Giant Salvinia

Beasts of the Southern waterways

by

The weekend is here, and you’ve been waiting all week to enjoy the temperate fall weather. Anticipation builds as you pull into the parking lot and exit your vehicle; the nip in the autumn air puts a zip in your step. After unloading your paddling gear, you walk to the edge of your favorite bayou only to realize what lies ahead of you looks more like a fairway than a waterway.

The surface of the water is covered in green. Although brightly verdant, this certainly isn’t what you expected to find. If you are fortunate, this greenery is just a thin coating of native duckweed. But fortune hasn’t found you today because the sheet of green is much thicker and more invasive than duckweed.

What you have encountered is most likely a bumper crop of Salvinia molesta; commonly called giant salvinia. You might as well pack up your kayak and find another paddling place because this ubiquitous nuisance isn’t departing any time soon, and you certainly can’t paddle through it. Even a motorboat operator would find it nearly impossible to navigate this waterway, akin to boating over shag carpet.

Native to South America, this invasive is speculated to have arrived here via the water garden trade. When plucked off the water’s surface and observed close up, these plants resemble tiny Venus flytraps and are really quite nice to look at, with their fuzzy exterior leaves. Obviously, somewhere along the way, someone thought this was a great pond plant … until it covered the host pond’s entire surface, killing all the koi. At that point, this adventurous horticulturalist must have yanked out the plants, tossing them into the ditch behind the house where they eventually found their way to open water; the rest is recent history.

In 1999, Toledo Bend was the first Louisiana body of water in which giant salvinia was recorded to have been discovered. Since then, the plants have spread to many of Louisiana’s fresh and brackish waters, resulting in federal law prohibiting their importation into the country or transportation between states. For the past decade, environmentalists have been challenged to find surefire ways to eradicate these fast-growing floating plants.

Under prime conditions, giant salvinia can double in as little as four days, eventually covering the entire surface area of smaller canals, ditches, or ponds. Once that happens, the plants may then thicken and stack up, creating mats one foot deep. By summer’s end, giant salvinia will have clogged bayous, ponds, and lakes across the state, bringing an end to boating and fishing in these areas. However, water sports and recreation aren’t the only activities that suffer—aquatic life is also impacted.

By blocking the penetration of sunlight into the water, these vegetative mats smother native plants and phytoplankton, causing the rapid depletion of dissolved oxygen and often killing fish. In some areas of our state, an infestation of giant salvinia can devastate crawfish, rice, and catfish farms. In more urban areas, drainage canals and pumps can become clogged, causing local flooding.

Many of Louisiana’s waterways continue to be infested by this fast-growing aquatic plant, causing negative economic impacts to fishing, boating, and waterfowl-hunting industries.  Outdoor enthusiasts often unknowingly contribute to the problem via their boats and trailers. The plants often bunch up at ramps, becoming lodged between boats and their trailers. If not removed before the next outing, the plants will transfer from the trailer to the body of water … and the cycle continues. Prevention involves thoroughly cleaning boats and trailers, disposing of the plants in a trash receptacle or compost pile.

What else can be done?

Because herbicides are relatively expensive and not always an ecologically desirable means of extermination, many of Louisiana’s lakes will be drawn down this fall and winter in an effort to destroy the floating ferns. Of course, we can always hope for a hard freeze, which seems to work well in keeping a bumper crop at bay—at least for a while. But, by far, the most inventive and organic way of depleting giant salvinia stocks is with the introduction of a lowly little insect.

Cyrtobagous salviniae is just a fancy name for a tiny South American weevil commonly called the salvinia weevil. Now grown in Louisiana for use as a control agent for giant salvinia, the weevils have been tried and tested and found to be quite successful in crippling the plant populations, especially in smaller, more confined waterways. The weevils work particularly well in the southern parts of the state, taking hold and eating their fair share of the water weeds. Do not fear, though, because researchers at LSU are hard at work bringing in and breeding more cold-tolerant varieties of this wonder weevil so that infested lakes in central and north Louisiana can also finally be free of this fast-growing menace. l

Details. Details. Details.

Do your part: If you encounter salvinia in an area where its never been seen before, you are encouraged to report the occurrence to the LSU Ag Center. For more information about weevils and how to obtain them for your waterway, contact Dearl Sanders at the Idlewild Research Station in Clinton, Louisiana. (225) 683-5848.