Trapping hogs one sounder at a time

New insights on managing wild pig populations

adult wild pig in the evening sun
Wild pigs have been in the U.S. since the 1500s, when Hernando de Soto brought them as pork on the hoof. However, their population has exploded over the past 40 years. Photo by Timothy Gonsalves, courtesy of Wikimedia Commons.

Wild pigs are the largest invasive species in the U.S., and cost billions of dollars in damage to ecosystems and farms each year. New insights from the Savannah River Site are leading to better ways of managing them: whole sounder trapping, baiting strategies, and timing trapping efforts so that pigs are absent during critical portions of other species’ lifecycles.

The Savannah River Site is a nuclear reservation that is operated by the U.S. Department of Energy. The site is in South Carolina and spans 198,000 acres. Most of that land – 170,000 acres – is forested and managed by the USDA Forest Service.

In 1998, about 330 acres of the Savannah River Site became a municipal landfill. In the U.S., up to 40 percent of the human food supply is thrown into trashcans and is taken to a landfill.

“Pigs began foraging there in 2001,” says John Kilgo. “By 2009 more than 100 pigs per night were observed foraging in the landfill.”

Kilgo is a USDA Forest Service research wildlife biologist who has been monitoring the pigs since 2013, along with colleagues from the Savannah River National Laboratory, the University of Georgia and other organizations.

The landfill affects the way pigs move across the landscape together, as Kilgo and his colleagues show in the journal Scientific Reports. Pigs are sociable and live together in matrilineal groups called sounders. The study shows that sounders are territorial and suggests that trapping whole sounders together is more efficient than trapping individual pigs.

color coded plot of where pigs were present on the landfill
The colors show sounder territories at the landfill. Forest Service image.

Eighteen sounders live near the landfill, and it provided so much food that nearby pigs became less territorial. When closer to the landfill, the sounders shared as much as 60 percent of their home ranges with neighboring sounders.

“Because sounders may sometimes share home ranges, you can’t trap a group of pigs and assume that you have removed pigs from that area,” says Kilgo. “Especially if you’re near a landfill or a crop field. In that case you’re more likely to have multiple groups of pigs using that area.”

Over the past forty years, the number of wild pigs in the South has exploded. They are present in every southern state. They are likely present in every national forest in the South. Although it’s difficult to estimate how many wild pigs exist, there are likely more than six million.

At the Savannah River Site, the landfill didn’t just affect pigs’ territoriality. It caused their bodies to get bigger, as Kilgo and lead author John Mayer document in the Journal of Wildlife Management. As individuals got bigger, so did the population size. Although almost half of all newborn wild pigs die, as Kilgo and lead author Sarah Chinn of the University of Georgia report in the journal Scientific Reports, their larger size allows females to give birth to more piglets.

Wild pigs can rip up a field of peanuts, corn, or other crops overnight. In addition to trash, they eat acorns, berries, roots, salamanders, seeds, and young lambs or calves. They eat eggs – whether of ground-nesting birds or sea turtles. They are the most opportunistic of all omnivores.

Trapping and removing whole sounders could be timed to protect sensitive species at critical times, as Kilgo and lead author Guillaume Bastille‐Rousseau of Southern Illinois University report in Pest Management Science. In that study, areas without landfills or other concentrated food sources stayed pig-free for at least 90 days after trapping. Although other sounders are likely to move into that territory, their temporary absence could help protect crops or wildlife species.

three wild pigs in a field
Wild pigs can carry diseases that affect humans (leptospirosis, toxoplasmosis, and salmonella) as well as domestic pigs (swine flu). Photo by NASA, courtesy of USDA APHIS.

“In the South, trapping appears to be the most effective way to control wild pig populations,” says Kilgo. “And whether using traditional methods or whole sounder trapping, you have to get the pigs to come to the trap.”

Corn is usually used as bait. Kilgo contributed to a study on effective baiting strategies, which found that if bait sites are three kilometers away from each other, 80 percent of the wild pigs in the area would visit the sites within nine days. The study was published in the Wildlife Society Bulletin and led by Jacquelyn McRae of USDA Wildlife Services.

“Wild pigs do a tremendous amount of damage,” says Kilgo. “And in most states, we’re not going to be able to get rid of them. It’s a matter of limiting the damage. We need to improve control techniques and develop management approaches that lower wild pig populations and associated damage.”

Read the full text of the studies:

For more information, email John Kilgo at john.kilgo@usda.gov.

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