Human–Wildlife Interactions 5(1):79–99, Spring 2011 Too many hogs? A review of methods to mitigate impact by wild boar and feral hogs Giovanna Massei, Food and Environment Research Agency, Sand Hutton, York YO41 1LZ, UK [email protected] Sugoto Roy, Food and Environment Research Agency, Sand Hutton, York YO41 1LZ, UK Richard Bunting, Department of Environment, Ministry of Agriculture, Land, Housing and the Environment, P.O. Box 272, Brades, Montserrat Abstract: Feral hogs (Sus scrofa) are among the most widely-distributed mammals in the world and have the highest reproductive output compared with other ungulates. Worldwide, feral hogs are increasing in range and numbers. Human–feral hog conflicts include impact on abundance and richness of plant and animal species, crop damage, predation on livestock, vehicle collisions, and disease transmission. We reviewed methods employed to mitigate the impact of feral hogs on human activities and discussed these methods in terms of effectiveness, feasibility, costs, and social acceptance. Traditional methods of control include trapping, angering, shooting, poisoning, and Judas hogs. Nonlethal methods of control include fertility control, fencing, repellents, diversionary feeding, and translocation.The review indicated that successful eradications of feral hogs from islands have been achieved by combining different control methods and by establishing post-eradication monitoring to ascertain that the eradication had been completed. Conversely, on the mainland and in countries where feral hogs have long been established, management of human–feral hog conflicts often relies on population size reduction through hunting and poisoning the animals or on exclusion fencing and diversionary feeding. In the majority of instances, population control is not based on previous knowledge of local densities or on predicted impact of control on population size. Based on these results, we propose a framework of criteria to guide decisions regarding the suitability of different options to manage human–feral hog conflicts in different contexts.
Key words: diversionary feeding, fencing, feral hogs, fertility control, human–wildlife conflicts, poisoning, population control, shooting, Sus scrofa, trapping, wild pigs
Wild boar and feral hogs (Sus scrofa, hogs cannot survive in areas where snow cover collectively (referred to as feral hogs unless otherwise persists for several consecutive weeks or where specified) are among the most widely-distributed droughts harden the soil. mammals in the world. Their natural range extends Throughout the world, feral hog populations are from western Europe and the Mediterranean basin to increasing in numbers and range. For instance, in the eastern Russia, Japan, and Southeast Asia (Sjarmidi late 1980s the number of wild boar shot annually in and Gerard 1988). In the northern hemisphere, this France was <100,000, but reached around 450,000 in species recently recolonized Sweden, Finland, and 2002 (Pfaff and Saint Andrieux 2007). Similar trends Estonia (Erkinaro et al. 1982) and was reintroduced were observed in many other European countries and in the United Kingdom (Wilson 2005). Feral hogs, in Australia, possibly due to a combination of socio- which are derived from domestic hogs, were economic and ecological changes (Sáez-Royuela and introduced to North and South America (Barrett Tellería 1986, Spencer and Hampton 2005). These 1978, Mayer and Brisbin 1991) Australia, and New changes include lack of predators, reforestation Zealand (Choquenot et al. 1996). of rural areas, reintroductions, limited hunting, Feral hogs are long-lived omnivores characterized supplementary feeding, translocations, and by the highest reproductive rate among ungulates, mild winters, which improved their winter with annual increases in population that may survival (Genov 1981, Erkinaro et al. 1982, exceed 100% (Katahira et al. 1993, Bieber and Geisser and Reyer 2005, Spencer and Hampton Ruf 2005). The species occurs throughout a wide 2005). In the United States, the number of states spectrum of habitat types, ranging from semiarid reporting the presence of feral hogs rose from environments to tropical forests, mountains, and 23 in 1988, to 30 in 2002, and 39 in 2004 (Hutton marshes. As monogastrics, hogs have a limited et al. 2006). Pimentel et al. (1999) estimated that capacity for digesting cellulose, and their survival 4 million feral hogs lived in the United States, and reproductive output depend on the availability while Muller et al. (2000) reported 3 million of high-energy food, such as acorns (Massei et al. feral hogs in Texas alone. In Australia, the 1996). Due to their habit of rooting for food, feral estimated number of feral hogs varies between 80 Human–Wildlife Interactions 5(1)
population control or eradication, which often are advocated by wildlife managers, veterinarians, farmers, and conservationists, can be opposed by local hunters and the tourist industry. Publications on feral hog control are almost invariably focused on single case studies. Exceptions include Choquenot et al. (1996), the State of Hawaii (2007) and West et al. (2009), who listed methods to control feral hogs in Australia, Hawaii, and the United States, respectively, and Campbell and Long (2009a), who focused on options to manage the impact of feral hogs in forested ecosystems. However, Figure 1. A group of trapped feral hogs. (Photo courtesy C. a comprehensive assessment of Wyckoff) the feasibility, humaneness, social 13 and 23 million (Spencer and Hampton 2005). acceptance, and costs of methods to control In the United States and Australia, feral hog hog populations and hog impact has not expansion was attributed to deliberate releases been produced. We present a critical review to create sport hunting opportunities, range of these methods and develop a framework expansion as population numbers increased, of criteria and recommendations to guide escapes from hog farms, habitat alteration decisions regarding the suitability of different due to human activities, milder winters, and options to mitigate human–feral hog conflicts. increased forage availability associated with agricultural development (Waithman et al. Methods to mitigate human– 1999, Hutton et al. 2006). In Europe, wild boar feral hog conflicts recently colonized suburban areas of Berlin, Although Hone (1995) showed that hog Barcelona, and Genoa, all of which reported abundance