THE MECHANICAL CONTROL OF BUSHPIG, Potamochoerus porcus, IN ZIMBABWE

Bushpig, Potamocheorus porcus, occurring naturally in the high rainfall areas of Zimbabwe, have become a major threat to maize producers in the country. Traditional means of control including hunting have been unsuccessful in keeping the numbers to a tolerable level owing to the secretive and cunning nature of the animal. The use of poisons has been discouraged because of indiscriminate use and problems of secondary poisoning, so alternative methods of mechanical control were sought. Several methods evolved during experimentation, producing a strategy to control bushpig throughout the year. INTRODUCTION Bushpig occur throughout Zimbabwe. Smithers (1966), Dorst and Dandelot (1970) show a wide distribution throughout Central and South East Africa, relating distribution to high forests and their fringes and thick bush country. Personal observations indicate that bushpig are more numerous in the higher rainfall areas of Zimbabwe near marshy seeps or vlei areas. Smither records the average mass of an adult pig to be 90.0 kg. Dorst and Dandelot record the average between 54.4 and 81.8 kg, while Kenmuir and Williams (1975) record an average mass of 80 kg. Bushpig are gregarious and occur in sounders comprising of 12 or more individuals in Zimbabwe. Each sounder is led by a dominant boar and defence behaviour in the protection of feeding ground has been recorded by Skinner et al. (1976). They noted some degree of tree marking, particularly near paths used by pigs in a study area in the Transvaal province of South Africa. Bushpig are omnivores, eating a wide range of food, such as berries, fruit, roots, insects and worms, keeping in constant communication with each other during foraging by emitting low grunts. Jones (1978) found that they are unable to root in dry ground. In his trials at Matopos National Park rooting increased 1 or 2 months after the onset of rains, which occur from November or December in Zimbabwe. This fact may be important in the distribution of bushpig, the animals favouring areas where natural seeps are more abundant in order to subsist during the dry months in Zimbabwe. During trials in 1981 observations indicated that bushpig movement increased 4 to 5 days after good soaking rains. Prior to this they remained near wet areas, foraging for food at night. Bushpig are nocturnal, lying up in dense undergrowth during the day. In Zimbabwe they take refuge in dense riverine thickets or at the base of small hillocks. Broken country comprised of shallow soil overlying rock with large natural seeps appears to be favoured with greater numbers of bushpig being recorded. When undisturbed they often rest in the mature maize during the day or the nearest suitable thicket to the food source. They are aware of the minute changes in their environment--for example, newly erected fences--which they will generally avoid. It is related how a domestic bushpig would investigate all new activity during family evening walks. Unlike most animals, bushpig avoid obvious paths when commuting and where possible they proceed under thickets or tall grass, presumably to take advantage of cover. They are wedge-shaped and move through thickets with comparative ease, forcing the vegetation over their bodies, where most other species jump or step over obstacles. Bushpig have a keen sense of hearing and smell, avoiding new areas or strange obstacles with 'foreign' odours. Recent trials indicate that even when attracted to bait they will abstain if they become suspicious. Bushpig give birth to 3 to 7 young during the early rains November to February. Nests are made of grass pushed into large stacks, under which the female gives birth and nurses a litter. Their success in the farming areas has been attributed to the lack of natural predators, such as leopards, and their secretive nocturnal habits. Increases in soil moisture resulting from irrigation overflow and the encouragement of bush encroachment may also be contributing factors. Two recent country surveys, Grobler (1973) and La Grange (1981) indicate an increase in bushpig over the last 10 years. This estimate is subjective and the apparent increase may just be a reflection of renewed interest caused by maize price increase which has led to an increase in maize production over the years and the degree of rainfall those seasons. Pressure has come from the commercial farming sector to find a satisfactory solution to the control of bushpig. The survey indicated that bushpig damage tends to be more severe in areas where lands are found in small pockets surrounded by numerous hills and vleis, for example, the Eastern Highlands of Zimbabwe. In more open highland areas near Harare, where undivided lands are large with fewer hills and vleis, damage appeared to be proportionally smaller. 