The Texas Parks and Wildlife Department releases giant salvinia weevils in an effort to control the plant on Caddo Lake. Along with spraying and cold weather, the amount of salvinia was reduced in early 2018, but had come back by fall. (TPWD/Courtesy)

Mother Nature teamed with hard work and herbicides to provide at least a brief respite from giant salvinia to fishermen in 2018 on Caddo Lake and other East Texas reservoirs. However, it was a short-termed relief.

“The extreme cold weather event in January 2018 reduced giant salvinia by 95 percent in Texas. This cold weather event was followed by spring flooding that flushed much of the giant salvinia from the backs of coves and creeks,” said John Findeisen, Texas Parks and Wildlife Department aquatic invasive species biologist.

Findeisen said the problem was that after a warm, dry summer, salvinia was rebounding on Caddo Lake, Toledo Bend, Sam Rayburn and B.A. Steinhagan, the four reservoirs with the largest infestations in the state.

Giant salvinia is a South American plant that was first brought into the United States for backyard ponds and fish tanks and was first discovered on Texas waterways in 1998.

Giant salvinia grows in floating mats in shallow water that chokes off light and oxygen below it, creating unlivable habitat for fish and making waters unnavigable for boaters. The concern is that it can double in mass weekly, meaning under perfect conditions 10 acres of giant salvinia could become 160 acres in a month.

Giant salvinia should be a Texas fisherman’s biggest fear. Conservation-wise, it is one of several water issues facing Texas waterways including water hyacinth and zebra mussels. There are 20 lakes in eastern Texas where giant salvinia has been found including Fork, Palestine, Conroe and Lake O’the Pines. In all, salvinia has been found on 22 lakes in East and South Texas.

Because it is shallow and usually slow-moving, Caddo Lake is perfect for giant salvinia. At its worst, the plant has covered as much as 6,000 acres of the 25,000-acre lake leaving some areas inaccessible for years.

Last year there was only about 1,500 acres of giant salvinia on the Texas side prior to the growing season. While cold weather like East Texas experienced recently can knock back the plant and slow the beginning of the summer growth, it is not going to stop the plant. Studies have shown it requires 3-degree temperature for 48 hours to kill it. In 2018, temperatures dropped into the teens and remained sub-freezing for 48 hours. This year it has dipped into the 20s several times, but warmed up quickly.

That leaves chemical treatment as the best control, but at about $50 an acre it does not come cheap. The department received $6.3 million in funding from the state for treatment of invasive aquatic species in 2018-19. Prior to that, the money came out of the agency’s budget along with funding from water authorities and local governments.

“Herbicide treatments have been conducted year-round at Caddo Lake since 2016. Wintertime treatments can be advantageous as salvinia growth and recovery from herbicide treatments is slowed due to the cold weather. These winter-time treatments have allowed TPWD to stay ahead of giant salvinia on many small reservoirs such as Murvaul, Sheldon, Martin Creek, Fork, Athens, Brandy Branch, Timpson, Raven and Naconiche,” Findeisen said.

He added they are an absolute must on Caddo to keep some open water. The problem is that even the airboats used by the contractors doing the spraying cannot get through all the timber and brush on East Texas lakes, meaning there is always going to be some giant salvinia that is not killed.

For those plants remaining, giant salvinia weevils have been brought in to devour the plant, but the same winter weather that knocks back the plant kills the weevils.

“These salvinia sanctuaries rapidly expand and continually feed salvinia to other parts of the lake. TPWD and the Caddo Biocontrol Alliance will continue to release giant salvinia weevils into the cypress breaks where herbicide boats cannot navigate. There is currently a more cold tolerant giant salvinia weevil in quarantine, which is standard protocol for all biocontrol agents to ensure they will not destroy natives and crops when released, and should be available later this year. This could be a game changer should these weevils be able to tolerate the colder weather at Caddo Lake,” Findeisen said.

A variety of herbicides have been used to treat giant salvinia on Caddo Lake since 2006. In recent years, a cocktail of glyphosate, glyphosate mixed with flumioxazin or penoxsulam mixed with flumioxazin have been used for spring-fall, with diquat being applied during the winter. Because of the amount of chemicals used over the years, there are concerns about water quality.

“The Caddo Lake Institute and the Northeast Texas Municipal Water District both conduct water testing and nothing has shown up. These herbicides are broken down in waters with high organic matter as well as by bacteria and sunlight,” Findeisen said.

The biologist explained that the herbicides are mixed with 100 gallons of water and sprayed directly onto the plants so little reaches the water. What does find its way into the water is quickly broken down and thus not detected.

The last thing boaters and fishermen are thinking of is watching the shoreline for invasive species or checking their boat trailers at the end of the day to see if they have brought any to shore. But to keep giant salvinia from becoming an issue on other lakes, it is a must.

“ The biggest problem with invasive species is their ability to outcompete natives and take over. If we did nothing about giant salvinia at Caddo Lake, it would rapidly take over and eventually kill much of Caddo Lake as well as prohibit recreational boating,” Findeisen said.

While TPWD continues to work with other agencies to find solutions for invasive species, public awareness and assistance in reporting outbreaks remains the first line of defense.

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