(04/12/17) ST. GABRIEL, La. – Landowners heard about how salvinia weevils can be used to control the invasive giant salvinia weed, which has clogged numerous Louisiana waterways, at an LSU AgCenter workshop on April 5.

Giant salvinia, which forms a mat on the surface of water bodies and blocks sunlight, can create low-oxygen conditions that are harmful to organisms such as fish. AgCenter scientists have been studying using salvinia weevils to rein in the weed as an alternative to repeated herbicide treatments.

Giant salvinia can smother out desirable native plants, said AgCenter plant scientist Chris Mudge. It blocks fishermen and duck hunters from accessing waterways by boat, a problem that could become worse as two other aquatic weeds come on the scene in Louisiana.

One is Cuban sedge, which can grow through the mat that giant salvinia forms, making boat access even more difficult, Mudge said. The sedge can be controlled with the herbicide 2,4-D.

But no herbicides or biocontrols are available to control crested floating heart, which Mudge called “our next problem plant.” He said it has already shown up in the Baton Rouge area.

It’s likely that crested floating heart and giant salvinia will be able to coexist, Mudge said, which would wreak even more havoc on Louisiana waterways.

To try to combat giant salvinia’s spread across the state, the AgCenter has created several salvinia weevil “rearing ponds,” including some at the Reproductive Biology Center in St. Gabriel. There, weevil populations are being established in giant salvinia that eventually will be harvested and placed in 20-gallon containers to be distributed to landowners.

At the workshop, AgCenter entomologist Rodrigo Diaz told about the success he had with salvinia weevils in a study from June to February at a wildlife refuge in Cameron Parish. Large portions of the salvinia quickly turned brown, then gave way to open waters and native vegetation.

“It was impressive to see this massive biomass going downhill,” Diaz said.

Dissolved oxygen levels also increased as the salvinia died off, he said.

It’s important to have a large weevil population early in the salvinia growing season, which begins in spring, Diaz said. In the study, most of the salvinia was cleared between June and September. That means good water conditions can be restored in time for duck hunting season.

Anyone interested in using the weevils as a biocontrol should remember that they will require time and effort, he said.

AgCenter research associate Charlie Wahl said precautions must be taken to keep the weevils alive during transport from the rearing ponds to the release site. They should be released the same day as harvest if possible.

Release sites have to be chosen carefully. The goal is to create a “weevil nursery,” so placing weevil-infested salvinia in a canal that can get washed out would be a poor decision, said entomology graduate student Alana Russell.

Once areas of the salvinia mat begin turning brown, weevils can be moved to portions that are still green, she said.

Monitoring progress so those kinds of adjustments can be made is critical to success, said entomology graduate student Lori Moshman. When the AgCenter begins distributing weevil-infested salvinia, landowners who join the program will be asked to regularly turn in documentation of the salvinia’s condition and how many weevils there are in samples.

“We’re not spraying a herbicide,” Moshman said. “It’s not going to have an immediate effect. With biological control, you have to have some patience, so it’s really important that we do some midseason monitoring so we know how the weevils are affecting the salvinia over time.”

Attendees of a workshop on using salvinia weevils to control the invasive giant salvinia aquatic weed stand near a pond infested with the weed on April 5 at the LSU AgCenter Reproductive Biology Center in St. Gabriel. This “rearing pond” is used to grow the weed and weevils. Photo by Olivia McClure/LSU AgCenter

LSU entomology graduate student Lori Moshman displays a handful of giant salvinia during a workshop on using salvinia weevils to control the invasive aquatic weed on April 5 at the LSU AgCenter Reproductive Biology Center in St. Gabriel. She is standing in front of a “rearing pond” that is used to grow the weed and weevils. Photo by Olivia McClure/LSU AgCenter

A pond infested with giant salvinia is seen during a workshop on using salvinia weevils to control the invasive aquatic weed on April 5 at the LSU AgCenter Reproductive Biology Center in St. Gabriel. This “rearing pond” is used to grow the weed and weevils. Photo by Olivia McClure/LSU AgCenter

A pond infested with giant salvinia is seen during a workshop on using salvinia weevils to control the invasive aquatic weed on April 5 at the LSU AgCenter Reproductive Biology Center in St. Gabriel. This “rearing pond” is used to grow the weed and weevils. The yellow boom pushes the weed closer to the shore so it is easier to harvest samples. Photo by Olivia McClure/LSU AgCenter