Invasive bug munching on the Louisiana delta survived colder-than-normal winter

Invasive bug munching on the Louisiana delta survived colder-than-normal winter
Scale on a stalk of Roseau cane (John Snell)

(WVUE) - Down Southwest Pass near the mouth of the Mississippi River, cattle rancher Earl Armstrong spots trouble on the Roseau cane, a tall grass species that holds together much of the delta.

"I just grabbed this one right here and there it is," Armstrong said as he inspected the stem for a tiny, invasive insect.

Scientists have identified the scale as Nipponaclerda biwakoenis, commonly referred to as Phragmites scale or Roseau scale. Native to Asia, the scale mostly likely hitched a ride on ocean-going ships that moved up the Mississippi River. About the size of a fingernail when fully grown, the scale attaches itself to the Roseau.

"The scale took the freeze with no problem by the looks of it," Armstrong said.

Researchers studying the scale had hoped the harsher-than-normal Louisiana winter might have killed off much of the scale, which was first discovered in lower Plaquemines Parish in 2016. However, Armstrong finds small, white blotches clinging to the plants on this year's growth of Roseau cane.

"At least two out of five might have it, or maybe four out of five. Depends on where you go," Armstrong said.

LSU researchers believe the cold never fully penetrated the thick stands of Roseau cane, and the the insect might have drawn insulation from the plant itself.

"The scales are quite well-protected in the Roseau," said Dr. Rodrigo Diaz, an LSU expert on invasive species ecology and control. "You know, they are in between the stem and the leaf sheet."

Diaz theorizes that the insects come from areas of Japan and China that are generally colder than South Louisiana and were able to withstand the colder temperatures. He points out the mystery surrounding the Roseau cane die-off is further complicated because very little research has been conducted anywhere in the world on the Roseau scale, including in Asia.

In fact, Diaz said they are not even certain the scale itself is killing the cane.

While the Louisiana Department of Wildlife and Fisheries estimates the insect has invaded thousands of acres of marsh, not all of those areas are experiencing the same issues. The scale is present, for example, in St. Bernard Parish, but appears to be causing little problem.

"We're not having the die-off like they're seeing in Plaquemines," said John Lane, The St. Bernard Parish executive director of coastal operations.

LSU researchers point out the problems in lower Plaquemines could involve other issues - or some combination of factors - from bacteria or a virus to weak soil conditions in the sinking and eroding delta near the mouth of the river.

"The new chutes that are coming now in May, for example, are coming in really weak, and on top of that the scale is attacking them," Diaz said.

It is also possible the scales invaded Plaquemines Parish first and are simply take time to start killing the Roseau cane.

The invasive scale also lacks predators in South Louisiana, such as wasps and birds that feed on it in its native Asia. Diaz said some Asian wasps seem to have arrived along with the scale, but apparently not enough to keep the scale population in check.

Congress recently approved $500,000 in research money, which will bring in experts from other disciplines to view the issue more comprehensively.

"We're a few years behind," said LSU Entomology Postdoctoral Associate Dr. Ian Knight. "To get where we want to be now, we should've been studying it a couple years ago."

In the controlled setting of an LSU facility, researchers are intentionally infesting some Roseau cane to isolate them from other factors stressing the plants, such as salinity.

"Then we can say with a little more confidence whether the scale is playing a role in these die-offs," Knight said.

Another critical question involves what risk, if any, the scale might pose to other grass species, such as sugar cane and rice. So far, there appears to be no sign of problems in crops that are critical to parts of Louisiana's economy.

"Based on the information from China, we suspect that this population may remain in Roseau," Diaz said.

Some of the suggested remedies for the infestation carry major disadvantages. The state has considered fire, but that would likely require some kind of accelerant since cane does not burn easily. In addition to the potential damage to marine life, the oil industry has also raised concerns about burning marshy areas near pipelines and other oil and gas infrastructure.

"We're working against the clock, trying to understand as much as possible," Diaz said.

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