Can anything stop the big pig invasion?

Scientists are scrambling to root out wild hogs, a destructive invasive species


A feral pig looks similar to the domestic pig from which it has descended. Each year, these animals are blamed for causing large and amazingly costly damage in the United States alone.


The call came one morning in the spring of 2013. The cemetery was a mess.

Charlotte Watson remembers it clearly. She works in the courts in New York City. She also runs an organization that protects a historic cemetery in rural Texas, where she grew up. Named Willow Wild, this cemetery sits on 36 hectares (90 acres) in Bonham. The site is about 130 kilometers (80 miles) northeast of Dallas. Someone in Bonham who regularly visited the cemetery was the first on the scene.

“Something terrible had happened,” Watson recalls — wild pigs!

Overnight, wild pigs can destroy a lawn, garden or field of crops.
USDA APHIS photo by Scott Woodruff/Flickr (CC BY 2.0)

They had barged in and uprooted wide patches of grass. It looked like someone had ripped out the grass and tilled the soil. No grave markers were knocked over, but “it looked really bad,” says Watson. “You couldn’t imagine [the grass] would grow back.”

For the next few weeks, wild pigs slept under the surrounding trees by day and slipped into the cemetery by night: They came to root in the soil for grubs. These thick white worms, which would grow up to become beetles, live several centimeters (a few inches) below the soil surface.

The invaders weren’t going to leave quickly on their own. Watson and her group had to face some tough questions about how to deal with these far-from-benign swine.

Texas is hardly alone in facing marauding pigs. These wild swine can be found in nearly every U.S. state. They’ve also been spotted in Canada, and many cross the border from Texas into Mexico. In the United States, they have become concentrated in southeastern states. They also wreak havoc in other countries, including the United Kingdom and Australia. In Germany, hordes of pigs dig up gardens in the suburbs of Berlin.

Wild pigs each year feast on crops and cause $1.5 billion in damage in the United States alone.
USDA APHIS photo Craig Hicks/Flickr (CC BY 2.0)

Wild pigs cause some $1.5 billion in damage every year in the United States, mostly to crops, according to the U.S. Department of Agriculture (USDA). They also pose a health hazard. Wild pigs carry at least 30 diseases and 37 parasites (organisms that live and feed on a living host). Some of these diseases and parasites can spread to other animals. They can also infect people who eat or breathe the germs. And when cornered, wild pigs can, though rarely, attack people. Last December, for instance, a feral pig attacked the German hunter who had shot it. The man would later die.

Wildlife biologists around the world want to understand these feral swine to halt the menace. They’re tracking the animals to understand their behavior and predict where they’ll go. Researchers are testing new traps, including some that send real-time video to smartphone apps.

Stopping the pigs is difficult, in part, because they’re canny. “They’re one of the smartest animals on the planet,” notes wildlife biologist Alan Leary. He works for the Missouri Department of Conservation in the state’s capital, Jefferson City. “We have to continually come up with new techniques to stay ahead of them,” he says.

Right now, the pigs are winning.

Swine invasion

They go by many names: wild pigs, wild hogs, feral swine, feral pigs and wild boars. But they’re all Sus scrofa, a pig species native to Europe, Asia and North Africa.

A group of wild pigs can devastate corn or soybean fields overnight. The swine can shred riverbanks and wreak havoc near cities, even in people’s yards. They destroy landscaping. The muddy mess they leave behind often looks like the crater from a bomb.   

In the last few decades, the pig menace has worsened in the United States because the animals don’t have any natural predators. What’s more, people haven’t found an effective way to stop them. In the first week after the fastest highway in the United States opened — south of Austin, Texas — three cars collided with wild pigs. And then there was that F-16 fighter jet, back in 1988, that collided with feral pigs on a Florida runway. The pilot ejected to safety. His $16 million jet? Destroyed.

This herd of wild pigs is roaming through Australia.

There’s a term to describe critters like wild pigs: invasive species. These organisms don’t cause problems in their natural habitats. But when people have introduced them into a new environment, either on purpose or by accident, they tend to cause problems. Sometimes big problems. Invading plants and animals can quickly gobble up available resources and make it harder for other species to thrive.

Invasives might outcompete native species, causing the natives to decline. Or the invasive species could damage crops and natural areas, such as woodlands. Invasive insects might kill trees, leaving a forest more likely to burn. One 2005 study estimated that invasive species cause $120 billion in U.S. damage each year.

Pigs are not native to North America. Spanish settlers who colonized Florida in the 16th century brought along swine. For the first couple hundred years, populations of these animals stayed small and contained. They rarely roamed beyond the Florida panhandle.

Then hunters became interested in wild pigs toward the end of the 20th century and everything changed.

“Their popularity spawned hundreds of commercial fenced operations of wild boar hunts,” says Jack Mayer. He’s a wildlife biologist at the Savannah River National Laboratory in Aiken, S.C., and has been studying wild pigs for more than 40 years. Ranchers and farmers began to keep wild pigs for hunters. Alas, he says, the animals couldn’t be contained. “Virtually every state has some of those operations.” Now, he says, “At least one or more of those operations in each state is leaking pigs.”

And their wild populations have exploded in the past 20 years. Partly that’s because pigs can live anywhere, eat just about anything — from acorns to small animals — and reproduce quickly. They can adapt to almost any climate. Mayer says they’ve been spotted in 48 U.S. states (including Hawaii and Alaska). These wild swine have established populations in 36. For now, only Wyoming and Rhode Island appear to be free of feral pigs, says Mayer.

