VENICE, La. — Louisiana’s coast was already facing deadly threats: drowning from rising seas, beatings from hurricanes, poisoning from oil spills. Now it is being eaten alive.

Vast stands of marsh grass have been transformed into empty mud flats and open water, and scientists believe that a plague of foreign insects is largely to blame. The tiny invaders have an insatiable hunger for roseau cane, a tall and hardy reed that binds together some of Louisiana’s most delicate stretches of coastline.

The insect, first observed in 2016, could not have chosen a worse target. As the Louisiana coast disappears at a rapid rate — about 10 square miles a year — roseau serves as a living, growing bulwark against land loss.

No one knows how or when the insect arrived, or what can stop it. It wasn’t until April that scientists could identify the pest: Nipponaclerda biwakoensis, commonly known as a scale. But by then it had spread over the lower Mississippi River Delta, consuming marshes that protect shipping channels, fishing and shrimping grounds, and hundreds of oil wells and pipelines.

The scale may reverse decades of coastal restoration and undercut major elements of Louisiana’s 50-year plan to slow land loss and limit damage from major storms.

“It very much looks like a cancer on our marshes,” said Todd Baker, a biologist who works for the state. “It should be a concern not only for the citizens of Louisiana but the country at large.”

Roots ‘Like Iron Claws’

Walls of roseau cane sweep along every bayou and canal in the Delta National Wildlife Refuge, a 75-square-mile marshland that spreads like a fan into the Gulf of Mexico. About 10 feet high with densely packed stalks, the plant resembles bamboo, a distant cousin.

For James Harris, the manager of the refuge, dying roseau means a dying delta.

“It’s what holds everything you see together,” he said from behind the wheel of a small boat. While many stalks stood tall, Mr. Harris said the scale was there, unseen, sucking the life out. He estimated that the insects were on 80 percent of the refuge. “Everywhere we’ve stopped, we’ve seen them.”

Known for its soil-building prowess, roseau is saltwater-tolerant and resistant to flood and drought. When conditions change, roseau holds firm. About 60 percent of its mass lies in its thick roots.

“Boom! Look at the density, the stability,” said Rodrigo Diaz, an entomologist at Louisiana State University, as he upturned a small stalk in a greenhouse. “No other plant grabs on like them. They’re like iron claws.”

The plant not only strengthens land but builds it. Thickets of roseau comb out river sediment, dropping it on tangles of roots that raise the soil.

Irving A. Mendelssohn, an emeritus professor of coastal sciences at L.S.U., said the plant was poised to withstand rising sea levels “more than any other marsh species.”

A Crawling Plague

“The bug — it’s everywhere,” said Earl Armstrong, a lifelong resident of the lower delta. To prove the point, he grabbed a dying stalk of cane in the fishing village of Venice. Cracking it open, he pointed to black stains and shells left by the insects.

Mr. Armstrong, a cattle rancher and former conservation official, said his family had survived floods and hurricanes for four generations. “But this cane problem is the worst we’ve ever had,” he said. “We’re going to lose land twice as fast.”

A July estimate from L.S.U. put the damage at about 225,000 acres. As the cane goes, the land beneath it may follow, exacerbating the coastal crisis.

About the size of a grain of rice, the insect gets its name from its shell’s resemblance to a fish scale. The scale works its way into roseau stalks and siphons the sap, weakening the plant or killing it if the infestation is large enough.

It starts life in a “crawler” stage, hitching rides on birds or floating debris. Once it finds a juicy stalk, it wedges itself inside. Females die loaded with eggs, and the hatchlings chew their way out in search of fresh roseau.

“They look like dust particles, but they’re moving,” said Trevor Victoriano, a state wildlife manager.

The insect mystified scientists for months before they had a breakthrough. Then, spotting the parasitic wasp that uses it for a host, they concluded that the scale probably came from China or Japan.

Pest Control

The scale follows a long line of nonnative insects whose numbers have increased along with international trade. Some have barely made an impact and have died out, while others have found the right conditions for a population explosion. In south Louisiana, for example, Formosan termites arrived from Asia in the 1940s but didn’t become a full-blown plague, causing millions of dollars of damage each year, until the ’80s.

Insects are not the only invaders destroying the Louisiana marsh. Nutria, an orange-toothed, semiaquatic rodent, was brought to the state from South America almost a century ago to be raised on fur farms. It gnaws marsh plants by the root, damaging about 6,000 acres a year.

Feral hogs, descendants of domestic pigs, also rip up wetlands, turning them into muddy wallows. They eat just about anything that moves or doesn’t move, including marsh plants, alligator eggs, young birds and even the occasional nutria.

Both invasive mammals have state control programs dedicated to them. The hog problem grows, with about a half-million thought to be in the state. Nutria are largely held in check through a program offering hunters $5 a tail.

But the scale has no dedicated program, nor has any government agency been tasked with leading a response.

Entomologists like Dr. Diaz are unsure what that response would even entail. “There’s so much we don’t know,” he said at his L.S.U. lab, where assistants were counting the number of scales on a stalk. Their record: 2,998. “We don’t know how to stop it.”

Widening Devastation

Speeding into a channel branching off Baptiste Collette Bayou, Eric Newman, a fishing guide, suddenly throttled back on his boat’s engine and removed his sunglasses. “It’s changed so much I don’t even know how to navigate it,” he said.

He passed muddy banks that had been lush with roseau two months earlier. Clumps of blackened roots were all that remained. “When I first saw what was going on, I thought a tornado touched down,” he said.

