Millions of marauding wild pigs have invaded large swaths of the southern U.S., eviscerating crops, gobbling up endangered sea turtles and trampling archaeological sites in a rampage showing no signs of letting up.

There are now six million of them in at least 39 states, and they are “rapidly expanding,” according to the U.S. Department of Agriculture. And they move in large groups called sounders. One instance in Ohio, 30 were caught in a single trap.

Feral hogs are invasive species that were brought to North America from Europe by Spanish conquistadors, and ever since, they have exploded across the country. Hogs use their snouts to dig through soil, leaving fields scarred and crops flattened. But they also kill livestock and reptiles.

The cost: $1.5 billion a year in damage and control spending, the USDA said in 2016. That has led to cottage industries of groups that exterminate the animals in all kinds of ways — including with firearms, though the hogs’ thick hides can help shield them from rounds fired from AR-15-style rifles.

Hogs are built to last. They produce large litters and wield stout tusks to defend themselves against cougars. “Hogs are tough, fierce, and hardy beasts,” Duke University professor Gabrielle Rosenberg wrote.

, and are helped by a general lack of natural predators and the ability to withstand different climates.

That leaves game officials, farmers and private industry to contend with the expanding pig crisis. Texas produced a uniquely Texan solution: Shoot them from low-flying helicopters. Sport hunters there can legally rent a helicopter jump seat and shoot fleeing hogs with AR-15-style rifles.

Of course, that has produced its own “Apocalypse Now” rip-off. In one video with a million views, a shooter fires at hogs with a semiautomatic shotgun, occasionally at nearly point-blank range, to the soundtrack of Creedence Clearwater Revival.

But Texas Parks and Wildlife has warned: if you’re going to hunt wild pigs, an AR-15-type of firearm may not be enough to pierce its tough hide.

“The best rifle calibers to use should be a .243 or greater to prevent wounding and loss of the animal,” the agency said, referring to a bullet used for hunting, which packs more of a punch than a typical .223 round associated with AR-15-style rifles. “Bowhunting, muzzleloading, and handguns are also popular among sportsmen to hunt feral hogs,” the agency said.

Since helicopter squadrons of rifle-toting hunters isn’t practical for the rumbling mass of feral hogs, Texas also tried to introduce pesticide to trigger a “hog apocalypse,” as the agricultural commissioner put it. But that plan had setbacks after 200 birds were found dead.

That leaves other methods, including elaborate camera-enabled traps and night hunts using infrared scopes.

Feral pigs are clearly dangerous to wildlife and agriculture. But are they a threat to people?

“In a natural state, feral hogs will prefer to run and escape danger and are not considered dangerous,” Texas game officials said. But people should still use caution, especially around wounded pigs, officials said. “Their razor sharp tusks combined with their lightning speed can cause serious injury.”