Gene W. Wood, Professor Emeritus
Dept. Forestry and Environmental Conservation
Clemson University
The issue
In 2025, the United States Department of Agriculture estimated that feral hog populations had caused $1.6 billion in agricultural losses in 13 states for which they had information. The combined feral hog population totals for the top ten states exceeds 8 million. Texas is the leader with an estimated 2.6-3 million, although Oklahoma (1.5 million), Louisiana (700,000–750,000), Georgia (600,000), Florida (500,000) and New Mexico (500,000) also have major population levels.
Other states with major hog populations include South Carolina, California, Hawaii, Oklahoma and Alabama with a combined estimate of 1.9 million animals. It is notable that South Carolina, with a fraction of agricultural and forest land acreage of any of these five states, leads the group with an estimated 450,000 feral hogs.
One of the most recent alarming situations is Henry County, Texas where there is an ongoing major invasion of suburban areas. Landscape damage in that area has been “spiking” in recent years. Urban sprawl has been blamed as a factor in the issue, but the difficulties are incurred when interacting with people and their pets living in relatively high densities exacerbate any control measures usually used in rural and wildland areas.
How Did This Happen?
Hogs roaming freely in the wildlands of the southeastern U.S. has gone on since the 1500s when the Spanish introduced them to support their attempts at colonization as well as a potential food supply for shipwrecked sailors, especially in the Caribbean islands. As eastern America became increasingly colonized, farmers commonly free-ranged their hogs and cattle. Free range remained a common and legal practice through the early 1900s. Farmers ear-notched their livestock to tell which animals belonged to whom. Early fences were mostly built to keep livestock out of crops and yards. However, by the mid-1900s almost all states had outlawed free range. By then, feral populations (populations of free ranging animals of domestic descent) of hogs, cattle and some horses were well established in some places, particularly in lower coastal plain ecosystems.
In addition to these feral hog populations, European wild boar were brought to the United States by wealthy hunting interests. The first and most notable of these importations was in the Smoky Mountains around 1912. Fencing was constructed around the large hunting preserve, but that was short-lived due to falling trees and rot. Escaped wild boar quickly interbred with free ranging hogs which developed crossbred populations. (European wild boar and domestic hogs are the same species, thus capable of interbreeding and producing offspring that can reproduce.) Today we have feral hogs and feral-wild boar cross hogs, referred to here as crossbred populations. It is doubtful that any pure wild boar populations are still in existence in the U.S., although Michigan may be an exception.
Extensive genetic studies currently being done by the USDA Branch of Wildlife Services involve the collection of 40,000 samples hair, tissue and blood. 24,000 of these samples from around the nation have been analyzed for DNA information. Research goals include:
- Mapping the illegal movement of feral swine from release points back to the original source populations.
- Distinguishing between feral swine and domestic pigs. This supports State efforts to enforce laws prohibiting the possession and/or transport of feral swine.
- Identifying the evolutionary processes that are contributing to make feral swine so profoundly invasive.
Samples from 387 feral hogs from South Carolina have contributed to this data set. These studies reveal that DNA in the South Carolina population is: 47.9% Western heritage domestic breeds, 28.8% European wild boar, 7.0% Berkshire, and 5.4% Hampshire.
In the mountains and more temperate parts of the South, there is substantial advantage to being cross bred. Both feral and wild boar stock have two layers of hair. The top layer is composed of relatively thick, long hair, called guard hair, that serves mainly for protection from mechanical damage to the skin. The second layer, referred to as the undercoat, is a soft dense fur that serves for conservation of body heat. Development of the guard hairs is similar in both feral and wild boar stock. However, the undercoat in wild boar and crossbred stock is far heavier than in pure feral stock. This gives the cross-bred stock a better survival chance in extremely cold weather.
Two other differences exist in hair coat features. First, the guard hairs along the top of the back, particularly of crossbred boars (males), are typically highly pronounced and stand erect, particularly when the animal is angered. Older feral boars have a similar display, but the characteristic appears to be more pronounced in crossbred stock.
