+ Wild Pig Information

The pig (Sus scrofa), was first introduced into Australia by the first fleet in May 1788, when 49 hogs were landed. As settlement spread, pigs were taken into new areas and turned out to fend for themselves, or escaped from insecure enclosures. Once established in the wild, populations of feral pigs rapidly built up and dispersed into favourable areas, usually following watercourses. Many feral pigs still exhibit some domestic breeding characteristics.

Feral pigs are now distributed throughout Queensland and are considered habitat generalists, colonising all bio-geographical regions including some urban areas. Population levels and distribution are influenced by environmental conditions (availability of water, food and cover) and the e effectiveness of control programs.

The total number of feral pigs in Queensland is not accurately known, but estimates range from 3–6 million, with the majority in North Queensland. In Australia, estimates range between 3.5 and 23.5 million feral pigs (most estimates are around 13.5 million), with the majority living in New South Wales and Queensland. Population densities in the wet tropics were estimated at 3.1/km2. Densities in the dry tropics range from 40/km2 in some coastal wetland areas in Cape York, to 4/km2 in freshwater lagoons and swamps, and 1/km2 in drier woodland and savanna areas.

The feral pig is a declared Class 2 pest under the Land Protection (Pest and Stock Route Management) Act 2002. Landholders must control declared pests on the land under their control. Penalties are applicable to landholders for non-compliance with these provisions. Feral pigs are a declared pest because of the large economic impact they have on agricultural industries and the environmental degradation they cause. Economic damage to Queensland agricultural industries is estimated at $80 million annually.

The general community attitude towards feral pigs varies considerably. Views range from pigs being a major agricultural and environmental pest and an exotic disease liability,to a food and recreational resource for aboriginal communities and an economic and recreational resource for rural communities. These contrasting opinions have sometimes lead to conflict within the community; however, there is an increasing acceptance that multiple-use management of feral pigs is both practical and appropriate, provided activities are undertaken in accordance with state legislation.

+ General Ecology

Feral pigs are found in all habitats throughout Queensland, from closely settled areas of the southeast, to the semi-arid channel country of the south-west and the savannas and the tropical rainforests of the north.

The main habitat requirements are for food, cover and a reliable water supply. Pigs are omnivorous, opportunistic feeders and can thrive in a variety of habitats and on a variety of diets. Feral pigs prefer dense cover to avoid direct sunlight and high temperatures. Because pigs have few sweat glands they tend to drink more often, and wallow in water or mud to cool o in high temperatures.

Feral pigs have a higher reproductive potential than other large mammals in Australia. In good conditions, populations may increase by 500% in a 12–15 month period as breeding can occur all year round. Piglets normally spend the first 1−5 days of life inside a grass nest, with the sow inside or close by. Weaning occurs after 2–3 months. Sexual maturity in sows is dependent on weight (25–30 kg) rather than age, similar to domestic pigs. Few pigs live longer than 5 years of age in the wild (usually males), but can live longer in domestic situations.

Adult females have a 21-day oestrus cycle, with a gestation period of about 113 days(3 months, 3 weeks and 3 days). Average litter size ranges between 4.9 and 6.3 piglets,but may be as high as 10 under favourable conditions and depending on the sow’s age, weight and food supply. The time between birth of a litter and the next fertile mating is about 2–3 months. Sows can produce 2 litters per year in good conditions. Mortality of juvenile pigs is very high if dietary protein intake is low. There may be a 90–100% mortality of young pigs (on low-protein diets) in dry seasons. Juvenile mortality in the wet tropics has been measured at 81%. The mortality of adult pigs does not vary significantly with seasonal conditions, as with juveniles, but ranges between 25–50% per annum.