Dying roseau is bad for business. The cane provides a habitat for young redfish, a top catch in a state nicknamed the Sportsman’s Paradise. The delta attracts thousands of anglers a year, fueling much of Louisiana’s $1.3 billion recreational fishing industry.

Hunters come in droves during the fall to bag ducks that use the delta as a migratory rest stop. During the spring migration, bird watchers flock there to see the millions of songbirds from Central and South America. For these hungry and exhausted animals, the delta is their first bit of land after a long flight. Every foot of roseau marsh lost makes the trip more perilous.

“The die-off’s impact on wildlife can’t be overstated,” said Mr. Baker, who manages a team of scientists for the state’s Department of Wildlife and Fisheries.

The speed of the scale’s spread is alarming, Mr. Baker said. He documented its eating through nearly six miles of roseau in two months. In June, the scale was found in only three coastal parishes. By late July, it was confirmed in 11. It is now in marshes surrounding New Orleans and 150 miles west, in Vermilion Parish. The bayous around the fishing town of Jean Lafitte are especially hard-hit.

“We’re going to lose a lot of wetland very fast,” Mr. Baker said.

Threats to Industry

Receding roseau may also bring problems for the delta’s two biggest industries: oil and shipping.

The lower delta is riddled with hundreds of active oil wells. Unlike the gulf’s hulking offshore rigs, built to endure hurricanes and a constant battering from waves, the wells in marshes are diminutive and low to the water. Long past their most productive and profitable years, some date to the 1940s and receive minimal maintenance. Dozens are abandoned.

The oil industry is confident the wells will hold up as roseau marshes erode. The Department of Natural Resources, the state agency regulating them, also expressed little concern. “The wells in south Louisiana are engineered to last a long time and to withstand storm surges,” said Patrick Courreges, a spokesman for the agency. “They’re built strong.”

Mr. Baker is not so sure. “This infrastructure is not designed to handle the gulf’s wave energy,” he said. Once the wells are exposed, he said, the coast might suffer more oil spills. In 2016, almost 480 spills were reported along Louisiana’s coast, releasing an estimated 880,000 gallons of oil, according to the National Response Center.

Bob Bea, a specialist in catastrophic risk management and former well manager for Shell, said the die-off was cause for worry.

“Wind forces, currents and wave forces definitely affect — very dramatically — the infrastructure designed for swamps and marshes,” he said. “These natural protections are pretty robust. When they disappear, you will see immediate changes.”

The shipping industry is taking the loss of roseau seriously. Last year, 7,500 oceangoing ships and barges passed through the delta en route to river ports as far north as Minneapolis. Industry groups said the die-off would dissolve river banks and clog and reshape navigation channels.

“The roseau cane is absolutely what holds the banks of the river together,” said Capt. Michael Miller, who maneuvers ships through the mouth of the Mississippi. “If the banks disappear, we’re going to have a much harder time getting through.”

No Perfect Fix

James T. Cronin has a solution, but he almost wishes he didn’t. In a garden behind a Baton Rouge office park, Dr. Cronin, an L.S.U. ecologist, nodded apologetically at a patch of reeds. Like the insects, they were invaders.

Multiple forms of roseau grow in the delta. The one thought to be native, gulf roseau, is being decimated by the scale. Another strain, from Europe, is considered an aggressive invasive species in the Great Lakes and on the East Coast but appears held in check in Louisiana, possibly by the other varieties. This strain, Dr. Cronin recently discovered, is resistant to the scale. “For some reason, scale leaves it alone,” he said.

Planting European roseau on Louisiana’s coast could restore what the insects appear to be taking away. But the idea has its downsides, he admitted. In Chesapeake Bay, the reed obstructs waterways, crowds out native plants and robs crabs, oysters and other wildlife of their habitats. East Coast states spend about $5 million a year to contain the plant, Dr. Cronin said. They mow it, poison it, dredge it, set cattle to graze on it — even smother it with plastic sheeting.

Other ideas for fighting the scale have sparked little hope. Insecticide would kill or sicken shrimp and other animals. In China, scale outbreaks are controlled with fire, but burns often require toxic accelerants, and the presence of oil wells and pipelines raises safety concerns.

The scale’s one known parasite, the wasp that traveled to Louisiana with it, is an unlikely savior. The wasps have so far been unable to halt the plague’s spread, and introducing more wasps isn’t feasible.

“Japan probably doesn’t have a batch of the parasite ready to ship,” said Joey Breaux, a specialist with the state’s Department of Agriculture and Forestry. “And if they did, it’d be expensive.”

Dr. Cronin and Dr. Diaz have been trying to drum up funding for a research and response plan. At about $400,000, it is a fraction of what the state pays for restoring a single beach. Yet most government agencies in the state have pleaded poverty. Louisiana’s congressional leaders have registered their alarm, but little federal support has emerged.

The state’s Coastal Protection and Restoration Authority, which might seem a natural partner, has repeatedly asked for more research before committing funds. “They have literally billions of dollars, and they’ve contributed the least,” Dr. Cronin said.

The authority wanted to know whether other factors — climate, environmental toxins, soil composition, water levels — were also contributing to the die-off. Dr. Diaz said that kind of research would take money and time and could lead to dead ends, whereas the scale was a clear threat.

Mr. Baker has resigned himself to playing slow defense against a fast attacker.

“It’s going to take a lot more money and time,” he said. “We don’t have money, and time is not on our side.”