Weevils losing battle against salvinia on Lake Bistineau

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Courtesy Photo

Written by Michelle Bates | Minden Press-Herald

The Louisiana Department of Wildlife and Fisheries dispelled rumors and misunderstandings about the weevil projects that have been ongoing since 2007, and the consensus is it is not working to control giant salvinia.

Jeff Sibley, LDWF biologist and director for Region 1, says since the studies began in 2007, they have learned that while the weevil projects are successful in some places, they are not on Lake Bistineau. It is largely due to climate, he said.

A study was done in Australia, and the weevils were successful in controlling giant salvinia, an invasive aquatic plant native to Brazil.

“Our climate is different than in Australia,” he said. “In Australia, year round, the water temperature falls within the range where the weevils can successfully reproduce. Here, we are below that during the winter. There’s only a portion of the year where we are in that range. We have a much colder and variable climate than what is found in Australia.”

For weevils to survive, he says, the water temperature must be between 70 and 86 degrees, with 77 to 86 being the optimal range. Weevil eggs fail to hatch above 96 or below 66.2 degrees, he said.

“What we see is they follow the same ebb and flow as the temperature,” he said. “In the fall, their numbers begin to decline. The larvae follows the same trend. What you see is during the summer, yes, we possibly exceed the population rate; as the temperatures rise in the spring, the number of larvae increase. But as soon as we approach fall and winter, reproduction tends to drop off, and our adult numbers soon follow behind.”

During a presentation by Sibley, he spent a good portion of the presentation showing why the weevil project is not working like they want.

For example, it takes approximately 680,000 adult weevils to control one acre of land. One acre of salvinia is approximately 18.75 tons based upon recent field sampling. Salvinia has been estimated to reach nearly 100 tons per acre under good growing conditions.

Other facts include the time frame for weevils to grow. From egg to adult ranges from 46 to 64 days, he says. Females lay two to five eggs per day on salvinia.

Since 2007, they have stocked 3,319,926 adult weevils in areas such as Black Bayou Lake, Black Bayou Reservoir, Caddo Lake, Clear-Smithport Lake, Cross Lake, Loggy Bayou, Saline Lake, Wallace Lake and Lake Bistineau.

According to the Julian Center and Tipping “Biological Control of Invasive Plants in the Eastern United States,” there are 12 other species of ferns in the genus “salvinia.” Researchers began looking for natural enemies as early as 1961.

Sibley says out of 49 insects, only 25 were found to eat only giant salvinia. Out of those 25 species, only four were deemed suitable for biological control; three of those species did not take hold.

All of this is based on research done in northwest Louisiana and research done all over the world for the last 50 to 60 years.

“In all of their research over the years, we have learned weevils are only able to reproduce in this area from about May 1 to Oct. 15,” he said. “They only live three to six months, reproduction is about six and a half months, and based on water samples from LDWF and DEQ, it’s going to be very close to Oct. 15 when the water temperature is going to be below 70 degrees.”

LDWF has partnered with LSU, the Red River Waterway Commission and the LSU Agcenter to research various ways for the weevils to survive colder temperatures.

“We have 10 plus entomologists and research students working on various aspects of it,” he said. “They are looking at direct cold tolerance and survivability, some things to help them survive in the water during the winter. Right now, the insect is not surviving as well as the plant is. This doesn’t mean we won’t have success, but these things take time. Research is ongoing; it’s trying to find one that is suitable for our location.”

Several things were also addressed in the meeting, such as spraying, the opposition to what LDWF is doing to treat the salvinia and other possible avenues to eradicate the weed.
LDWF officials say they understand the opposition, and they know they care about the lake as much as they do.

“Our main goal is to extend the life of the lake,” one official said. “Giant salvinia is not the only problem we’re having on the lake. We’ve got a lot of organic matter at the bottom of the lake, and we’re trying to deal with that too. There’s a lot of things going on on Lake Bistineau that we’re trying to control and keep from harming the lake.”

Drawdowns were another concern addressed, and Sibley says drawdowns have more benefits than just drying out the salvinia. While they are trying to conduct drawdowns when it is not peak recreational season, he says July and August are the best two months to have the lakebed dry in order to deal with the salvinia and other biomass at the bottom of the lake.

However, funding dictates that they focus their efforts on boat access areas and the channel. The state allots about $7 million to $8 million statewide each year, he says, and about $500,000 of that is spent on Lake Bistineau alone.

Herbicide treatment is ongoing, officials say, and crews are spraying somewhere every day. They may not be on Lake Bistineau every day, but they are conducting herbicide treatment every day possible.

Researchers at Stephen F. University in Texas have developed endocide, which means in laboratory conditions they have found a chemical in salvinia that will make it turn on itself, Sibley said. While that research is still ongoing, Sibley says they are open to options that work and will try it.

“There is a likelihood of it being turned into a herbicide, but that hasn’t been done yet,” he said. “They still have the EPA guidelines and things they must follow. Right now, that is actually the only new technology that I’m aware of, and it has its limitations in its current form. It has promise for the future.”