+ Feral Pig Control

The feral pig is an opportunistic omnivore and consumes a wide range of foods. The diet varies from region to region and the potential food sources are limited by availability rather than preference for any one food type. Pigs have a high energy requirement, particularly during lactation and for the growth of young pigs. Sows require about 15% of their diet to be crude protein in order to successfully suckle their young. This protein requirement can be obtained from plant material, but is more commonly acquired from animal matter such as earthworms, carrion, arthropods, frogs and reptiles (although animal matter rarely exceeds 5–18% of a pig’s diet). Feral pigs will relocate in response to low food availability, when protein and energy requirements associated with reproduction and growth are not being met.

The social structure of feral pigs is based on a matriarchal society with the most common group (called a sounder) consisting of related sows (mother, daughters, sisters, aunt’s etc.) and their young. Bachelor groups (usually siblings) form when sexually mature males leave or are chased from the group. Older males operate alone or in pairs (siblings) and join the female groups for mating purposes. Group size varies with age, gender, food and water availability, and disturbances (such as hunting or other control measures). Group size can range from solitary boars to groups of 100 or more sharing a scarce resource such as a single waterhole during droughts.

Feral pigs have a defined home range and habitually make use of trails, shelter areas, feeding and watering areas (subject to availability), rubbing and tusking trees, and wallows. Home ranges of groups and individual boars overlap considerably. There is no evidence that feral pigs, of either gender, actively defend territories.

The size of a feral pig’s home range depends on a number of variables, including gender (males tend to have larger home ranges than females), resources and seasons. Generally, pigs in the tropics have a significantly larger home range size in the dry season compared to the wet season. Food availability and quality, and availability of water are thought to be the main determining factors influencing home range size. Home range size varies from as little as 0.16 km2 for furrowing sows, to greater than 40 km2 for individual boars in the semi-arid rangelands. Feral pigs are most active at dawn and dusk (called crepuscular activity) or during times of cooler temperatures (at night, during rainy or overcast conditions). They may also become less active during periods of disturbance from hunting or other human activities such as stock mustering.

+ Impacts of Feral Pigs

The potential for pig populations to rapidly increase in good seasons, combined with the pig’s omnivorous feeding behaviour, has resulted in most agricultural industries being affected by pig damage. Pigs will feed on all types of seed, grain, fruit and vegetable crops, with the exception of safflower.

Grazing industries are affected by predation on young stock, damage to pasture by grazing and rooting, or damage to fences and watering facilities. Feral pig damage in Queensland is estimated to be tens of millions of dollars annually. Damage caused to all Australian agricultural industries is estimated at $106 million annually.

The impact on livestock industries is basically limited to predation on lambs. Research has shown feral pigs can take as many as 40% of lambs. Some reports indicate that mature boars may also predate on calves and goats.

In the wet tropics, most damage to agriculture is on banana and sugarcane plantations. The pigs prefer older cane with a high sugar content (which is available during the dry season). Pigs can camp in a paddock for several weeks causing substantial damage, as they can obtain sufficient moisture from the cane.

The impact of feral pigs on banana and sugarcane plantations can be substantial

Individual farms can suffer substantial crop losses while neighbours receive little or no damage. In the southern and western Darling Downs, large-scale grain production is accompanied by suitable refuges for pigs. Feral pigs are a major pest in these areas. Pigs can also transport weeds and their diggings provide ideal conditions for weed establishment.

Mesquite seedlings (a woody weed species) germinating from feral pig faeces

Weed seeds have been known to survive up to 8 days in a pig stomach. During this time, feral pigs could potentially move considerable distances and deposit seeds into new areas.

+ Feral Pig Control

Very little quantitative information on the ecological impacts caused by the feral pig throughout Australia is available. Degradation of habitats is probably the most obvious environmental impact of feral pigs. Soil disturbance caused by feral pigs searchingfor food is the most visual impact. This disturbance may also cause hidden ecological damage such as disrupting soil nutrient and water cycles, changing soil micro-organism and invertebrate populations, changing plant succession and species composition patterns, and causing erosion. Diggings may also spread undesirable plant and animal species and plant diseases. Feral pigs physically destroy vegetation by trampling, wallowing, digging up, tusking, rubbing and eating plants.

