HELENA, Mont. — Ranchers and government officials here are keeping watch on an enemy army gathering to the north, along the border with Canada. The invaders are big, testy, tenacious — and they’ll eat absolutely anything.

Feral pigs are widely considered to be the most destructive invasive species in the United States. They can do remarkable damage to the ecosystem, wrecking crops and hunting animals like birds and amphibians to near extinction.

They have wrecked military planes on runways. And although attacks on people are extremely rare, in November feral hogs killed a woman in Texas who was arriving for work in the early morning hours.

“Generally an invasive species is detrimental to one crop, or are introduced into waterways and hurt the fish,” said Dale Nolte, manager of the feral swine program at the Department of Agriculture. “But feral swine are destructive across the board and impact all sectors.”

Wild pigs occupy the “largest global range of any nondomesticated terrestrial mammal on earth,” researchers in Canada recently concluded. They have roamed parts of North America for centuries.

But in recent decades, the pigs have been expanding their range — or more accurately, people have been expanding it for them.

“It’s not natural dispersion,” Dr. Nolte said. “We have every reason to believe they are being moved in the backs of pickup trucks and released to create hunting opportunities.”

In the United States, their stronghold is the South — about half of the nation’s six million feral pigs live in Texas. But in the past 30 years, the hogs have expanded their range to 38 states from 17.

Eurasian boar first arrived in Canada in the 1980s and 1990s, imported as livestock or for hunting. They escaped or were released, and sometimes mated with domestic pigs. Their descendants have become common across the Canadian prairie.

Many experts thought the pigs couldn’t thrive in cold climates. But they burrow into the snow in winter, creating so-called pigloos — a tunnel or cave with a foot or two of snow on top for insulation. Many have developed thick coats of fur.

Now they are poised to invade states along the border, threatening to establish a new beachhead in this country.

“It’s concerning that Canada isn’t doing anything about it,” said Maggie Nutter, one of 80 concerned ranchers and farmers who met recently near Sweet Grass, Mont., to discuss the potential swine invasion. “What do you do to get them to control their wild hog population?”

States and federal agencies are monitoring the border. Should the pigs advance, wildlife officials plan an air assault, hunting the pigs from planes with high-tech equipment like night-vision goggles and thermal-imaging scopes. They’re testing waterways for pig DNA, and turning to more traditional approaches — hunting dogs and shotguns.

Why the worry? The harm caused by snuffling, gobbling wild hogs is the stuff of legend. The damage in the United States is estimated to be $1.5 billion annually, but likely closer to $2.5 billion, Dr. Nolte said.

They are very smart and can be very big — a Georgia pig called Hogzilla is believed to have weighed at least 800 pounds — and populations grow rapidly. Each female is capable of birthing at least two litters a year of six or more piglets.

“Nature’s rototillers,” experts have said. Feral pigs don’t browse the landscape; they dig out plants by the root, and lots of them. Big hogs can chew up acres of crops in a single night, destroying pastures, tearing out fences, digging up irrigation systems, polluting water supplies.

“Pigs will literally eat anything,” said Dr. Ryan Brook, a professor of animal science at the University of Saskatchewan in Canada.

“They eat ground-nesting birds — eggs and young and adults,” Dr. Brook said. “They eat frogs. They eat salamanders. They are huge on insect larvae. I’ve heard of them taking adult white-tailed deer.”

A recent study found that mammal and bird communities are 26 percent less diverse in forests where feral pigs are present. Sea turtles are an especially egregious example.

“Feral swine dig up nests and eat the eggs or consume the baby turtles,” Dr. Nolte said. “We have taken feral swine and in necropsies shown their entire stomach and intestines are full of baby sea turtles.”

Feral swine have caused extensive damage to cultural and historical sites. The invaders cause $36 million a year in damage to vehicles alone.

“Hitting a two- or three-hundred-pound pig on a highway is not that much different than hitting a two- or three-hundred-pound rock,” Dr. Nolte said. Two F-16 fighter jets have crashed after they hit pigs on the runway.

The swine are also reservoirs for at least 32 diseases, including bovine tuberculosis, brucellosis and leptospirosis. Outbreaks of E. coli in spinach and lettuce have been blamed on feral hogs defecating in farm fields.

There are reports that people have contracted hepatitis and brucellosis from butchering the animals after hunting.

“If an animal disease like African swine fever or hoof-and-mouth gets into these animals, it will be almost impossible to stop,” said Dr. William Karesh, a veterinarian who works for EcoHealth Alliance, an organization that studies animal disease. “It will shut down our livestock industry.”

Many countries are frantically trying to contain a global outbreak of African swine fever, which may necessitate the slaughter of a quarter of the world’s domestic pigs. Denmark has built a pig-proof fence along its border with Germany to keep wild boar from entering and infecting domestic pigs.

In the United States, pig hunting is a popular sport, but biologists caution that it is not always the solution to the nation’s feral pig problem.

In states where populations are not established, hunting creates an incentive for people to distribute feral pigs for sport. Hunting makes the animals warier and scatters sounders, or family groups, which go on to multiply in new family groups.

But where pigs are already well established, hunting can reduce their numbers. Gunning feral pigs from helicopters with semiautomatic weapons is a popular sport in Texas. (There are no hunting seasons for feral swine in the state; the animals, which cause $400 million in crop damage in the state annually, can be shot year round.)

Feral hogs are the descendants of swine brought by European explorers in the 16th century; Hernando De Soto, the Spanish explorer, is credited with introducing them to the New World.

Explorers released pigs as they traveled, and then hunted them for food when they returned to the area. Later, Eurasian wild boar were imported to North America for hunting. In many places the boar and the feral domestic hogs interbred.

This crossbreeding has resulted in a well adapted animal, Dr. Brook said: “It’s created a super pig.”