An Overview of Wild Pigs
Wild pigs are non-native invaders in North America and their introduction causes problems for native plants and animals. Domestic pigs were first transported to this continent from Europe in the early 1500s by early explorers such as Hernando de Soto. Originally introduced as free-ranging livestock, some animals escaped and eventually established feral (wild) populations. Eurasian or “Russian” boar were also introduced to the continent in the late 1890s and early 1900s primarily for hunting purposes. While initial releases of Eurasian wild boar were on fenced properties, many later escaped confinement, resulting in hybridization with wild pigs originating of domestic stock. Intentional (illegal) and accidental releases of wild pigs since that time have further contributed to their numbers and distribution in North America. Today wild pigs can occur in three varieties: feral hogs (wild pigs of domestic origin), Eurasian/domestic hybrids, and Eurasian boar. Few, if any, true Eurasian boar remain from early stockings.
Biology and Behavior
Interbreeding among feral domestic pigs and Eurasian wild boar resulted in the wild pigs now found on the landscape. With an average lifespan of 4-5 years, sows (adult females) are capable of producing 1-3 litters per year. Females are typically sexually mature at 1 year, but pregnancy has been documented as early as 6 months. As sows mature they can produce larger litters, but those of 4-6 offspring are most common. Boars (adult males) average around 220 pounds in size, while sows average around 170 pounds. Adult sows and juvenile males and females commonly travel together in groups called “sounders,” while boars (males) are typically solitary. Adults rarely exceed 400 pounds without access to supplemental or other food sources meant for livestock.
Wild pigs lack sweat glands and must therefore modify their behavior in order to keep cool. Wallowing, occupying shaded areas, restricting day time activity, and remaining near water sources are ways in which wild pigs accomplish thermoregulation. While such modified behaviors help to keep wild pigs cool, they also act as biological drivers which contribute to water quality and riparian ecosystem impacts including bacterial impairments in areas where wild pigs are present.
Classified as opportunistic omnivores, wild pigs are known to consume a wide variety of food items in competition with livestock and native wildlife. While the vast majority (>80%) of their diet consists of vegetation, opportunistic predation of livestock including juvenile sheep and goats as well as wildlife species including reptiles, amphibians, ungulate fawns and the eggs of ground nesting birds including quail and wild turkeys is not uncommon. Predation of juvenile wild pigs by animals such as coyotes, bobcats, foxes, and others does occur, however humans and mountain lions (Puma concolor) remain the primary predators of adult wild pigs in North America. The lack of significant predation, combined with the high reproductive output makes intentional management and sustained abatement programs essential to reducing wild pig populations.
Wild Pig Sign
Wild pigs are a highly mobile species and will routinely move to different areas in their habitat. As they move about, they will generally leave visible signs. The most noticeable and easily recognizable sign is damage cause by their destructive rooting behavior as they search for food. Another highly visible sign are wallows. During warm months, wild pigs create wallows in moist areas near ponds, creeks, and sloughs to access mud, which helps them cool down and ward off biting insects. After wallowing, wild pigs will rub on objects to remove dried mud, hair, and parasites. Mud and hair can be found on trees, fallen logs, fence posts, rocks, and utility poles, particularly those near water or wallows.
Other sign, wild pig tracks, can sometimes be hard to distinguish from native species, such as white-tailed deer. Wild pig tracks will have blunted or rounded toes, whereas deer tracks are typically heart or spade shaped with sharp pointed toes. As they move across the land, wild pigs will frequently use the same path and create a well-worn trail. Where these trails cross under fences, the pigs often leave hair and mud on the wires as they pass underneath. Wild pig droppings can vary in shape and consistency depending on the main component of their diet. Their droppings are often tubular, filled with mast (such as acorns and pecans) and other vegetation, but it can range from resembling those of domestic dogs to those of horses. A diet of young grasses and shoots results in loose tubes and formless patties.
One of the more difficult wild pig signs to identify are beds. They will create shallow beds by overturning the soil to expose the cool dirt to lay in and will spend significant amounts of time in these beds during the heat of the day. They typically bed in dense vegetation such as vines, fallen trees, and other thick or thorny plants, since these areas offer security and shade.