is not necessarily related linearly to increasing numbers of sightings (Walker 2008). impact, a high level of impact is usually regard- Throughout their range, feral hogs have ed as an indicator of overabundant population. a substantial impact on human interests, Human–feral hog conflicts traditionally have including damage to crops and livestock, spread been managed through culling and poisoning of diseases, and vehicle collisions (Engeman et the animals. More recently, however, commun- al. 2004, Conover 2007, Conover and Vail 2007, ity opposition to lethal methods to manage Mayer and Johns 2007). Feral hogs may also wildlife has become widespread because of cause reduction in plant and animal abundance animal welfare issues, concerns about human and richness, particularly where they occur as safety in urban settings, and environmental non-natives; they are regarded to be among the impact of toxicants (Beringer et al. 2002, McCann worst 100 invasive species in the world (Hone and Garcelon 2008, Reidy et al. 2008). As a 2002, Massei and Genov 2004, Seward et al. 2004, result, many state agencies and local authorities Engeman et al. 2007). Current trends of human are under pressure to consider safe, effective, and hog population growth and landscape nonlethal options to resolve human–feral hog development indicate that human–feral hog conflicts. Management efforts, thus, have been conflicts are likely to increase in the near future. redirected toward protecting resources, such as However, feral hogs also are important as game, valuable crops or livestock, by using methods, and in some parts of the world, they provide such as exclusion fencing and fertility control, a valuable source of protein (Waithman et al. to reduce population size. We summarize the 1999, Milner-Gulland et al. 2002). Therefore, advantages and disadvantages of lethal and Too many hogs • Massei et al. 81
Table 1. Lethal methods to manage human–feral hog conflicts. Method Advantages Disadvantages Trapping • Hogs are easy to trap • Trap shyness and • Humane, if frequently checked • Impractical on high slopes or very dense euthanasia • Selective removal of age or sex vegetation classes • Labor-intensive due to building, baiting, and • Species-specific removal checking traps • Low social disturbance • Requires euthanasia • Fast-acting at population level • Effective only when natural food availability • Hogs can be removed alive is limited • Usable in residential areas • Applicable on a small scale • Can provide meat • Encourages animal translocation • Traps can be moved and re-used • Traps prone to human interference Snares • Effective if correctly set • Often regarded as inhumane • May target localised problems • May affect nontarget species • Can provide meat • Remove relatively small numbers • Labor-intensive to set and check • Prone to human interference • Illegal in some countries Ground • With several teams, cost-effec- • May encourage hogs to avoid people shooting tive in areas of high densities • Changes in spatial and temporal behavior (with or • Selective removal of age or sex • May cause social perturbation and increased without classes contact rate dogs) • Fast-acting at population level • Inhumane if shooters are inexperienced • Can provide meat and trophies • Difficult to use or illegal in residential areas • Useful for inaccessible or re- • Dogs may be injured or killed by hogs mote areas • Hunters may be shot • Dogs can be used to flush hogs • Untrained dogs may attack other species in dense vegetation Aerial • Cost-effective in areas of high • May encourage hogs to avoid helicopters shooting densities • Changes in spatial and temporal behavior • Selective removal of age or sex • Increase in unit costs as hog numbers de- classes crease • Fast-acting at population level • May cause social perturbation and increased • Can provide meat and trophies contact rate • Useful for inaccessible or re- • Inhumane if shooters are inexperienced mote areas • Difficult to use or illegal in residential areas Poisoning • Cost-effective • Often regarded as inhumane • Can be used on a large scale • Hogs might not eat poisonous baits • Fast-acting at population level • Can affect nontarget species • Can be used to target trap-shy • Bait shyness animals • May cause social perturbation in feral hogs • Unfeasible in residential areas • Toxicants are not approved in many countries • Requires banning on meat consumption Judas hogs • Can be effective for removal of • Used only with other control methods remnant animals • Labour intensive due to trap and release • Expensive due radiotracking equipment nonlethal methods to control the impact of feral of animals in areas of high hog density (Figure hogs (Table 1). For each method, advantages and 1; Table 1). disadvantages should be regarded as relative to Many trap designs are available, ranging those of other control options. from those that can capture single hogs or small groups of them to corral types that can capture Lethal methods of control large groups (e.g., Saunders et al. 1993, Caley Trapping and euthanasia. Traps are widely- 1994, Choquenot et al. 1996, Sweitzer et al. used to control feral hog populations (West et 1997, West et al. 2009). The majority of traps are al. 2009). When the availability of natural food made of mesh frames with drop gates and side- is low, feral hogs are relatively easy to trap, and hinges or top-hinged spring-gates that hogs trapping can effectively remove large numbers must push to gain access to the food placed 82 Human–Wildlife Interactions 5(1) inside the trap. Corrals have similar gates but of feral hogs should be discouraged (see below). are larger and may have a funnelled entrance In addition, traps can be easily damaged by to guide animals toward box traps that are people who are opposed to culling. used to remove hogs. The food most frequently Trapping has been employed in many feral used to attract feral hogs to traps is maize, hog eradication projects. In the Pinnacles fermented wheat, vegetables, fruit, blood, fish, National Monument, California, trapping animal parts, or carcasses (Choquenot et al. removed 70% of the hog population in the first 1996, Cruz et al. 2005, Twigg et al. 2005). Hog- 3 months, and the combination of trapping specific baits are also commercially available, and opportunistic shooting increased the and attractants have been developed to increase efficiency of hog