Sometimes uninvited guests just don’t know when to leave. That’s the case with two invasive plants in the state, giant salvinia and hydrilla. But LSU AgCenter scientists are trying to give the eviction notice to these two aquatic weeds with herbicide and biological controls.

Entomologist Seth Johnson is working on a biological control for giant salvinia. He is rearing the salvinia weevil in 10 above-ground tanks with each tank holding 100-200 weevils. When the weevils reach the adult stage, he releases them (250 at a time) in infested areas. He has made releases at two sites in Louisiana, Toledo Bend Reservoir and in Cameron Parish.

“Both areas are showing damage,” Johnson said. “What you will see is the salvinia actually turn brown from the weevil feeding on the buds of the plant and the larvae tunneling into the rhizomes.”

Over time the salvinia will die and sink. The weevils are placed in 1-meter square containment areas. Johnson says this is done to mark the release point but the weevils can move outside these areas.

After giant salvinia was discovered in Cameron in 2001, larvae of the salvinia weevil were released there in December 2001. When Johnson returned to monitor the progress of the weevils, he found none.

“A hard freeze in early January could have been lethal to the larvae. If they survived, adults could have left the area after it was drained,” Johnson said. “There is not a lot of good information on weevil dispersal and exactly how much salvinia they consume. We hope to be able to answer those questions from our research.”

Johnson also hopes for efficient production of weevils in a controlled environment. They take five weeks to reach maturity at 80 degrees F with an additional two weeks required before egg laying by a female, and a weevil lays approximately 375 eggs. The weevils have a life expectancy of 16 weeks and lay approximately 23 eggs per week almost continuously during their lifetime.

A confirmed report of giant salvinia near Houma prompted Johnson to seek a permit from the U.S. Department of Agriculture to release the weevils in this area. Johnson expects permission, which can take a while, sometime in 2003.

Herbicide can be used to control giant salvinia, but it is extremely costly. Weed scientist Dearl Sanders is looking for alternative herbicides that kill salvinia, but are more affordable than the diquat formulas.

“I did a single trial using a glyphosate as a replacement herbicide for diquat. It provided better than 90 percent control at 8 pounds of active ingredient per acre. This is a high rate but still less expensive per acre than diquat,” Sanders said.

Hydrilla, an invasive species native from South America, has been in Louisiana for quite a while. Efforts to control this nuisance plant have included lake drawdowns, introduction of sterile carp that feed on the plant, and herbicide applications. Sen. Craig Romero of New Iberia helped secure funds to treat Lake Henderson, a popular fishing and tourist destination near Lafayette. Contractors applied Avast! in July and August of 2002 to kill the hydrilla and under the agreement would not be paid until an 80 percent kill rate was achieved.

“It basically starves the plant to death. Avast! inhibits chlorophyll and the plants don’t produce food,” Sanders said.

At the end of October 2002 a final determination was made with a kill rate of 90 percent.

A big hurdle facing the state is the cost of treating hydrilla. Controlling nuisance weeds such as water hyacinth costs around $8 per acre. Treatment for Lake Henderson costs nearly $100 per acre not including the application costs. With hydrilla present throughout Louisiana, treatment costs would run into the millions.

While it may be impossible to rid the state of all the hydrilla and giant salvinia, herbicides and biological controls can reduce their impact. AgCenter scientists are working on the best methods to accomplish this.

Writer: Craig Gautreaux

(This article appeared in the winter 2003 issue of Louisiana Agriculture.)