The first domestic pigs came to the U.S. on the boats of Spanish explorers. Swine were carted along as hedges against starvation and to seed New World settlements. They could be unruly companions. When pigs went feral in the West Indies, they "often attacked Spanish soldiers hunting rebellious Indians or escaped slaves, especially when ... cornered," writes Mayer in Wild Pigs of the United States.

In 1539, Hernando de Soto brought a gaggle of hogs to Florida; then he marched through the Southeast, shedding escapee swine along the way. By 1562, Florida Indians were hunting established wild populations. A few years later, a Spanish crew brought over 400 more, and so on.

Back then, and on Western homesteads centuries later, pigs often roamed freely and were hunted when meat was needed. Today, many feral pigs are descended from these free-range domestics. Others are part Eurasian wild boar, released in some states for sport. Some have pure Eurasian lineage, and all belong to the species Sus scrofa.

They are a hardy and adaptable bunch, with undiscerning  appetites: ground-nesting bird and turtle eggs, frogs, lizards, acorns, tubers, grubs. Their foraging snouts turn ground upside down, facilitating weedy invasions. They degrade water quality, and decimate and contaminate crops. (Feral pigs were partly to blame for the 2006 E. coli outbreak in California's spinach fields.) They carry diseases such as pseudorabies, which can be fatal to livestock, and swine brucellosis, which gives humans a flu-like illness and causes abortions in animals. Feral hogs have few natural predators. And they are piglet-making machines: Studies show a population must be culled by 50 to 70 percent annually just to remain level.

Conventional wisdom used to be that once you had feral hogs, you were stuck with them. The same thinking applied -- and in many cases still does -- to many invasive species, whether mammal, plant or mollusk. Eradication efforts are expensive and biologically complex. And some infamous failures have made them politically risky. (A long and pricey campaign to eliminate fire ants from the Southeast in the '60s and '70s was more effective at wiping out the ants' predators. Biologist E.O. Wilson dubbed it "the Vietnam of entomology.")

But an increasing number of successful counterinsurgencies in recent decades, particularly against island-invading mammals, have begun to shift attitudes. In 2006, for instance, feral goats were wiped off the largest island in the Galapagos using sophisticated "Judas" techniques, which employed artificially sexed-up but sterile females wearing radio collars to lead gunners to the last holdouts. "With each (eradication) that's successful, people think, 'Why can't we do this one, and that one?' " says Dan Simberloff, a University of Tennessee professor.

Pigs have also been eradicated from islands and some inland locales, most notably in the U.S. from Santa Cruz Island off the California coast in 2007. Because of the island's size and rugged terrain, "most people said we couldn't do it," says Norm Macdonald, president of Native Range (then called Prohunt), the private contractor hired to do the job. But it was relatively straightforward, he says: After fencing the island into five zones, they used traps and aerial shooting to remove most of the population, then hunted with dogs and released Judas pigs. The team killed 5,036 hogs, most in a little more than a year.

Enthusiasm for bigger and tougher eradications has been slower to reach the mainland due to technical, financial and political constraints. Killing animals, even invasive ones, invites controversy. But some states with relatively new, small or isolated colonies are beginning to accept the challenge. Kansas has already eradicated four local populations, and the rest could be purged from the state within two years, says John Johnson, one of two USDA wildlife biologists charged with eradication there. In 2006, Oregon's Invasive Species Council advised lawmakers to dedicate $1.29 million over four years and $50,000 annually after that to eliminating hogs. The Forest Service just floated an eradication proposal for the Cleveland National Forest in California. And a group of state and federal agencies finalized New Mexico's eradication plan last fall.

These schemes follow a basic tenet of invasive species management: If you can't keep interlopers out, find them early and attack aggressively. In practice, though, it's not easy to do. Pigs, like many invasives, leave evidence of their presence: wallows, rubbed trees, rooted ground. "But likely as not, you're not going to see any pigs," says Mayer. "(People think) it must not be a problem. That's not the case."