Linda F. Benedict, Reed, Donald P.
Michael D. Kaller and Don Reed
In the eyes of many conservationists, feral swine are among the most damaging invasive species around the world. Farmers, ranchers, foresters and landowners consider the feral swine to be a nuisance, at best, and, more commonly, a grave threat. However, feral swine also may be culturally and economically important to hunters and other enthusiasts. Consequently, the invasive feral swine is one of the most controversial animals on the landscape.
The first documented introductions of swine into what would become the United States occurred in 1539 in Florida by Spanish explorer Hernando de Soto. In early settlements, it was common practice to allow domestic swine to range freely on public or common lands to be recovered for slaughter. This practice was extensive across the United States until the 1930s, with some exceptions that exist today.
In addition to domestic swine, the European boar was introduced into the United States by sport hunters. Although the European boar is in the same species as domestic swine, it has a number of characteristics different from domestic swine, including a more distinctive hair pattern and a tendency toward larger tusks that make it a more desirable hunting quarry. The bloodlines of these various swine have joined over the years, creating individuals with mixed genetic makeup. Therefore, feral swine in a given area may be descendents of domestic stocks, pure wild boar or a mixture of the two strains. As a result of these introductions, feral swine are believed to be present in 40 of 50 states. Although the genetic composition of Louisiana feral swine has not been investigated, the introduction of European boar in Louisiana is believed to be limited, and most Louisiana feral hogs are likely descended from domestic stocks.
In Louisiana, feral hogs can be found in a wide variety of habitats ranging from tidal marshes to timbered areas. They prefer hardwood forests that produce mast, such as acorns, but will frequent pine forests. In remote areas or where human disturbance is minimal, they can often be found in open ranges or pastures. Although feral swine generally prefer less interaction with humans, feral swine are adaptable and have successfully colonized parks in large cities, including Dallas and New Orleans.
Feral hogs seek areas with access to water, cover and a dependable food supply. They are omnivorous in their feeding habits, consuming anything from grain to carrion. Heavy mast crops will often hold them in smaller areas during the fall and winter, but human disturbance and lack of food are the two greatest factors in causing nomadic behavior. Areas in Louisiana that may never have had populations of feral hogs will suddenly begin to see the rooting, wallowing and other telling signs of their presence. Other areas, where feral hogs have occurred in the past, may go for years without a sighting when suddenly individuals begin to be seen once more.
Feral hogs are the most prolific large mammal in North America, and given adequate nutrition, populations can double in only four months. Sexual maturity is reached as early as six months, with sows producing two litters per year. Litter size varies with age and nutritional intake but five to six young are produced on average. Estimates from Australia suggest that based on reproductive capacity, feral hog populations can rebound from a 70 percent cull in only two and a half years. Other studies suggest that reductions of adult females greater than 90 percent may be ineffective because of the ability for juvenile females (6-12 months old) to reproduce. Therefore, where food and cover are available, feral hog populations can explode and spread quickly.
Beyond their remarkable reproductive capacity, feral hogs have had assistance in their spread. Legal and illegal introductions have extended the range of invasive feral hogs beyond the historic southern and western distribution (North Carolina, South Carolina, Florida, Georgia, Alabama, Mississippi, Louisiana, Arkansas, Missouri, Texas, New Mexico, Arizona, California and Hawaii). West Virginia introduced the European boar to supplement whitetail deer hunting opportunities, and legally permitted introductions have occurred in other states. More frequently, however, introductions for hunting purposes have been illegal and clandestine. Because these introductions are unauthorized, they sometimes go unnoticed by state and federal agents until problems occur. For example, the first reports of escaped feral hogs from hunting reserves in Michigan occurred in 2004. By 2008, hogs were reported in 61 of 83 counties, including counties in the snowy Upper Peninsula heretofore thought to be too cold for feral hogs. Because feral hogs can expand rapidly, it is likely that they were present but not actively being sought by state wildlife agents. Consequently, the seemingly rapid expansion was more probably a matter of finally documenting existing populations. Similar illegal introductions associated with game ranches and private hunting groups have occurred in Colorado, Illinois, Iowa, Ohio, Kansas, Kentucky, New Hampshire, New York, Oregon, Pennsylvania, Tennessee, Virginia, Vermont and Wisconsin. These states are implementing eradication and educational programs aimed at halting the spread of feral hogs.
Nationwide damage from feral hogs has been estimated to be more than $1.5 billion dollars annually. These estimates include damage to agriculture and forestry operations and public health and control costs. They do not include the damage effects to other wildlife, including predation and displacement of irreplaceable endangered species, aesthetically important species and recreationally important wildlife. The damage estimates also do not include impairments to aquatic ecosystems and water quality or property damage to vehicles from collisions, home gardens and other personal property.
Feral hogs have been documented to destroy shrub-nesting and ground-nesting bird nests and to consume turkey poults, white-tailed deer fawns, small mammals, turtles and their eggs, fish, snakes, frogs, and salamanders, oysters and other mollusks. Further, feral hogs displace and out-compete adult wildlife, such as white-tailed deer and wild turkey, from desirable habitats because of their more aggressive nature. Wildlife-related effects are difficult to quantify, and the real monetary damage from feral hogs is probably far greater than official estimates.
Feral hog damage to agricultural and forested lands is better documented. Recent estimates of damage in east Texas range from $3 million to $6 million per year. In 2008 and 2009, the LSU AgCenter conducted surveys along with the Louisiana Department of Wildlife and Fisheries to determine the extent of the distribution and damage from feral pigs. In 2008, 80 percent of survey respondents in 50 parishes reported feral hogs. Of those respondents, 95 percent reported damage to their property, crops or forests. Results from the 2009 survey have not been completely tallied, but the trends are similar to 2008.