eradication (McCann and trapping success (Cowled et al. 2006, Campbell Garcelon 2008). In Hawaii, trapping failed to and Long 2009b). If they can be checked at least remove feral hogs at low densities because once per day, traps are generally considered these animals became trap wary (Reeser and to be humane for feral hogs and for nontarget Harry 2005). On Santiago Island, Ecuador, species, such as other wildlife and livestock Coblentz and Baber (1987) found that trapping that can be released. Large traps that allow the was ineffective, due to a combination of poor whole social group to be captured are likely to trapping success and lack of sufficient staff have little impact on social behavior. The latter required to check traps. However, McIlroy is particularly important as social perturbation (1983) and O’Brien et al. (1986) used trapping may lead to increased contact rates, with the as the main method to eradicate feral hogs from potential risk of increasing disease transmission study sites in California. On Santa Cruz Island, and may encourage long-distance movements, California, 16% of the 5,036 hogs removed to thus, extending the impact to neighboring areas achieve eradication were caught in 102 traps (Sodeikat and Pohlmeyer 2002). Maintaining that were set for 1,660 trap-nights (Parkes et and regularly checking traps can be expensive al. 2010); by comparing hog home range size in staff time and can be applied over only and trap distribution, researchers were able to relatively small areas. However, traps can be predict the efficacy of each trap. moved and redeployed to other areas, and Snares. Snares consist of an anchored cable trapping can be fitted around other routine or a wire noose set to close around the neck control activities. or a foot of an animal. These devices may Trapping success depends on a variety of have stops that prevent them from closing factors, including topography, time of year, and strangling animals of a certain size or type of trap used, number and density of traps break-away locks that allow larger animals to deployed, trap location, number of nights each escape. The effectiveness of this method greatly trap is used, type of bait used, and duration of depends on snare design, although snaring has pre-feeding before the traps are set (Hone et al. been criticized as inhumane to both target and 1980, Choquenot et al. 1996, West et al. 2009). nontarget species (TWDMS, 1998). For instance, in New South Wales, Australia, The use of snares is regulated in many parts Saunders et al. (1993) found that season and of the world and is illegal in most European trap location affected trapping success and that countries. Snares have been used extensively in placing traps in areas with recent hog activity Hawaii to remove large numbers of feral hogs or along the treeline, rather than in the forest or (Anderson and Stone1993). For instance, snares in the clearings, increased trapping success. accounted for 55% of the feral hogs removed Traps are difficult to transport and use on in Hawaii during 1983 to 1992 (Jeffery 1999). sloping or rough terrain; conversely, they can be Snares were also used to complement shooting easily deployed to remove hogs from residen- and achieve hog eradication on Sarigan Island, tial areas. Compared to poisoning as a method Western Pacific Ocean (Kessler 2002). of control, trapping has the advantage that the Snares are inexpensive and easy to set in number of animals captured is known and large numbers. However, they can target only carcasses can be safely removed. The fact that 1 animal at a time and should be checked at live traps may encourage translocation should least once per day to monitor whether target be regarded as a disadvantage, as translocation and nontarget species have been caught. This Too many hogs • Massei et al. 83 clearly increases the cost of programs based Namadgi National Park, Australia, hunters on this method. In the Haleakala National with dogs on 19 occasions passed within 100 Park, Hawaii, Anderson and Stone (1993) used m from hogs that carried radiotransmitters, approximately 2,000 snares, set at a density of and they found and killed a hog only once 96 to 200 snares per km2, for a feral hog control (McIlroy and Saillard 1989). Although hogs program. After 45 months, hog density was were active when the hunt started, they became reduced from the initial 6 to 14 hogs per km2 to stationary when the hunters moved closer, and an estimated 1 hog per km2. However, snares most animals did not leave their home range. were checked every 3 months, which meant Dexter (1996) suggested that the impact of that hogs caught in the snares were left to die shooting on feral hogs' behavior might depend for lack of water and food. At present, this on the level of human disturbance that animals approach would be deemed unacceptable due have experienced. Where hunting pressure is to its lack of humaneness. constant and high, hogs may learn to cope with Ground shooting. Shooting has long been the disturbance by hiding or lying still until the established as a control method for feral hogs hunters have moved away. (West et al. 2009). In many parts of the world, Poorly-trained dogs may pursue and kill recreational hunting is carried out by shooting other animals, thus, causing serious disturbance from the ground or from high seats at bait to local wildlife and increasing the staff effort stations. Hunters may hunt alone, in small to achieve eradication. (Massei and Toso 1993, teams, or in large groups to carry out drive Cruz et al. 2005). Other disadvantages of hunts, in which animals are driven toward hunting include potential social disturbance a line of hunters by people walking along a and animal welfare issues. If hogs leave their front to flush hogs from cover; often they use normal home range, they can potentially dogs trained to flush hogs. In Europe, the use increase their contact rate with other hogs and, of hunting dogs is widespread, particularly thus, extend their impact to other areas. Animal in areas with dense vegetation (Geysser and welfare issues concern hogs that are injured but Reyer 2004). Dogs are also used by hunters in not killed and dogs that can be severely injured Australia, New Zealand, and the United States by hogs. Controlled shooting by experienced (McIlroy and Saillard 1989, Campbell and Long staff can overcome this problem, and dogs 2009a). trained in flushing but