Second, while adults of crossbred stock are most typically black, all of those with significant amounts of wild boar DNA produce horizontally striped piglets (brown with white stripes). These piglets lose their stripes within a few months of life. Feral sows bred by crossbred boars are likely to have striped piglets. Feral sows bred by feral boars, produce colors ranging from varying shades of red to brown, shades of white, black, and mixtures of black and white.
The mixtures of colors are indicative of the wide range of breeds in the ancestry of feral and crossbred stock over many years. While the free-range stock origins are historically documented, intentional transport of feral and crossbred stock to new areas for sport hunting became a particularly major problem in the last part of the 20th century. This practice, while legal, exacerbated the developing ecological disaster we now face. In addition, in the last part of the last century, sport hunting plus intentional population control measures were totally inadequate as population suppressants. Natural population growth dramatically expanded the distribution and abundance of wild hogs.
Why Hogs
Once the basic biology of the hog is understood, the answer to the question of “Why Hogs?” becomes obvious. First, why was the hog chosen by humans to be a valuable domestic animal? Thousands of years ago in a period when certain wild species were being chosen for domestication by humans, observation showed hogs to have a very wide foraging amplitude. That is, they could eat an extraordinary array of both plant and animal tissues, including terrestrial and aquatic vertebrates and invertebrates, and convert those sources of energy and nutrients into their own body tissue which a human could consume. They were collectors and concentrators of environmental energy and nutrients that could save humans a lot of work at gathering.
In modern times, researchers have used these characteristics in hogs (mini-pigs) to study obesity as high digestion and metabolic efficiency in these animals easily produced large amounts of fat tissue. Hogs also have been used to study alcoholism as they are the only species other than humans known to voluntarily imbibe alcohol, sometimes showing a high predilection for intake.
Second, the hog has the highest reproductive capacity of any large mammal. In domestic swine operations, a sow is expected to produce at least two litters of 10-12 piglets per year. Although she is only likely to successfully nurture 10 offspring as she has 10 mammary glands and the piglets are territorial for each of those glands. Late born piglets in litters of more than 10 have a poor chance for survival.
Litter size in feral and crossbred populations in the U.S. is believed to average around 5 fetuses in utero (in pregnant sows). An extensive study in coastal South Carolina which included fetus counts in over 100 sows showed an average of 5.1. (Baruch Forest Science Institute, Clemson University) Studies involving fetal counts in European crossbred stock in Great Smoky Mountains National Park found an average of 4.9.
Sows and boars are sexually mature at six months of age. Even in situations of high mortality rates, hogs will survive through natural reproductive pressure that resists natural population expansion suppressants.
A case history is developing in Japan that illustrates the potential for rapid development of crossbred populations. The Tohoku earthquake and subsequent tsunami in 2011 caused the Fukushima Daiichi nuclear reactor disaster. The area surrounding the reactor quickly became contaminated with radioactive isotopes at levels harmful to human health. The original contamination zone was 312 square miles, but by 2017 that had been reduced to 143 square miles.
Farms and forest lands were extensive in the area. The forest lands had a population of European wild boar while the farms had substantial populations of domestic swine. The area contained a population of 164,000 people which had to be evacuated. Farm livestock were either slaughtered on site, died of starvation due to human abandonment, or escaped after human abandonment.
Today, a large, thriving population of wild hogs roams the abandoned area. Scientists have demonstrated a substantial occurrence of mitochondrial DNA from domestic sows. Some believe that hybrid vigor of the crossbred stock is an important factor in the rapid growth and vigor of this new population. The crossing seems to have combined the natural instincts for survival of the European wild boar with the capacity for high levels of reproduction from the sow ancestry.
A second population expansion scenario of great concern is occurring in the southern portions of central Canada. A crossbred population referred to as “super pigs” is expanding rapidly and is expected to invade adjacent areas of the northern U. S. Midwest in the future. Again, hybrid vigor and the presence of a critically important undercoat needed to withstand very low temperatures are considered to be primary factors in resistance to environmental suppressants.
The high intelligence level of wild hogs is another important factor contributing to their efficacy in adaptation and survival. Hogs are ranked among the five most intelligent animals. They are more intelligent than dogs, and better than dogs at solving puzzles and spacial memory. Field observations by Clemson scientists working in the South Carolina coastal plain noted excellent nest-building capabilities in feral hogs building elaborate nests for protection from cold in winter and a different type of nest for protection from biting insects in summer.
