Researchers Look to South America for Help in Controlling Invasive Water Weed Giant Salvinia

Bruce Schultz  |  4/27/2016 5:45:00 PM

The idea sounded simple enough: To rid Louisiana of giant salvinia, an invasive species choking the waterways, return to the plant’s origin and find the insect that feeds on this weed.

In South American countries where giant salvinia is native, the salvinia weevil feeds on the plant. So the insect was brought to Louisiana where it could feed on the aquatic weed.

Giant salvinia is an invasive species that came to the U.S. in the 1990s from South America. It grows rapidly and forms dense mats on the water surface, creating a cover that shuts out sunlight.

The weevil plan, started in 2008, worked well. Its successes include monitored sites in Lafourche Parish where infestation levels of giant salvinia were reduced by 90 percent within three years. But there was one hitch: Salvinia weevils can’t tolerate the cold weather of north Louisiana as well as the plant can, according to LSU AgCenter entomologist Rodrigo Diaz.

So now Diaz has developed a new strategy to return to areas of South America that have winters similar to north Louisiana to find weevils that can survive cold temperatures.

Alana Russell, a graduate student, has been collecting the insects from Uruguay and northeast Argentina, which are along the same latitude as the Shreveport area.

Weevils found in Louisiana can survive 32 degrees Fahrenheit for eight days, but the weevils from Argentina can withstand freezing for 11 days. “The hypothesis is the weevils in Argentina have adapted to colder temperatures,” Diaz said.

Australia has been using the weevil to control giant salvinia, Diaz said, and the insects have been able to adapt Down Under through three decades of natural selection.

But the control efforts for north Louisiana are not resting solely on developing a cold-tolerant weevil. To help the insects survive winter, AgCenter entomologist Steve Micinski is working on a plan to cover several patches of giant salvinia with pine straw to protect the insect from the cold with the goal of helping enough weevils survive the cold to keep pace with the rapidly growing giant salvinia.

Diaz is working on a similar project, but with a fabric covering over patches of giant salvinia during the winter to see if it is effective at insulating weevils from cold temperatures.

“So when spring comes, we’ll have surviving weevils,” Diaz said. “Biological control of salvinia is a numbers game. Salvinia grows fast, and weevils can’t keep up.”

In the cold, weevils become dormant and stop laying eggs. Giant salvinia also becomes dormant but starts growing as soon as warmer temperatures arrive. “The rate of growth of salvinia is so fast, and that is the problem,” Diaz said.

Weevils also don’t move fast, he said, and they seldom fly. They usually move by catching a ride on floating mats of giant salvinia.

Diaz is also looking at more efficient methods of distribution to maximize successful colonization. Until now, the weevils have been released at one location in an infested lake, but it’s possible that a wider release would be more effective, he said.

Even if the new population of weevils from South America is more cold-hardy, that doesn’t mean they can immediately be released into the affected area. Federal approval must be obtained first for their distribution, Diaz said. “It’s difficult to say when this is going to happen.”

Also, export permits must be obtained from the countries where the weevils are found.

Adult weevils feed on the growing buds of giant salvinia, Diaz said. “But it’s the larvae that do most of the damage.” Larvae burrow into the plant and cause it to collapse and sink, he said.

Herbicides have limited success on giant salvinia. If an herbicide only reaches the top part of a plant, it will die, but the rest of the plant can regrow, Diaz said. And it’s physically impossible to get to every location where giant salvinia is growing, he said.

Giant salvinia can be controlled in lakes with a drawdown of water, but that’s not an option for Cross Lake because it is Shreveport’s water source.

To produce a stock of weevils, the insects initially were grown in a pond on private property in Terrebonne Parish, but it has been replaced by a site on LSU AgCenter property in south Louisiana.

The weevils are not sold. Instead, the LSU AgCenter provides the insect to the Louisiana Department of Wildlife and Fisheries for distribution in affected areas.

Bruce Schultz is a writer and photographer with LSU AgCenter Communications.

(This article appeared in the winter 2016 issue of Louisiana Agriculture.)

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The salvinia weevil is that tiny speck on the giant salvinia leaf. Photo by Johnny Morgan

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Salvinia weevil. Photo by Scott Bauer, USDA Agricultural Research Service

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