not attacking feral hogs Hunting is effective in areas with high are less likely to be injured. densities of hogs, as many animals can be culled Ground shooting has been employed in a in relatively short periods (Table 1). Hunting large number of projects aimed at eradicating or may allow selective removal of specific age or sex controlling feral hog populations. For instance, classes and provide hunters with the additional on Santiago Island (Ecuador) Coblentz and incentive of meat and trophies. Feral hogs can Baber (1987) found ground shooting effective, learn to avoid hunters by becoming more active but time consuming. On the same island, Cruz during the night and by avoiding areas where et al. (2005) found a rapid increase in effort hunting occurs. However, the effects of hunting required to remove hogs in the final stages of the on the spatial behavior of feral hogs are still eradication campaig; in 2000, the effort required unclear. For instance, in France, hunting with to remove each hog was 450 times greater than dogs caused wild boar to increase home range it was in 1998. However, the authors mentioned sizes (Calenge et al. 2003). In Germany the that opportunistic hunting over bait sites was home range of 6 wild boar groups out of the 9 particularly useful as a secondary technique groups monitored increased from 183 ha (pre- to reduce feral hog numbers after trapping. In hunt) to 299 ha after a drive hunt, and 3 groups Switzerland, Geisser and Reyer (2004) showed also moved up to 6 km outside their previous that hunting was more effective in reducing range (Sodeikat and Polheimer 2002). However, damage to crops than fencing or supplementary 2 other studies, in Germany and Australia feeding, although shooting was also regarded (McIlroy and Saillard 1989, Keuling et al. 2008), as time consuming. In California, Barrett (1978) found no effect of hunting on spatial behavior found that hunting with dogs throughout the of feral hogs. During a study carried out in the year removed approximately 20% of the feral 84 Human–Wildlife Interactions 5(1) hog population. In Australia, McIlroy and appears unable to control feral hog densities, Saillard (1989) reported that sustained hunting as evidenced from current trends in feral hog throughout the year reduced the population numbers in Europe, Australia, and the United density to 3 to 8 hogs/km2 compared to 43 hogs/ States (Choquenot et al. 1996, Hutton et al. 2006). km2 in a nearby area with low hunting pressure. In Hawaii, Reeser and Harry (2005) showed Similarly, in Hawaii, shooting by hunters with that volunteer hunting or public hunting failed dogs that varied the routes hunted, the time of to remove feral hogs at the required rate, while the day when the hunt started, and the interval professional hunters were more successful. In between hunts led to eradication of hogs from France, Toigo et al. (2008) found that between large, fenced compartments or reduced densities 1984 and 2004 the number of wild boar culled in to less than 1 hog/km2 (Stone and Keith 1987). the study area by recreational hunters rose from Ground shooting also was used as the main 200 to 1,000 and that the propensity of hunters technique to eradicate hogs from the 5 km2 to target adult males instead of females and Sarigan Island, Pacific Ocean; circa 2,000 man hoglets reduced the effectiveness of population hours were required to remove 68 feral hogs control. Conversely, recreational hunting offers and 904 feral goats in 2 months and achieve the opportunity for hunters to be directly eradication of both species (Kessler 2002). involved in participatory management of a About 50% of this effort was taken by follow-up sustainable resource. In this capacity, hunters surveys to ascertain complete eradication. may also volunteer precious skills and free Recent studies suggested that targeting labor that can benefit the often tight budgets of a particular sex or age class could improve projects aimed at mitigating feral hog impact. hunting efficiency. For instance, reducing Aerial shooting. Shooting from helicopters juvenile survival has the largest effect on is relatively common in countries such as the population growth rate, and increasing United States and Australia, which have vast, hunting pressure on adult females, particularly uninhabited areas of sparse vegetation where in years of low food availability, appears to it is relatively easy to locate groups of animals. be the most effective approach to population This method can achieve quick decreases in control (Sweitzer et al. 2000, Bieber and Ruf hog abundance over large areas. Thus, one of 2005). However, compensatory responses to its greatest advantages is that it allows large- culling, such as increased immigration and scale coordination of effort among several reproduction, can limit the success of hunting landowners (Table 1). (Hanson et al. 2009). Aerial shooting in areas of high hog densities Ground shooting has been employed to has a relatively low cost per hog killed and control disease outbreaks, such as classic swine allows population control in inaccessible fever. In this context, the hunting rate is usually areas. Besides having similar advantages and assumed to be constant over time. However, disadvantages of shooting from the ground, a recent cost analysis model showed that, by however, aerial shooting can disperse animals, implementing flexible hunting strategies that is ineffective in areas with dense vegetation, vary according to the density of hogs and and, as hog numbers decline, the cost of aerial disease prevalence, managers can minimize the shooting increases relatively more than the cost cost of hunting and the sanitary costs associated of ground shooting (Choquenot et al. 1999). For with the infection over a specific period of time instance, in Australia, Choquenot et al. (1999) (Bolzoni and De Leo 2007). These results can demonstrated that, as aerial shooting reduced be used to design cost-effective contingency hog populations below threshold densities of plans to control feral hog populations in case of circa 2 to 6 hog/km2, the number of hours to disease outbreak. cull individual hogs increased exponentially. Intensive, sustained hunting can eradicate In another area of New South Wales, Australia, feral hogs from vast areas. In many European aerial shooting did not affect the home range countries, wild boar went extinct due to hunting size and