Finally, wild hog populations may carry 30 viral/bacterial diseases and 40 species of parasites. Among the diseases, brucellosis and pseudorabies (PRV) may be the most widespread and important to humans and their domestic livestock and pets. Brucellosis is a bacterial disease most commonly contracted by humans from unpasteurized dairy products. However, it can be contracted by direct contact between a skin abrasion or open wound of a human or domestic animal with raw meat of an infected hog. It can be devastating to commercial swine farming by causing “abortion storms” in the herd. Brucellosis has been found often among slaughterhouse workers in the past.
All states strive to be declared by the USDA as brucellosis-free. South Carolina was on the cusp of that designation in the mid-1970s when the disease was demonstrated to exist at a significant level in a coastal feral population by Clemson University scientists. At that time, Florida had already demonstrated substantial levels of brucellosis in its widespread feral hog population. The testing of over 1500 feral hogs killed in the past 10 years in South Carolina during control measures undertaken by federal APHIS (Animal and Plant Health Inspection Service) personnel found antibodies for brucellosis in 15% of animals examined.
Brucellosis is a disease dreaded by the domestic swine industry. If this disease is found on a farm, all swine must be quarantined and culled, but they may still be sold directly for slaughter.
Swine are the host of Pseudorabies virus (PRV). PRV does not affect humans, but it is lethal to pigs (particularly piglets), dogs, cats, cattle, sheep and goats, as well as a number of wild mammalian species. The same federal control program being conducted by APHIS and referred to above for brucellosis found 21% of 1500 hogs tested to have antibodies for this virus.
Wild hogs host a wide range of tick species of which two are exceptionally important to human health. Deer tick is infamous for its transmission of Lymes disease to humans. The brown dog tick is well known for its transmission of Rocky Mountain spotted fever.
Trichinosis is a human disease caused by Trichinella round worms that may be found in the muscle tissue of hogs. It is contracted by humans through consumption of inadequately cooked meat. Symptoms range from mild flu-like reactions to serious damage to major organs and brain tissue.
Population Control Measures
In the late 1970s and continuing into 1993, Clemson University scientists, publishing in scientific journals, warned of the ecological disaster developing with feral hog populations. At the time, these warnings went unheeded at best. Today, virtually no land manager finds the issue to be unimportant.
Traditionally, the most extensively used attempts at control have involved trapping and immediate euthanasia by whatever means proved to be most financially expedient and within manpower and budget limits. Intensive trapping efforts generally were limited to areas to which traps could be transported to and carcasses removed from by motorized vehicle.
Sport hunting can contribute to control measures, but it also has important limitations. First, rifle hunting is sometimes constrained by state laws to the deer season. Therefore, even for hunters that may want to shoot a hog, the opportunity to do so is seriously time constrained. Second, even within the deer season, some hunters will refrain from shooting a hog as the action might preempt their opportunity to shoot a deer.
Where legally allowed, hunters are now using night vision scoped rifles to shoot hogs at night. An important technological step has been taken in the use of drones with night vision cameras to locate hogs that hunters can stalk and shoot.
Hunting with hounds and catch-dogs is usually open year-round, although very few hunters have the dogs needed for this sport. In addition, the spectacle of a hog-dog fight is among the most violent that occurs in any recreationally legal endeavor in the U.S.
The good news is that control measures have vastly improved in the last couple of decades. First, and by far foremost, has been the development of the National Feral Swine Damage Control Program which was inaugurated by congressional funding of the 2014 Farm Bill. The program, which today receives annual funding of around $20 million is managed by APHIS. It aims to minimize and ultimately prevent the excessive damage done by wild hogs to agricultural and forest crops and wild ecosystems. Program personnel, including professional trappers, are located in all states with substantial wild hog populations.
Federal initiatives have provided for hugely improved control measures. One of the greatest advances has been funding required to support projects in which large numbers of wild hogs are shot from helicopters. This technique has proven to be highly efficient and efficacious. It is an approach that was impossible in the old days due to prohibitive costs.