The damages associated with wild pigs are complicated by their destructive feeding habits, high fecundity, adaptability, biological drivers, ecology, potential for disease transmissions, popularity as an exotic game species and growing populations. They negatively affect water quality, habitat, native wildlife species, livestock production and threaten both human and animal health. As populations continue to expand, issues with wild pigs in urban/suburban areas are also a growing concern. In 2007, the estimated economic toll of these animals in the U.S. exceeded $1.5 billion; an economic impact likely to be much larger today. In Texas, wild pigs are estimated to cause $52 million in annual agricultural damages alone. Examples of wild pig damages include: bacterial impairments, vehicle collisions, habitat loss, proliferation of invasive plant species, urban/suburban (turf grass, landscaping, etc.), livestock depredation, disease transmissions, opportunistic consumption of native wildlife species, rooting of pastures/rangeland and others.
Wild pigs negatively impact water quality largely due to behavioral drivers related to their physiology. Because wild pigs do not have sweat glands, they commonly wallow in and near water sources to keep cool. This process covers their skin with mud that they rub off on trees and utility poles to remove external parasites. However, wallowing damages riparian zones, and has been shown to increase sedimentation and cause major changes to native plant communities by creating more disturbance for exotic and invasive species to establish themselves. At the same time, wild pigs defecate in and around water sources, thereby increasing levels of bacteria (E. coli) and nutrients including nitrogen. Increased nutrients in the water can lead to excessive algae growth, altered pH levels, reduced dissolved oxygen levels and sometimes complete oxygen depletion (anoxia).
In some areas, pigs are contributing to water quality degradation so severely that the waterbody cannot support human recreation (swimming, wading, etc.) or aquatic life, such as freshwater mussels and aquatic insects. Without consistent and widespread abatement efforts, wild pigs will likely increase their numbers; thus compounding their impacts on water quality in the future. Reducing wild pig populations has been shown to improve water quality and riparian system processes. Local ecosystems benefit through reduced bacteria levels, reduced spread of invasive species, increased plant cover, decreased erosion and decreased sedimentation. These benefits have been shown to increase the health and function of watersheds, improve riparian habitat and reduce the potential disease transmission to humans and livestock.
Wild pigs cause tremendous damage within many agricultural sectors including row crops, pastures, livestock production and others. Agricultural crops serve as an easy source of food for wild pigs and they will commonly consume a variety of types including sugar cane, corn, grain sorghum, soy beans, wheat, oats, peanuts, rice, lettuce, spinach, melons, and pumpkins. As they forage through both row crop and pasture fields, they can damage plant roots through eating, rooting, wallowing and trampling. The disturbance and loss of crops or grasses in turn often encourages the growth of undesired invasive or weed species. Wild pig rooting and wallowing behavior can create sizeable holes or ruts, making it challenging or sometimes impossible for farm equipment to navigate the area and creates potentially hazardous situations for equipment operators.
Wild pigs are capable of transmitting a variety of pathogens to livestock, causing financial losses due to reduced productivity or mortality, and increased veterinary costs for producers. They also may opportunistically prey on newborn livestock including sheep, goats, lambs or calves. Lastly, direct competition with livestock for use of supplemental feed, mineral supplements and water sources can deplete and/or potentially lead to the contamination of these resources.
Wild pigs are capable of carrying and transmitting many different diseases, including swine brucellosis (Brucella suis), pseudorabies (Herpesvirus suis) and tick-borne diseases (Rocky Mountain spotted fever (Rickettsia rickettsia), Tularemia (Francisella tularensis), Lyme disease (Borrelia spp.), Ehrlichiosis (Ehrlichia spp.), and Southern tick-associated rash illness (STARI) in humans, and Anaplasmosis (Anaplasma spp.) in cattle, among others.)
Swine brucellosis is a bacterium transmitted among wild pig populations through direct contact including breeding (semen, reproductive fluids) and ingestion of the bacteria (placenta and aborted fetuses, milk and urine). When humans contract swine brucellosis it is called undulant fever because body temperature rises and falls along with flu-like symptoms. In wild pigs, symptoms include abortions, lameness, arthritis, abscesses, infertility, and sometimes mortality. Swine brucellosis is of concern to the cattle industry because this bacterium can cause a false positive test for bovine brucellosis (Brucella abortus). When a positive test for bovine brucellosis is found, the cattle herd is often quarantined, leaving the rancher with significant economic loss.