movements by feral hogs, possibly pressure when wild game was regarded as one due to the dense vegetation where hogs could of the few sources of protein (Saez-Royuela and hide as the helicopter approached (Dexter Telleria 1986). Nowadays, recreational hunting 1996). The availability of shelter could explain Too many hogs • Massei et al. 85 differences in hog behavior among studies attempted, poisoning should be combined with carried out in different environments, and this at least 1 other population control method. should be taken into account when planning The success of a poisoning campaign hog population control. depends on many factors, including time of the Aerial shooting has been used as the main year, bait composition, adequate distribution method to eradicate feral hogs; for instance, and abundance of baits, type of toxicant, and 77% of the 5,036 hogs in Santa Cruz Island, hog density. In the Namadgi National Park, California, were shot from helicopters in 15 Australia, McIlroy and Saillard (1989) found months (Parkes et al. 2010). Shooting from that the success of poisoning depends on helicopters can be a valuable tool to control adequate distribution and abundance of baits disease outbreaks because it provides a quick and on timing, as bait consumption by hogs reduction of hog density. During a simulated varied greatly throughout the year. In the same exotic disease outbreak in New South Wales, area, the use of poisoned baits in autumn, when Saunders and Bryant (1988) used this method hogs were attracted to baits because of limited to evaluate the effectiveness of plans to natural food supply, reduced hog numbers by eradicate feral hogs. The results indicated that, 91% and 100% in 2 study sites (McIlroy et al. although 80% of the hogs were removed in 5 1989). days of aerial shooting, some hogs modified The humaneness of toxicants used in hog their behavior to avoid detection. One year control is increasingly being questioned, and later, due to reproduction and immigration, the the possibility of affecting nontarget species, population had recovered to 77% of the pre- and the environmental fate of toxicants can control population; Saunders (1993) concluded pose serious constraints on the application that, at least in the local conditions, eradication of this technique. At present, poisoning is of hogs was an unrealistic goal and that efforts carried out in Australia and New Zealand, would be better directed toward eradicating the but there are no toxicants registered for use disease rather than the host population. on feral hogs in either the United States or Poisoning. Poisoning can achieve rapid Europe (Cruz et al. 2005, Campbell and Long reduction in the number of feral hogs on a large 2009a, West et al. 2009). In Australia, sodium scale and at moderate costs and has been used monofluoroacetate (1080) is incorporated into extensively to control feral hogs (Table 1). For baits and is considered to be one of the most instance, on Santiago Island, Ecuador, Coblentz effective toxicants for quickly reducing feral and Baber (1987) found that poisoning was hog numbers (Hone 1983, Twigg et al. 2005). far more efficient than shooting or trapping to The relatively large doses required to kill feral reduce the hog population size. On the same hogs implies that the use of 1080 carries a high island, Cruz et al. (2005) used spot-poisoning to risk of poisoning nontarget species (Kavanaugh complement ground shooting in the final stages and Linhart 2000). However, 1080 has been of the eradication campaign when shooting had employed mainly in areas where nontarget become too inefficient due to the low density species were absent or where bait uptake by of hogs. Spot-poisoning consisted of leaving nontarget animals, such as livestock, were meat chunks or entire goat carcasses laced with prevented by building hog-specific bait stations. a poison where signs of fresh hog activity had In northwestern Australia, 1080 poisoning for 8 been observed and where hunters had failed to 9 days caused a 89% decrease in the numbers to cull the hogs. In this study, the effectiveness of feral hogs (Twigg et al. 2005). Twigg et al. of using toxicants as a supplementary method (2005) recommends this method to meet the was demonstrated as the last hog was poisoned requirements of disease-containment strategies 6 months after the last hog was shot (Cruz et based on significant density reduction within a al. 2005). In New South Wales, Hone (1983) few weeks from a disease outbreak. demonstrated that 9 days of pre-baiting, The anticoagulant warfarin also is used to followed by 3 days of poisoning over a 50-km2 poison feral hogs in Australia. Warfarin and area, killed 73% of the feral hogs. Aerial hunting 1080 have been employed to eradicate feral hogs was then used to kill 95 of the 98 feral hogs in the from Santiago Island in the Galapagos (Cruz et area. The study suggested that, if eradication is al. 2005) and to reduce the feral hog populations 86 Human–Wildlife Interactions 5(1) in Australia by 73 to 96% (Saunders et al. 1990). when the density was very low. On Santa Cruz Besides their environmental impact, toxicants, Island, only 9% of the 5,036 hogs removed to such as 1080, have also the potential to induce achieve eradication were dispatched as a result bait shyness because hogs that ingest sublethal of their association with Judas hogs (Parkes et doses are less inclined to feed again on the al. in press). However, once all hunting had same bait (Hone and Kleba 1984). In contrast ceased, hogs equipped with radio collars found to 1080, warfarin is slow-acting and symptoms 3 out of 7 of the remaining hogs and were of intoxication appear long after a lethal dose responsible for the dispatch of the last hogs in 2 is ingested, thus, reducing the chance of hogs compartments (Parkes et al. 2010). acquiring bait-shyness (Cruz et al. 2005). Using Judas hogs can also be employed to identify warfarin in eastern Australia, Saunders et al. areas frequently used by the hogs so that (1990) reduced the local feral hog population baiting with toxicants or hunting can be by 99% in 3 months. However, one of the 2 redirected toward these sites. McIlroy and sows that survived produced 2 litters, which Gifford (1997) suggested that, to decrease highlighted the importance of maintaining a the cost and time required to trap the last control program in years following the initial few animals, hogs