The program has also promoted and supported use of a new and highly innovative trap commercially named the “Pig Brig.” It is a net trap that is relatively easily transported and erected, and which is highly efficient at multiple animal captures in one setting. The device is expensive ($2500 each), but affordable under federal program support.
In the last 10 years since the National Feral Swine Damage Control Program was established, APHIS and partners have successfully eliminated feral swine from 8 States (Colorado, Idaho, Iowa, Maine, Maryland, Minnesota, New Jersey, and New York), and recognizes 4 States (Indiana, Vermont, Washington, and Wisconsin) in detection status.
Other ideas for population control include drugs that control reproductive physiology and use of toxicants. Chemically induced suppression of reproductive potential of females is a procedure that has worked in humans. Females set the timing and frequency of conception in all species. Wild hogs are not seasonal breeders mimicking native wildlife, thus they are similar to humans in this respect. The primary control for human population growth is through drugs administered to adult females on a continuing periodic basis. There is no way to accomplish such a procedure with wild hogs.
Virtually no population control is accomplished by attempting to remove the wild hog male contributions to reproduction. Dominant males do almost all of the breeding, One male can breed many sows. It is impossible to remove enough males from the population of breeders to have a measurable effect on pregnancy rates.
The next most common idea is about toxicants. The overwhelming problem confronting implementation of the use of toxicants for wild hog control is that of impacts on non-target species. In recent years, Louisiana State University developed a toxicant proven to be highly palatable to hogs and highly lethal. However, the substance has not been licensed for landscape scale use because there is, so far, no delivery system that will prevent the toxicant from being ingested by non-target wildlife, such as racoons and bears.
A Model for the Future
The state of Missouri has likely set an example for truly meaningful control of wild hog populations. In fact, their goal is total eradication. They have made sport hunting of wild hogs on public lands and lands leased for conservation purposes illegal, thus there is no incentive for people to be transporting and releasing hogs for sport hunting. Furthermore, such transport and release activities are illegal. Landowners may shoot wild hogs on their own lands, but they cannot conduct commercial sport hunting projects.
Missouri, working closely with APHIS and an array of other partners, takes full advantage of all federal support they can get, including removal by helicopter shooting projects. The result has been to demonstrably reduce wild hog populations and concomitant agricultural damage in the state. All other states need to study the Missouri model and appropriately adapt it to their own socioeconomic and ecological conditions.
Summary
In summary, the catastrophic increase in distribution and abundance of wild hogs in the U.S. has been the greatest invasive vertebrate species catastrophe in the history of the nation. Annual agricultural damages are measured in billions of dollars. Hogs are likely the most adaptable of large vertebrate species. They also have the greatest reproductive potential of any large vertebrate species. The most important population and damage control measure is the National Feral Swine Damage Control Program instituted by the 2014 Farm Bill and managed by APHIS at a current funding level of approximately $20 million annually. While all states with significant wild hog populations have begun to take the issue seriously, Missouri appears to be the model for making demonstrably major progress in population and damage control. In the last 10 years APHIS and partners have successfully eliminated feral swine from 8 States, and recognizes 4 States in detection status.
Acknowledgement
Information about and gathered under the APHIS National Feral Swine Damage Control Program was supplied by James E. Combee, ER Aviation Logistics Manager, USDA Wildlife Services, Savannah River Ecology Lab, Aiken, SC 29802. Current wild hog population numbers were obtained via Google Search and verified at State websites (such as Texas Parks and Wildlife).
Scientific articles on this topic by the author
Wood, Gene W., and Reginald H. Barrett. “Status of Wild Pigs in the United States.” Wildlife Society Bulletin (1973-2006), vol. 7, no. 4, 1979, pp. 237–46. JSTOR, http://www.jstor.org/stable/3781857
Wood, Gene W., and Ronnie E. Brenneman. “Feral Hog Movements and Habitat Use in Coastal South Carolina.” The Journal of Wildlife Management, vol. 44, no. 2, 1980, pp. 420–27. JSTOR, https://doi.org/10.2307/3807973.
