spread throughout the south. It was first found in on Caddo Lake, in 2006, in Jeem’s bayou. Weevils were introduced, but did not become established, and questions were raised about their ability to overwinter. Although extended periods of freezing weather can kill the weevils, a population released in the summer of 2012 did survive the winter on the lake. There were over 30,000 weevils present in the greenhouses at CLNWR that were released in the spring. Unfortunately, Caddo already had 3000 acres of salvinia, and the growing season was just getting underway. Herbicide funding is at an historic low, and herbicides alone have not been shown to be effective at Caddo, because there are so many areas that cannot be sprayed.
The weevils are not a silver bullet. They can be killed by drought or cold weather, or washed over the spillway during heavy rains. They may fail to become established, or it may take 1.5-2 years to see results. However, the weevils can be re-introduced, and will not do any harm to the environment. All considered, the weevils are the best tool we’ve got. How quickly we can expect to see results depends on the resources that are available and the efficiency with which they are used. It will depend, to a large extent, on local input and energy. This situation did not occur, nor will it be resolved, overnight. We’ve got to be in this for the long haul.