(11/14/22) BATON ROUGE, La. — An invasive insect that could be weakening stands of roseau cane across the Mississippi River Delta has spread farther east along the Gulf Coast, according to surveys performed by LSU AgCenter researchers.
Native to China and Japan, the roseau cane scale was initially found in Louisiana in 2016. Researchers have implicated the insect in the die-off of stands of roseau cane, a vital component of the coastal ecosystem.
“It is definitely a stressor, if it’s not outright killing the plants themselves,” said Tanner Sparks, a doctoral student in AgCenter entomologist Rodrigo Diaz’s biological control and invasive species lab. “It’s definitely a heavy stressor for weak plants.”
Surveys along the coast in the fall of both 2021 and 2022 indicated the roseau cane scale expanded its range since the invasive insects were first found in Louisiana and southeast Texas in 2016 and 2017. The insect’s range now stretches from Bayou La Batre, Alabama, to Sabine Pass, Texas.
Roseau cane stands run along the East Coast and Gulf Coast, providing habitat for birds, fish and shrimp. These stands establish deep roots in fragile soil, protecting shipping channels from waves and limiting erosion caused by major storms, Diaz said.
Scientists found the first evidence of the roseau cane scale in Louisiana in 2016 while studying previously healthy stands of the cane that collapsed and died. Other stressors, such as saltier water and extended periods of flooding, may contribute to the decline of roseau cane, but Diaz said a diverse team of scientists thinks the scale insect plays a role in the die-offs.
“Just by feeding, they are stealing nutrients from plants,” Sparks said. “A lot of the carbohydrates in the sap are not digested, so scales excrete them as honeydew.”
This honeydew allows sooty mold to grow, affecting the plant further.
A plant sap feeder, the roseau cane scale spends most of its lifecycle immobile while feeding on a host plant. As a result, the insect eventually does away with legs, eyes and antennae. However, the immature scale, called a crawler, is mobile and will search for the right feeding location before settling there for the rest of its life.
Diaz and other researchers have speculated that strong winds can blow the crawlers to new roseau cane stands, and storms can wash infested plants into debris piles. Pregnant females can hatch crawlers where the debris piles settle.
The roseau cane scale does not feed on rice, sugarcane or other grass species, Diaz said.
Bayou La Batre, southwest of Mobile, Alabama, appears to be the leading edge of the expansion because research teams only found immature specimens of the insects and no mature females.
The roseau cane die-offs have been limited to the Mississippi River Delta so far. Diaz said larger populations of the scale insects may be required to stress roseau cane stands to the point of dying.
“It may take several generations, maybe several years of that scale being present at a site, until they build numbers and the stress is more noticeable,” he said.
Diaz and a team of graduate students have traveled the coast in the fall to examine stands of roseau cane. In the summer, Sparks stopped on his way home from a beach trip to inspect cane near the interstate highway in the Mobile Bay area. He found evidence of the insect and returned with the research team later.
Interested amateurs have assisted in the search for the insects by uploading photos of infestations to iNaturalist, a social network that allows citizen scientists as well as professional researchers to share observations of plants and animals. Diaz encourages anyone with an interest to search roseau cane stands along the Gulf Coast for the scale insects.
“We suggest that people in Alabama, Florida, Georgia, they should be checking the cane to see if they have infestations there,” Diaz said.
Because the roseau cane scale is a native of colder climates, Diaz hypothesizes that the scale could survive farther north along the Atlantic Coast of the United States if it continues its spread.
Graduate students working in Rodrigo Diaz’s lab searched the Gulf Coast for evidence of an invasive insect, the roseau cane scale, in the fall of both 2021 and 2022. Photo by Rodrigo Diaz
Viewed through a microscope, a mature female roseau cane scale can be seen clearly. The mature stage of the insect, which is a plant sap feeder, has done away with legs. Photo by Tanner Sparks