Humans do not contract pseudorabies, however, domestic livestock like sheep, cattle and some wildlife can be affected. This disease is not a form of rabies as the name implies. Pseudorabies is spread by direct contact (nose-to-nose or sexual), as well as ingestion or inhalation of the virus. Symptoms include abortion, mortality among piglets, coughing and fever among adults. Cattle and dogs experience intense itching and may incessantly scratch and bite at the skin. Other neurological symptoms may occur along with the potential for mortality in livestock and other animals that contract the disease. Tularemia is commonly known as rabbit fever. Humans are susceptible to the disease by direct contact through a wound, eating infected meat, and by carrier external parasites including ticks and biting flies. When humans contract tularemia, flu-like symptoms occur along with swollen lymph nodes. Severe cases can potentially result in pneumonia, blood infections, or meningitis. This bacterium can survive for several weeks in wet environments. In 2011, researchers at Texas Tech University tested 130 wild pigs in Texas and found that 50% of those tested in Crosby County and 15% in Bell and Coryell counties showed past exposure or were currently infected with tularemia.
Wild pigs are capable of living in a variety of habitats and will compete directly with native wildlife for resources. They are capable of shifting their diet throughout the year to take advantage of available food sources. Many native species have very specialized diets and are unable to do the same when resource availability is low. Wild pigs will also compete with native wildlife for supplemental food sources provided by landowners (protein supplements, bait used by hunters, etc.). Texas spends an estimated $50 million annually on shelled corn to feed white-tailed deer (Odocoileus virginianus) populations, and non-targeted species such as wild pigs can significantly impact these resources as well.
While wild pigs are opportunistic predators and take many different types of native wildlife species. These animals have been shown to consume eggs of ground-nesting birds such as northern bobwhite (Colinus virginianus) and wild turkeys (Meleagris gallopavo). Although not considered significant predators of white-tailed deer fawns, wild pigs will also consume these and other ungulate fawns. Research indicated that reptiles and amphibians are one of the groups most threatened by wild pig predation. These species are often consumed incidentally as wild pigs forage, however there is evidence of wild pigs will actively seek out reptiles and amphibians to consume. In a food habit study conducted 2010, one wild pig had 49 eastern spadefoot toads (Scaphiopus holbrookii) in its stomach. Reptiles that nest on the ground can also lose their eggs to opportunistic predation, as wild pigs have been shown to be major nest predators of threatened and endangered sea turtles such as loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) sea turtles.
Native Plant Communities
Due to their high adaptability and a wide-ranging diet, wild pigs have the potential to affect many types of native plant communities. Riparian forests, upland forests, native grasslands, streams, and wetlands are among the various native plant community types that are negatively impacted by non-native wild pigs. During the late summer and fall, wild pigs will consume large amounts of mast (nuts, fruits and beans) in upland forests and ultimately reduce the number of large-seeded tree species, such as oak (Quercus sp.) and hickories (Carya sp.) in direct competition with native wildlife species. Their rooting behavior disturbs the soil and can lead to conditions that are favorable in proliferating exotic and invasive plant species such as Chinese tallow (Triadica sebifera) and others. Wallowing and rooting can also reduce the amount of vegetative ground cover and litter layer that is essential for seedling establishment and growth. The potential for wild pigs to alter native plant communities is compounded by their destructive wallowing and rooting behavior, omnivorous diet, prodigious foraging practices and expanding populations.
Wild Pig Control/Damage Abatement
Legal methods for reducing wild pig populations in Texas include trapping, aerial gunning, shooting, snaring and the use of trained dogs. In urban/suburban areas, wild pig control is often limited to nonlethal control (exclusion fencing, habitat manipulation etc.) and trapping. As of 2018, there were no toxicants labeled for legal use on wild pigs in Texas. Effective abatement often involves a combined and sustained approach, and landowners should consider working cooperatively with adjacent landowners to gain increased success and more widespread benefit. This can be accomplished by sharing traps, head gates, snaring sites (i.e., fence crossings), aerial gunning costs, property access for the use of trained dogs and other collaborative efforts.
It is important to understand that not all abatement strategies are independently capable of reducing wild pig populations. While techniques such as corral trapping (remote, suspended and animal-activated) as well as aerial gunning can effectively reduce populations, other techniques such as shooting, snaring, and the use of trained dogs can alter the movements of wild pigs and force them to abandon areas where they are causing damage. For example, research indicated that agricultural damages in areas that allowed the shooting of wild pigs was less than half than in areas where this practice did not occur. Studies have also shown that successful abatement efforts often follow a defined sequence, where large-scale population reduction practices are followed up by other techniques such as snaring and shooting, with the use of trained dogs being employed as a final measure to remove wild pigs missed by previous strategies.