captured and kept in density reduction. captivity at the beginning of a population Studies are currently being carried out in control program could be used as Judas hogs. Australia to identify more humane, fast-acting To improve the efficiency of this method to toxicants that can be used to control populations achieve eradication, McCann and Garcelon of feral hogs (Cowled et al. 2008). However, (2008) suggested that all Judas hogs should be Fagertsone et al. (2008) reported that, in the surgically sterilised before release. When using United States, companies average 11 years and Judas hogs, Parkes et al. (2010) sterilized all spend approximately $22 million to develop males prior to release, and induced females into and bring new animal drugs to the market. estrus to enhance their attractiveness to males, Registration costs and growing public concerns showing that these females were significantly toward use of toxicants on wildlife suggest that, better than the males at attracting other hogs. at least in Europe and in the United States, it is unlikely that poisoning will be used to manage Nonlethal methods feral hog populations. Fertility control. Chemical sterilization Judas hogs. Judas hogs are animals that are to reduce overabundant wildlife has been trapped, equipped with a radio-collar, and discussed for at least 2 decades (Fagerstone released so that they rejoin conspecifics. The et al. 2002). For many years the lack of long- whole group can then be located and culled acting, safe contraceptives, the practicality of by hunters. This technique was tested in delivering oral contraceptives in baits, and the Australia and indicated that the best results potential effects on nontarget species prevented were achieved by releasing sows captured in the use of this method. Recently developed the same area where they had been trapped immunocontraceptives have reawakened (McIlroy and Gifford 1997). Out of the 15 Judas interest in this technique to control feral hogs. hogs released, 12 established contact with ≤12 Immunocontraceptives act by causing the other animals; hogs released in the same site of production of antibodies against hormones or capture rejoined their group within 1 week. proteins essential for reproduction (Miller et This method can be employed to locate the al. 2008). These compounds recently have been last few trap-shy or poison-shy hogs once the formulated as single-shot vaccines, capable population density has been drastically reduced of inducing long-term infertility after a single through trapping or shooting (Table 1). The main injection. For instance, the Gonadotropin- advantage of using Judas hogs is quick detection Releasing-Hormone (GnRH) vaccine stimulates of animals; using this technique, Wilcox et al. the production of antibodies against GnRH, (2004) showed that hogs were detected within which is, in turn, responsible for the production 1 hour compared to 4.1 hours to locate hogs of sex hormones that lead to ovulation and without telemetry when the population was spermatogenesis. Animals injected with this at its maximum density, and almost 60 hours vaccine can be rendered infertile for 1 to 5 years Too many hogs • Massei et al. 87
Table 2. Nonlethal methods to manage human–feral hog conflicts. Method Advantages Disadvantages Fertility control • Humane • Slow-acting at population level (injectable contra- • Long-term effectiveness • Requires trap-inject-and-release ceptives) • No social disruption • Applicable to small scale • Usable in residential areas • Expensive due to trapping effort • Species-specific • Can decrease disease transmis- sion Fencing • Very effective when well-con- • High initial set-up costs structed • High maintenance costs, including • Humane replacement • Short-term protection of vulner- • May interfere with public access able crops • May increase damage in adjacent areas • Long-term protection of live- stock or areas • Useful to partition areas and facilitate eradication • May be fitted with one-way gates to allow animals to exit • Fences can be moved and re- used Repellents • Humane • Short-term duration • No social disruption • May concentrate damage in adjacent • Usable in residential areas areas • No repellents registered for hogs Diversionary • Humane • Efficacy depends on availability of feeding • May concentrate hogs for a diversionary food short time • Labor-intensive if diversionary food is • Fast-acting to alleviate damage provided continuously to crops or areas • May increase reproductive output and thus population size • May attract and affect nontarget spe- cies Translocation • Perceived as humane • Labor-intensive due to building, bait- • Fast-acting at population level ing, checking traps, and transporting • Usable in residential areas hogs to new area • Effective only when natural food avail- ability is limited • May translocate pathogens and dis- eases • Animals may suffer during trapping, translocation and post-release • May encourage illegal or irresponsible introduction of hogs
(Killian et al. 2008, Miller et al. 2008). GnRH Fertility control has a high level of public vaccines have been tested extensively on many acceptance and could be used to decrease wildlife species, including feral hogs. In most numbers of feral hogs, particularly for isolated species, these contraceptives have been found populations where immigration and emigration to be safe and effective for many years without do not affect the population dynamics. side effects on the animals’ behavior, welfare, However, managing feral hog populations or physiology (Killian et al., 2006, Massei et by using injectable contraceptives could be al. 2008, Table 2). Immunocontraceptives also more expensive than trapping, as the costs of have been proposed as a possible means of contraceptives will add to that of trapping, and, decreasing transmission of several wildlife thus, will more likely to be confined to small- diseases by reducing the abundance of newborn, scale, specific contexts where lethal control susceptible animals within the population is not feasible or desirable. Examples of the (Killian et al. 2007). latter are urban areas or national parks where 88 Human–Wildlife Interactions 5(1) hunting is not allowed (C. Gortazar, National once local eradication has been achieved; and Research Institute on Game Biology, personal (3) to partition an area, typically a large island, communication) or where lethal control