EAST LANSING, Mich. » At first he thought it was a deer, perhaps the big-antlered buck he had seen before and hoped to bag someday.

But the closer Steve Davenport got, the more unfamiliar the looming dark mass in the cornfield behind his house seemed.

At 15 feet, he saw the long, bristled snout. Then he saw the hoof.

"It just kept looking more and more like a pig," he recalled. "I had never heard of anything like that. I was just kind of in shock."

In Southern states like Texas, backyard encounters with feral swine have become routine. The pigs — ill-tempered eating machines weighing 200 pounds or more — roam city streets, collide with cars, root up cemeteries and provide plot lines for reality TV shows like "Hog Hunters."

But the pig wars are moving north. In Michigan, New Hampshire, New York, North Dakota, Ohio, Oregon and Pennsylvania — states where not long ago the only pigs were of the "Charlotte’s Web" variety — state officials are scrambling to deal with an invasion of roaming behemoths that rototill fields, dig up lawns, decimate wetlands, kill livestock, spread diseases like pseudo-rabies and, occasionally, attack humans.

In 1990, fewer than 2 million wild pigs inhabited 20 states, according to John J. Mayer, the manager of the environmental science group at the Savannah River National Laboratory in Aiken, S.C., who tracked the state populations. That number has now risen to 6 million, with sightings in 47 states and established populations in 38 — "a national explosion of pigs," as Mayer put it.

The swine are thought to have spread largely after escaping from private shooting preserves and during illegal transport by hunters across state lines. Experts on invasive species estimate that they are responsible for more than $1.5 billion in annual agricultural damage alone, amounting in 2007 to $300 per pig. The Agriculture Department is so concerned that it has requested an additional $20 million in 2014 for its Wildlife Services program to address the issue.

There is wide agreement that the pigs are undesirable — like the Asian carp that is threatening to invade the Great Lakes, but far bigger, meaner and mounted on four legs. But efforts to eradicate or at least contain them have been hampered by the lack of a national policy to deal with invasive species as a whole, the slowness of states to recognize the problem and the bickering between agencies about who is responsible for dealing with them.

"As a nation, we have not thought through this invasive species problem, and we just have disaster after disaster after disaster," said Patrick Rusz, the director of wildlife services at the Michigan Wildlife Conservancy. Rusz, who travels around the state educating farmers about the menace posed by the wild pigs and encouraging them to set traps on their land, is so avid a hog-hater that in the early stages of Michigan’s invasion, he went to bars to eavesdrop on hunters who might have spotted the porcine invaders.

At least in Michigan, Rusz said, the pigs appear to be winning — their numbers are estimated at 3,000 to 5,000 and growing. Wild pigs are virtual Houdinis, able to dig or climb over almost any barrier; pig experts are fond of saying that "if a fence won’t hold water, it won’t hold a wild pig."

Allowing hunters to shoot them in the wild all year round, as Michigan and other states do, is not in itself enough to limit the population, Rusz said. So trapping is an important component of wild pig control, as are bans on owning or breeding the animals.

But state bans like an invasive species order issued by Michigan in 2011, which prohibited ownership of Russian wild boar and other feral swine, have been opposed by shooting preserves and other businesses with a stake in keeping them.

"The conundrum is that you’ve got one of the world’s hundred worst invasive animals, and at the same time you’ve got a highly desirable game species," Mayer said. "It’s a real Jekyll and Hyde type situation with wild pigs."

In the United States, "the pig bomb went off after 1990," Mayer said, when Northern states began holding hog hunts, which had long been popular in Southern states. Among other reasons, the pork is tasty, hunters say. In Texas and Florida, most feral swine are descendants of domestic pigs released into the wild or hybrids. Michigan’s wild pigs came primarily from escaped Russian wild boars imported from Canada for hunting on private game ranches.

Political battles over how best to control the pigs can become vicious. In Pennsylvania, the State Game Commission was scheduled to take a final vote this month on a regulation to prohibit private shooting preserves from owning feral swine. The regulation, said Cal DuBrock, the director of the State Bureau of Wildlife Management, was intended to keep the trickle of wild pigs from turning into a deluge.

"All of our counterparts across the nation have said, ‘Nip it in the bud, otherwise it will get away from you,"’ DuBrock said.

But state lawmakers, prodded by shooting preserve owners and some hunters, are on the verge of passing legislation redefining the term "wild animal" to exclude wild boar kept behind a fence, effectively removing them from the commission’s purview. The commission is now amending the regulation and will take a final vote in June.

In Michigan, the bruises still hurt from a fierce clash over the state ban that pitted farmers (afraid for their crops) and pork producers (worried about disease) against the owners of about 60 private game ranches that offered wild boar hunts and a few small pig farmers who kept the boars. The fight grew so heated that at one point, rock musician Ted Nugent, who owns a game ranch near Jackson, jumped into the fray, as did a conservative commentator based in Arizona, Mike Adams, who claimed that the ban meant Michigan regulators would be "kicking in the doors of all these farmers, shooting the pigs and then arresting all of these farmers as felons."

Boris, the wild boar Davenport shot behind his house, played a prominent role. His huge head and bristled hide were displayed at legislative hearings on a bill that would have nullified the ban, substituting regulations for fencing and health checks on penned wild boars. (The bill died in the Senate.)

The issue has been complicated here by a state-issued description of the physical characteristics of Russian boar. Just as different breeds of dogs have a common ancestor, all pigs are descended from the wild boar, and all fall under the species sus scrofa. But biologists say that generations of selective breeding have resulted in domestic pigs that look very different from their ancestors.

The differences, Rusz said, are as clear as those between a pit bull and a poodle.

"It’s simple," he said. "Russian boar — global track record of destruction, property damage, bankruptcy and spreading disease. Porky Pig on the farm — none."

But five lawsuits filed by game ranches and small pig farmers have challenged the Michigan invasive species order, in most cases putting forward some version of the argument that a pig behind a fence is by definition a domestic pig.

Meanwhile, the Michigan Department of Natural Resources is suing a shooting preserve, Renegade Ranch, for keeping wild boar after the ban went into force last spring.

Ron McKendrick, the owner of the 300-acre ranch, said that wild boar were cheaper for hunters than the deer and bison he stocks and made up 80 to 85 percent of his business, money he would lose if the ban holds. (A court ruling allowed him to keep the boars and offer hunts until the suit is resolved.) "I’m done if I don’t have hog hunts," he said.

On a recent Friday, a group of hunters pursued a wild boar for several hours on McKendrick’s ranch, through a pine forest still wrapped in a deep blanket of snow. They eventually shot the animal.

McKendrick argues that he should not be penalized for the actions of irresponsible ranch owners who have inadequate fencing — a hole in his 10-foot reinforced fence that in one instance allowed some boar to escape was made by vandals, he said.

Mayer, who consulted with Michigan officials on the ban, said that in battles against invasive species, some people are bound to lose out.

"Unfortunately, this law, which is for the greater good, is going to end up hurting people. There’s no question about that," he said. "It makes me glad I’m not a politician."

© 2013 The New York Times Company