+ Environmental Impacts

Diggings destroy soil structure; upset nutrient and water cycling; kill native animals, soil invertebrates and plant species; allow the introduction of weed species; change species succession patterns; and reduce biodiversity.

Feral pigs are known to prey on a wide range of native animal species including earthworms, insects, amphibians, reptiles, ground birds, small mammals, freshwater crayfish, frogs, and marine and freshwater turtles. There have been reports of tortoises killed in large numbers by feral pigs in receding swamps in the Northern Territory. Marine turtle nests suffer 100% predation rates on some beaches on the west coast of Cape York. Other potential impacts include pigs becoming hosts or vectors of endemic or exotic diseases, and the effects of pig control (particularly hunting) on non-target animals.

The indirect impact on native species is also di cult to quantify. Feral pigs compete for resources with native species—competition with endangered or rare native species is of particular concern. The endangered southern cassowary (Casuarius casuarius),a specialist frugivore (primarily fruit-eating animal), is considered vulnerable to competition from feral pigs.

Pig activity also has a dramatic effect on creeks and lakes. In many areas concentrated rooting ploughs up the area around the waterline. Such disturbance of the natural vegetation affects not only water quality but the habitat of small aquatic and terrestrial animals. It also creates erosion and allows the establishment of exotic weeds. Feral pig control

+ Feral Pig Diseases

Feral pigs can carry many infectious diseases and internal and external parasites. Some are endemic (native) to Australia, while others are exotic. Many of the diseases can not only spread to domestic pigs but to other livestock and humans. Diseases naturally transmitted from animal to man are called zoonoses. Some of the more serious diseases include the following:

• Brucellosis is a bacterial disease causing severe long-term illness, undulating fever and possible infertility. Both strains are contracted by handling raw meat.

• Sparganosis is a parasite that can infest the muscles of humans, forming cysts. It is common in pigs from swampy areas and is contracted by ingesting raw meat.

• Melioidosis is a serious bacterial disease that causes abscesses and, in some cases, death. It can be contracted by handling infected animals.

• Leptospirosis is a serious bacterial disease. In humans it is called Weil’s disease— causing very high temperatures, kidney trouble and jaundice—and it can be fatal. It is found in up to 20% of feral pigs in Queensland.

• Q fever is a disease that occurs in all animals and is well known to abattoir workers. It can cause a very high temperature resulting in heart problems, and can be fatal. Most infection can occur through contact with pig blood, meat or urine through soresor broken skin; by consuming infected food or water contaminated by people that have handled feral pigs; by inhaling infectious airborne organisms during the butchering process (especially opening the abdominal cavity); or by eating undercooked feral pig meat. It is advisable to use suitable protective clothing and gloves when butchering feral pigs, and to wash hands thoroughly after handling feral pigs.

A major concern with feral pigs is their potential to harbour or spread exotic diseases. The introduction of foot and mouth disease to Australia would cost the Australian community an estimated $9 billion in lost export trade, even if the outbreak was eradicated immediately. This would result in a major social upheaval in rural Australia.

+ Other Exotic Diseases

• Swine vesicular disease is a viral disease that only affects pigs.

• Aujeszky’s disease is a highly contagious herpes viral disease that affects several animal species, and kills up to 100% of affected piglets.

• African swine fever is a highly contagious viral disease that only affects pigs.

• Classical swine fever (CSF) or hog cholera, is a highly contagious viral disease in pigs.

• In acute form it kills up to 90% of infected animals.

• Japanese encephalitis is a virus spread from pigs to humans by mosquitoes, causing severe acute problems in the nervous system.

• Rabies is a serious disease affecting the brain that can be fatal to animals and humans.

• Screw-worm fly maggots can attack healthy flesh and, if untreated, can cause massive wounds to animals and humans.

• Trichinosis is a helminth (roundworm). All mammals are susceptible, with humans infected by eating improperly cooked meat.