Feral pigs will travel extensively for food, including hay, small grains, corn and peanuts. Similarly, tree seedlings are an attractive food resource, possibly for saps and starches. Vegetables, watermelons, soybeans, cotton and tree fruits are reported to be less attractive.
Beyond direct consumption of agricultural crops and seedlings, feral hogs can be highly destructive to roads, ditches, levees, outbuildings and other physical structures, as well as agricultural infrastructure, such as airplane landing strips and irrigation systems.
Adult livestock are sometimes preyed upon by feral hogs. Feral hogs are attracted to birthing grounds, and young goats, lambs and cattle, as well as the occasional livestock giving birth may be killed. Feral hog depredation is difficult to document because the hogs completely consume the carcass. Further, pigs scavenge animals killed accidentally by disease or from other predators. Lastly, rooting and wallowing behaviors destabilize the ground and can lead to accidental livestock death and injury, reductions of beneficial soil insects and microbes, and compaction of soils reducing agricultural and forest productivity.
Disease is another critical issue associated with feral hogs. Feral hogs carry diseases that may infect humans including brucellosis, leptospirosis, salmonellosis, toxoplasmosis, sarcoptic mange, Escherichia coli-related illness and trichinosis. Most human infections from feral pigs occur among hunters, and precautions such as wearing rubber gloves and masks while cleaning feral hogs are important to adopt by hunters and landowners removing feral swine. Humans also may contract diseases from feral pigs from contact with water that feral pigs frequent. More commonly, feral hogs infect livestock they encounter at watering sites and feeding areas with pseudorabies, swine brucellosis, tuberculosis, vesicular stomatis and classical swine fever.
Because feral pigs cannot be vaccinated or readily destroyed when disease is detected, they often act as disease reservoirs re-infecting livestock in a cyclic pattern. Further, feral pigs in Mexico and Central America are known to carry foot-and-mouth disease, which has been eradicated in the United States since 1929. Estimates suggest a loss of $14 billion to $21 billion to the U.S. agricultural industry if feral hogs were to become infected or carry foot-and-mouth disease into the country. The U.S. Department of Agriculture Animal and Plant Health Inspection Service, Louisiana Department of Wildlife and Fisheries, and Louisiana Department of Agriculture and Forestry conduct disease surveillance on feral hog populations in the wild and on feral hogs brought to livestock sales. These agencies also respond to hunter concerns about harvested feral pigs.
Because of Louisiana’s extensive and ecologically and economically important wetlands and aquatic resources, it is the ideal location to study the impact of feral pigs on wetlands and waters. Feral pigs have an affinity for wet areas and are well-known for their wallowing in wet soils and shallow waters. Further, in warm climates feral hogs spend most of their time in close proximity to water sources. In a 1998 study, scientists at the U.S. Geological Survey National Wetlands Center in Lafayette concluded that feral pigs had similar negative impacts to marshes and wetlands as nutria.
More recently, LSU AgCenter scientists studied the effects of feral swine on aquatic ecosystems and concluded that feral swine introduce pathogenic bacteria Aeromonas, Staphylococcus aureus and Shigella and reduce beneficial aquatic insects and mussels. In another study conducted in cooperation with the U.S. Army at Fort Polk, AgCenter scientists concluded through use of DNA source tracking that the excessive E. coli levels in a stream in western Louisiana were from local feral pigs. Feral swine impacts to small streams and rivers, specifically the reductions in beneficial insects and mussels, have important implications downstream with reductions in the appropriate nutrients for coastal estuaries and wetlands that may further threaten coastal ecosystems. Therefore, management of feral hogs is another important aspect to conserving Louisiana’s wetlands and aquatic resources.
Control and management of feral pigs is difficult. There are no repellents or toxicants registered for control of feral hogs. Fencing is generally impractical because of the large areas that often must be protected and the persistent nature of feral hogs in their rooting and feeding habits. Trapping and shooting are the most effective control measures. Where feral hog densities are high, trapping is the most effective control method. Stationary corral-type traps and moveable box traps are two designs that can be used. Research suggests that large corral-type traps that are baited to habituate the feral hogs are more effective than box traps. Corn is the preferred bait to use with either trapping method.
Feral hogs are classified as outlaw quadrupeds in Louisiana, and until new laws were passed in 2010, they could be hunted at any time of the year during daylight hours on private lands and during legal hunting seasons on public land. Because many landowners requested night hunting opportunities, which are allowed in other states, the Louisiana Legislature passed a law in 2010 allowing nighttime hunting from March 1 to Sept. 1.
In 2008, surveyed landowners reported that a combination of daytime shooting and trapping was the more effective management activity. Unfortunately, 25 percent reported no success in managing their hog populations with any method. Nevertheless, landowners should continue management efforts. A Texas study indicated that every dollar invested in technical assistance to landowners returned $11.42 in increased revenue for the landowner.
In another law passed in 2010, the Louisiana Legislature removed the exception to the free ranging of feral hogs where the practice is still allowed. In no area of Louisiana is it legal for swine to run at large upon public property or the property of others.
Feral hogs have been in Louisiana since the earliest settlements. They were a source of meat for the region’s emblematic gumbos, sausages and cochon de lait and remain a popular sporting animal among some Louisianans. However, despite their historical and cultural significance, the feral hog of today is a threat to productive farms, livestock operations and forests. Further, feral hogs have tremendous negative ecological impacts to wildlife, aquatic life and wetlands. Therefore, despite a few benefits to some people, the invasion of feral hogs into Louisiana has been a loss to the state.
Michael D. Kaller, Assistant Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.; and Don Reed, Professor, Bob R. Jones-Idlewild Research Station, Clinton, La.
(This article was published in the fall 2010 issue of Louisiana Agriculture.)