Feral pigs ramble freely in most U.S. states. Traps may help control where they roam.
USDA APHIS photo by Clint Turnage/Flickr (CC BY 2.0)

Leary, in Missouri, says people can be part of the problem. Maps show pig populations separated from each other by hundreds of kilometers (miles). The pigs probably didn’t hoof it all that way. People must have transported them. “We know that pigs don’t fly, and they had to get there somehow,” he says. Some people intentionally release wild pigs into an area to create a hunting ground, even though it’s illegal. Such actions give rise to new pig populations.

The problem isn’t going away. The Texas Department of Agriculture predicts that if nothing is done, the pig population in that state will triple within five years. A federal program, the National Feral Swine Damage Management Program, has been created to curb the invasive species’ expansion. Already, it estimates, the United States host some 5 million or 6 million feral pigs. And their numbers are growing.

Indeed, that growth shows no signs of slowing down, according to a 2017 study in the Journal of Applied Ecology. USDA researchers studied pig populations from 1992 to 2012. If they continue to spread at the same rate, it estimates that most counties across the United States will be plagued by wild pigs within 30 to 50 years. 

Hunting — sometimes even from helicopters

Wild pigs can run as fast as 48 kilometers (30 miles) per hour and scamper over fences a meter (three feet) high. These swine can reproduce once or twice every year, and a typical litter includes five or six piglets. (Some people in the South even joke that “pigs are born pregnant.”) A single pig may grow to weigh hundreds of kilograms (pounds).

Scientists have a lot of information about the habits and behaviors of wild pigs, says Mark Smith. He’s a wildlife biologist at Auburn University in Alabama. “Everybody’s staring at the same science,” he says. “Our role is to get the best information out there, see it and make good judgments off good science.”

Some scientists have run computer models of pig populations. Then they analyze what control tactics might prove most likely to bring those populations down. To completely rid an area of pigs, more than two-thirds of the animals have to be removed every year, those computer models suggest. And that removal rate would need to be continued year after year after year — until there were no more pigs.

How might that removal be accomplished? These are, after all, wily animals.

Some states have established hunting seasons. Others have brought in sharpshooters, or trained hunters. Others offer rewards for feral-pig carcasses. Texas passed a law in 2011 that allowed people to shoot the pigs from helicopters. Now some people pay thousands of dollars for the experience.

Beyond hunting

Smith doubts that hunting will ever solve the problem. Most hunters stop after they get one or two pigs. What’s more, some scientists have observed that pigs can learn from the hunts. They may adapt their behaviors to avoid hunters. Some might move away from sites where people prefer to hunt. Or the animals might eat at night, instead of by day. That could make them harder to find. Hunting and sharpshooting will likely only work for the last few pigs of a sounder. (Sounder is the name for a group of wild pigs.)

Leary says trapping offers the best chance of catching the most pigs. But the traps have to be smarter than the animals.

A motion sensor camera took this night time image of wild pigs caught in a trap in Tennessee.
Rodney Woodson

Pigs can climb, so the traps must be tall and not have sharp corners that can give a pig a hoofhold to climb out. And traps have to be able to catch all of the pigs in a sounder. If any get away, they’ll know enough to not return to this trap site. Then, unless they’re tracked down some other way, these pigs may colonize a new area. 

Newer traps incorporate new technologies. Some include motion-sensor cameras that connect to smartphone apps. The cameras watch the trap, which looks like a big ring of tall metal fencing. There are one or two open gates to the enclosure. When pigs arrive, the camera alerts the landowner or ranger. Then, someone can watch the scene in real-time, from wherever they are. Once all the pigs have wandered into the fenced pen, the trapper can drop a gate through the app with a swipe of a finger.

It’s not cheap, though. A basic trap will cost a farmer hundreds of dollars. With the sensors, cameras and app, that cost can climb into the thousands.

Traps also won’t be able to get all the pigs, says Mayer. So scientists are looking at other approaches. Biologists in Alabama and Colorado are studying possible poisons. But there’s no guarantee that only a pig will consume it. Texas, for example, has black bears. They will eat almost anything that pigs eat. Livestock also might take the bait. Researchers will have to figure out how to poison wild pigs without harming bears or other animals.

Meet the HOG-HOPPER. Animal-control experts in the United States and Australia are developing devices like this one. They use bait to draw wild pigs into a trap.
USDA APHIS photo Tyler Campbell/Flickr (CC BY 2.0)

At Auburn, Smith says veterinarians are also working on pig birth-control strategies. These are drugs or devices to prevent reproduction. Researchers have developed such drugs that work. But here’s the snag: Someone would have to inject it directly into each pig. And that isn’t practical for wild animals, which could be anywhere — and hiding. 

Such efforts to get rid of pigs have the best chance of working where the animals are new, say experts. But the challenge of removing every pig, permanently, is daunting. So scientists want to focus their efforts on reducing pig populations and limiting the damage they cause.

Smith says the way to reduce and control the wild-pig problem will take a combination of methods. First, though, people have to be convinced that their moving and releasing pigs is a serious problem. Traps may then be useful to get most of the pigs. Birth control or poisons, if they don’t cause extensive harm, may help. And sharpshooters may be able to get the last few. “Those last pigs are where you’re spending all your money,” says Smith.

Charlotte Watson, at the cemetery in Texas, went through her own ordeal to get rid of the pigs. First, she hired someone to set up traps. “Ideally, the pigs run in there and they can’t get back out,” she says. Then a trapper would come and get the pigs. The cemetery would pay for every animal caught.

Except that it didn’t work.

“They didn’t pay any attention to the traps,” she says of the pigs. “Of course, hogs are very smart.” A few weeks later, though, the pigs moved to another neighborhood. They haven’t returned. Though Willow Wild may have been spared for now, there’s no guarantee the swine won’t be back wreaking havoc once more.

Correction: The text has been adapted to note that explorers and settlers did not carry pigs to North America until the 16th century.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

More Stories from Science News Explores on Animals