could into smaller units and to facilitate eradication affect contact rates and spread of diseases. from each unit (Table 2). Compared to trapping or shooting, fertility Many types of fencing, simple or electrified, control is relatively slow in decreasing local are available and often consist of woven wire abundance because the benefits of this method mesh 65- to 80-cm-high with strands of barbed can be accrued only after several years or if wire strung along the top, bottom, and above fertility control is applied in conjunction with the woven wire to create a fence of 110 to 120 other population control option. For instance, cm in height; the fence also often is buried to a fertility control could be used to keep the depth of 40 to 60 cm to prevent hogs from forcing density of feral hogs at a set level once lethal their way through it (Hone and Atkinson 1983, control has been applied (Cowan and Massei State of Hawaii 2007, McCann and Garcelon 2008). More research is ongoing to develop oral 2008). Fences also can be fitted with one-way contraceptives to widen the spectrum of con- gates to allow animals to exit an area but not texts where fertility control could be applied. If to reenter it. Several electric fencing designs oral, nonspecies-specific contraceptives become also have been developed and tested to exclude available, the possibility of affecting nontarget feral hogs; these usually consist of 2 to 3 strands species must be addressed. Hog-specific of electrified fencing spaced 15-30cm apart. feeders have been designed and evaluated for In Australia, different fence designs have bait uptake by target and nontarget species. been tested to protect crops and lambing For instance, the Boar Operated System paddocks (reviewed in Hone and Atkinson (BOS™) is an effective, species-specific device 1983). In California, electric fencing had been developed to deliver contraceptives and other used to prevent feral hogs from entering pharmaceuticals to feral hogs (Massei et al. irrigated summer pastures (Barrett 1978). In 2010). In Europe and the United States, BOS France, steel-wire electric fencing was used has been used successfully to deliver baits to extensively to prevent damage to valuable feral hogs only, unless bears (Ursus americanus), crops over relatively small areas (Vassant which also can feed from the BOS, are present and Boisaubert 1984, Vassant 1994), although (Long et al. 2010; M. Avery, National Wildlife Geisser and Reyer (2004) noted that it may cause Research Center, personal communication). a shift in damage to adjacent, nonfenced fields. Mathematical models designed to evaluate The general conclusions from many studies are the effect of fertility control on population that fence design affects the effectiveness of the dynamics of feral hogs, indicate that a relatively method and that electrification significantly small proportion of females in a population reduces the number of feral hogs crossing must be rendered infertile to reduce population the fences, although the cost of maintenance size (Cowan and Massei 2008). According to is higher for the electric fencing (Hone and these models, treating 30% of the adult females Atkinson 1983, Reidy et al. 2008). To prevent every year with contraceptives that induce overgrown vegetation from damaging the fence permanent infertility, would lead to halving the or interrupting the circuits and to maintain the female population in 5 years. Although more functionality of the fence, herbicides or manual research is required to test these predictions in clearance of vegetation must be used regularly field trials, these results confirm the potential (Littauer 1993). of fertility control to play an important role in Recently developed polywire electric fencing feral hog population management. that uses conductive wires incorporated into Fencing. Fencing has been used in 3 different ribbons or ropes is now available. Compared scenarios: (1) as a preventive measure, to to fixed-steel wire electric fencing, the newer reduce feral hog impact into economically or designs have the advantage that they can be conservation sensitive areas, such as nesting easily set up, removed, and reused so that they grounds, threatened habitats, wildlife refuges, can be employed temporarily. Using portable farms and agricultural fields; (2) as a reactive polywire, electric fencing, Reidy et al. (2008) measure to protect an area from feral hog impact found that 2 strands at 20 and 45 cm from Too many hogs • Massei et al. 89 the ground excluded 75% of hogs that visited from predation by wild boar and found both bait stations in Texas. As most of the hogs that compounds ineffective. Thus, the evidence so crossed this fence were juveniles that slipped far suggests that repellents are unlikely to be under it, the authors concluded that the fence effective in reducing the impact of feral hogs. was more effective in preventing access by Diversionary feeding. Diversionary feeding, adult hogs. In Switzerland, Geisser and Reyer also referred to as supplementary feeding, (2004) found that the 2-strand electric fencing often is carried out by hunters to concentrate locally used to protect crops was not as effective densities of feral hogs in the forest and optimize as shooting the feral hogs to decrease damage culling effort and to decrease crop damage to crops. However, in Slovenia, a combination (Geisser and Reyer 2004; Table 2). To remain of polywire-polytape electric fencing reduced effective, supplementary food must be available damage to maize fields by 100%; but, the continuously, which makes this method researchers observed an increase in damage to expensive in terms of staff and resources, neighbouring arable fields (Vidrih and Trdan however, these costs can be absorbed by hunter 2008). groups and volunteers (Vassant et al. 1987). When permanent fencing is used after The effectiveness of this technique in reducing eradication to keep an area free of feral hogs, crop damage is controversial. While some its efficiency depends on both the type of studies reported that diversionary feeding was fencing and the perimeter (size of patches and successful (Andrzejewski and Jezierski 1978, length of fencing) ratio (Hone 1995). The main Vassant 1994, Calenge et al. 2004), others found disadvantage of permanent fencing is the initial limited or no effect on crop damage (Hahn setup costs and subsequent maintenance costs. and Eisfeld 