(02/22/23) BATON ROUGE, La. — For six and a half years, LSU AgCenter scientists and their counterparts at other agencies have been searching for clues as to why large swaths of roseau cane are dying along Louisiana’s coast — and how to restore these areas to prevent further land loss.
They quickly identified roseau cane scale, an insect native to Asia, as the culprit. But they now believe the scale is likely just one of many problems affecting the cane, which is a key defense against coastal erosion.
This week (Feb. 20 to 26) is National Invasive Species Awareness Week. Invasive species are non-native species that spread aggressively, often endangering features of the local ecosystem. AgCenter scientists study a variety of invasive species — from insects and weeds to feral hogs and Asian carp — and ways to control them.
The scale has expanded its range in recent years, taking out large stands of roseau cane from Port Arthur, Texas, to Mobile, Alabama. A statewide survey in 2022 revealed that the largest scale populations as well as the worst roseau cane die-offs have been observed in the Mississippi River Delta, where the problem originated in 2016.
The loss of roseau cane means the loss of its deep root-and-rhizome system that helps hold fragile coastal soil together. In some places, the cane has been replaced by open water. In others, taro — an invasive species commonly known as elephant ear — has moved in. Taro has a much weaker root structure and creates competition for struggling roseau cane populations.
“Our research is focused not only on what is killing the cane, but how we can restore areas that are dying and maintain the ecological and economic services of these marshes,” said LSU AgCenter entomologist Rodrigo Diaz. “To keep the Bird’s Foot Delta functional, we need to have roseau cane. It’s habitat for wildlife, it holds sediment, it maintains navigation channels.”
As Diaz and others working on the roseau cane effort have learned, mitigating the effects of an invasive species requires teamwork and an understanding of other factors that can compound damage or impede control solutions.
“Each scientist observes the problem with a different angle,” Diaz said. “We were focused on the scale at the very beginning, but then we started bringing people in from different disciplines.”
In addition to AgCenter researchers, the roseau cane initiative includes various LSU departments; Southern University; the Louisiana Department of Wildlife and Fisheries; and the U.S. Department of Agriculture.
The team has found that prolonged periods of flooding, lack of rainfall, high salinity levels and certain soil types can all exacerbate damage from the scale insect and contribute to die-offs.
What has proven more complicated is coming up with options for restoration.
“There is no single solution,” Diaz said. “Roseau cane grows in a complex and changing environment. The Bird’s Foot Delta is sinking in some areas; in other areas, you have river diversions providing fresh sediment.”
As is the case with many invasive species, “we have to learn to live with the scale,” Diaz said.
There is encouraging news, including the discovery of four small wasps that are natural enemies of roseau cane scale. Diaz said scale infestations likely would be much worse without these wasps, which represent a potential biocontrol option.
Scientists also are studying genetic variations in roseau cane in hopes of finding particularly hardy plants. Some stands of cane seem more resilient than others; it’s possible that these plants have genetic traits that make them more tolerant of stressors such as the scale or less-than-ideal environmental conditions.
And studies are underway to find out the best way to replant roseau cane from cuttings in various soil and water types. Many factors need to be considered, such as how deep and how far apart to plant the cuttings, to successfully reestablish a stand.
Scientists met last month at a summit where they shared the latest information on topics including microbes, plant defenses, ecological interactions, restoration effects, river hydrology, remote sensing and the scale insect. Summaries of these projects can be found at https://bit.ly/3xLHiT8.
“All of this wealth of knowledge that we are gathering is going to allow us to customize our recommendations for restoration in different areas of the coast,” Diaz said.
For more information, go to www.lsuagcenter.com/roseaucane.
Dead roseau cane stems. Photo by Rodrigo Diaz/LSU AgCenter
Mature female roseau cane scales. Photo by Ian Knight/LSU AgCenter
Taro replacing roseau cane after die-off. Photo by Madeline Gill/LSU AgCenter