Lake Bistineau is under attack by giant salvinia (as are many of the lakes in north Louisiana & east Texas). Giant salvinia (hereafter referred to as salvinia) is an invasive aquatic weed with an explosive growth rate that enables it to outcompete native plants and form a floating mat on the surface of a water body. These mats can cover thousands of acres; they can destroy wildlife habitat, degrade water quality, clog water intakes and irrigation ditches, and eliminate nearly all opportunities for hunting, fishing or boating. Fish can’t live in the oxygen-poor water under a salvinia mat. Ducks won’t land on it, and most boats can’t push their way through it. Local businesses suffer as well.
Control efforts have centered on herbicide application. Drawdowns and mechanical harvesting are the other tools in the toolbox. Louisiana Wildlife and Fisheries has been fighting hard against giant salvinia; they’ve spent millions on herbicide treatment at Bistineau, over a million dollars in the last year alone. In spite of these efforts, giant salvinia has continued to spread. Each year, when salvinia covers 1500 acres of Lake Bistineau, a drawdown is initiated. While this does dehydrate and kill a lot of salvinia, there are backwater areas where salvinia can survive. Also, a great deal of salvinia flows out through the dam and ends up scattered around the Red River basin. Since salvinia is easily spread by boaters, it can easily be re-introduced to Lake Bistineau. Drawdowns also favor the growth of alligatorweed, another aquatic invasive that become more common at Bistineau. Mechanical harvesting has been around for a long time, and has never been shown to be an effective treatment against salvinia. The best harvesters can process 10-14 acres a day. A thousand acres of salvinia will grow faster than that. Unfortunately, any plant debris that is not removed will enrich the water and cause the salvinia to grow faster. Many backwater areas are inaccessible to spray boats or harvesters.
Coincidentally, these actions-herbicides, drawdowns, and mechanical control (harvesters)-were the same ones undertaken by the Australians in 1954, when they first recognized that giant salvinia was becoming a big problem in their water bodies. An aggressive herbicide program was initiated that lasted for 24 years. Harvesters and drawdowns were also used. By 1978, giant salvinia had spread to “vast areas” of Australia and it was clear that their efforts had failed. They turned to biological control-the use of organisms to control other organisms. Australian scientists went to salvinia’s home range to see what kept it under control there. They discovered a weevil that fed exclusively on salvinia, and was a good candidate for biological control. When a small number of these weevils (5500) were introduced to a 1000 acre infestation at Lake Moondarra, in Queensland, Australia, the size of the salvinia mat was reduced by about 99% in 11 months. Perhaps more importantly, the remaining salvinia had a population of weevils on it, which provided continuous control. The weevil was less effective in the cooler climate of Australia. Since then, salvinia has invaded more than twenty other countries, mostly in the warmer regions of the world. In 14 of those countries, salvinia is considered to be under control due to biocontrol with the salvinia weevil. Unfortunately, the U.S. is not one of them, for the same reasons there were problems in northern Australia. The weevils are most effective in tropical climates, where populations of the cold-blooded weevil continue to grow year round, and our temperate climate can be challenging to them.
The salvinia weevil will only feed on salvinia. In order for scientists to import the weevil, it had to undergo testing in a quarantined lab. The salvinia weevil was given access to plants from 70 different plant families to feed on. It was not able to survive and reproduce on any other plant besides salvinia. It has been used throughout the tropical and subtropical regions of the world for over 30 years, and has never been shown to harm any other plant. In fact, the weevil has never been shown to do any harm at all. Unfortunately, eradication is not a realistic goal, and we should concentrate on management or control.
If salvinia weevils are used in north Louisiana, it will be necessary to have greenhouses in order to address the shorter growing season. A weevil population normally declines in the winter, and starts to grow again in the spring when the water temperature reaches the upper 60’s. In one lab study, the female salvinia weevil has been shown to lay over 200 eggs during a five month adult stage (under warm conditions). Weevils no doubt have a shorter lifespan and lay fewer eggs in the wild, but it would not be unreasonable to suggest that females can lay one hundred eggs over their lifespan. This would allow a population to grow at an exponential rate-something which has been observed under natural conditions. When weevils are pond or greenhouse-raised, the salvinia can be fertilized to provide more nitrogen and make the population grow faster. Temperature-controlled greenhouses can also provide a growing weevil population earlier in the spring (before the salvinia has had much of a chance to grow) and later in the fall, so more releases can be done in a given year.
It takes about 6 weeks for the weevil to develop from egg to adult. About half of that time is spent in the larval stage, during which it feeds by boring down the stem. Adults feed on buds and leaves. Together, the two life stages can do a lot of damage. When the population grows to the point where there are 40 adult weevils per kilo of salvinia, the mat will become waterlogged and sink. There is no easy way to know how long it will take to clear a given area, because the salvinia could be an inch thick or a foot thick. Once they have been released, the weevil population will continue to grow on its own, and slowly spread throughout a salvinia mat.
The weevils are not a silver bullet-but there is no silver bullet. It will take years to see results-but lakes have sprayed for years with no long-term results. The weevils could be wiped out by unusually cold winters-but they can be released against a salvinia population that has also been reduced by the cold. It will take a fairly substantial sum of money to get a weevil production facility up and running. The alternative would be to watch the lake smother under a mat of salvinia, or go through yet another drawdown. All considered, the weevils are the best tool we’ve got for long term control.