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 two 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 six million, with sightings in 47 states and established populations in 38 — “a national explosion of pigs,” as Dr. 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. Dr. 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.