1986, Proceedings Twelfth Vertebrate Pest Conference 215 (T.P. Salmon, Ed.). Printed at Univ. of California, Davis, Calif. QUESTIONNAIRE RELATING TO BUSHPIG PROBLEMS A questionnaire was sent to prominent maize producers early in 1981 to obtain basic facts on the pig problem. One hundred one were returned, giving a 78% return, from which the following information was extracted: Total hectarage of maize under consideration = 13,785 Total pigs destroyed during 1980/81 season + 942 Number of pigs destroyed ranged from 0-84/farm/season Average kill was ± 9.3 pigs/farm or 1 pig/15 hectares Most of the pigs killed were by crop guards armed with 12 bore shotguns and during concentrated drives in maize lands and adjacent thickets. The estimated loss per pig ranged between 100 kg and 3,000 kg of maize per season, producing an average of 800 kg per season. (The upper limit is probably exaggerated.) The figure represented a loss of $100 by each pig during that season. Traditionally, bushpigs have been hunted at night by crop guards specifically employed by individual farmers. The crop guards patrol the lands at night, usually commencing along paths outside the maize or paths within the maize until they hear bushpigs breaking the stalks or sounds of feeding pigs. The guard then attempts to approach the pigs through the maize using the wind to advantage. The pigs are approached to within 10 metres when the guard fires in the direction of the noise. Reports indicated that firing this way in total darkness there is less than a 10% chance of killing the pig. The more successful guards developed techniques such as covering themselves with cattle dung which has enabled them to approach to within a few metres. The survey indicated skepticism in the success of the method. However, sources indicated that on three occasions kills up to 60 pigs were achieved during the 1980/81 season. Bushpig easily succumb to 12 bore shotguns using cartridges heavier than AAA, even when the shot is poorly placed. Experienced trackers had more success by tracking down the animals from where they left the lands early each morning. Drives were also arranged to drive the pigs out of the lands or anthills towards concealed hunters in ambush. Hunting success was increased by mowing the grass in a 100-metre strip around the maize which bushpigs are forced to cross. Dogs have been effectively used where they have been trained to corner pigs which then were destroyed using a small-calibre weapon or even spears. However, most farmers are reluctant to have pack dogs on their farms as the dogs are nonselective when chasing animals. Only one farmer on the survey was satisfied with these methods of control; the remainder felt that more efficient methods were needed. PREVIOUS TRIALS IN ZIMBABWE Work was carried out in Matopos National Park by Grobler (1973) who investigated the feasibility of a fenced boma for the control of bushpig. An area 100 x 100 metres was fenced using a fence 2.5 metres high. A 2-metre section was removed and a fenced corridor erected into the boma from this position for 2 metres, tapering to an 800-mm gap where a sturdy gate was placed. Several types of bait were used, ranging from offal to cattle cubes. The system proved successful and 28 pigs were caught on one occasion. Above-average rainfall was experienced in the area that year. The Commercial Grain Producers Association gave a grant of $5,000 to the Department of National Parks and Wild Life Management in 1981 for the development of a bushpig boma based on the results of Grobler's work and similar trials conducted in South Africa. This was increased by a further $2,400 in 1982. At the time of experimentation, only Telodrex (Telodrin or Isobensan) was effective in the control of large numbers of bushpig. This poison, however, was highly persistent with high risks of secondary poisoning, particularly to avian species as carcasses were rarely recovered. This method of control was therefore discarded in compliance with a resolution passed by the world conservation agencies. The Department formed a Problem Animal Research and Control Unit, (P.A.R.C.U.) based at Mvurwi, now known as Management Unit No. 1, tasked to undertake research into the problem commencing in April 1981. AREAS OF RESEARCH Although considerable work has been carried out on the basic biology of bushpig with particular emphasis on feeding, little literature was available on control techniques. The unit decided to approach the problem on a wider basis than originally dictated in the terms of the grant, not only investigating the boma system but it also looked in all possible avenues of mechanical control other than poisons. It was reasoned that control could be effective in three areas where pig movement took place: 1) In maize lands on which they depredate; 2) Within wet vleis or seeps where pig are found during the dry season; and 3) Along paths to and from these areas and in daily places of refuge. This approach resulted in the following possibilities of experimentation: a) Use of fixed boma near commercial lands; b) Portable bomas in "wet" areas during the dry season: 216 c) Protection fences around lands; d) Development of trap guns; e) Modification of shotguns to improve upon shooting blind at night.