1998, Geisser and Reyer 2004). In some areas, such as the Hawaiian rainforest, For instance, in France, Vassant et al. (1987) wire fences erected to exclude hogs from used maize, distributed daily along transects sensitive area required monthly inspections and in the forest from late June till early August had to be replaced every 5 to 15 years (Katahira and concluded that, although this method was et al. 1983). In addition, fences also had to be effective to reduce crop damage by wild boar, repaired following storms or earthquakes. its actual cost was similar to that of replacing However, fencing can be employed successfully crop losses. In Switzerland, Geisser and Reyer to control impact by feral hogs. For instance, 42 (2004) found that in September and October, km of fence were used in the Pinnacle National when maize and wheat are ready to harvest Monument, California, to surround an area and particularly vulnerable to damage, wild of 57 km2 and eradicate hogs (McCann and boar hardly visited the feeding stations where Garcelon 2008). supplementary food was provided, irrespective Repellents. A large number of olfactory, of the type of food these stations offered. In acoustic, and gustatory repellents has been another French site, Calenge et al. (2004) used developed to decrease the impact of wildlife corn as dissuasive feeding to protect valuable on human activities (Conover 2002; Table 2). vineyards and reported a 60% reduction in both In a study aimed at identifying deterrents for the proportion of damaged vineyards and the wild boar, Vassant and Boisaubert (1984) tested level of damage, with a net financial benefit 25 potential chemical repellents and acoustic for the farmers. In many European countries, scarers, such as cannons firing at random, practitioners spread corn throughout the year to electronic sound generators, and wild boar attract boar to their hunting grounds. However, alarm calls. The results showed that wild boar several authors (Andrzejewski and Jezierski became habituated to all repellents within 1978, Geisser and Reyer 2004, Schley et al. a few days. In China, Cai et al. (2008) found 2008) warned that this practice could enhance similar results with several repellents used reproductive success and survival of feral hogs by local farmers to protect crops against wild and, thus, contribute to long-term increase in boar and concluded that the only effective damage to crops. measure was the presence of humans in the Even when it is cost-effective, diversionary field. In France, Vilardell er al. (2008) tested 2 feeding should be regarded only as a short-term potential repellents to protect tortoise nests solution to protect crops (Conover 2002). When 90 Human–Wildlife Interactions 5(1) used as a deterrent, diversionary feeding might and consequences of carrying out this type be employed to decrease damage to localized, of control. However, as illegal translocations valuable crops, such as vineyards, for very short are regarded as one of the main causes of the periods. If the amount of diversionary feeding increase of feral hog range, it is unlikely that provided is small compared to the availability this method is proposed to mitigate human– of natural food (5 metric tons of corn versus feral hog conflicts. 900 to 1,500 metric tons of naturally available acorns [Calenge et al. 1994]), and the feeding Monitoring effects of population is localized in time and space, the effect of this control method on feral hog population dynamics Sustained monitoring is critical to determine would be negligible. the effectiveness of the methods used to de- Translocation. Translocation of problem crease feral hog population size or impact. The animals is increasingly advocated to mitigate greatest challenge for managers of eradication human–wildlife conflicts, even if the choice of programs is deciding whether the inability to using this method over other management op- detect hogs indicates that the species has been tions often is dictated by public pressure rather eliminated. Cessation of monitoring too soon than by scientific or economic reasons (Beringer risks declaring eradication incorrectly, but et al. 2002, Conover 2002). Translocations may monitoring for too long wastes resources if the encourage irresponsible introductions, and eradication is complete (Morrison et al. 2007, in many countries translocating feral hogs is Ramsey et al. 2008). The majority of eradications illegal, particularly where the species is non- of non-native mammals from islands remains native (Hutton et al. 2006). Several authors (e.g., unpublished, and many of these data have not Gipson et al. 1998, Spencer and Hampton 2005) been collected (Simberloff 2003). This makes it indicated that transport and release of feral hogs impossible to evaluate the efficiency of these by hunting clubs was the most important factor eradication programs and to learn lessons for explaining the marked increase in distribution future control options. of this species throughout the United States and Several methods are available to monitor Australia. the effects of population control on feral hog A recent review of translocation of problem numbers. Because feral hog absolute numbers animals found that, despite their perceived are notoriously difficult to assess (Sweitzer humaneness, translocations may have a et al. 2000), many estimates rely on indices of detrimental impact on survival rates and lead abundance, such as passive tracking indexes to extreme dispersal movements (Massei et al. derived from activity signs, such as tracks, 2010). In some species, individuals that survive pellet groups, and rooting (Engemann et al. a translocation may suffer from malnutrition, 2001). Other methods are based on monitoring dehydration, decreased immunocompetence, bait uptake at baiting stations or on aerial and predation. In addition, some animals and ground surveys. For instance, using bait resume the nuisance behavior at the release site. uptake to monitor reduction in hog abundance More importantly, in the context of feral hogs, achieved by trapping, Choquenot et al. (1993) translocations have the potential to spread found that trapping had reduced the numbers diseases to conspecifics, humans, domestic of feral hogs in 2 areas by 93 to 100%. However, animals, and livestock. Very few studies an alternative monitoring method based on reporting the costs of translocations neither spotlight counts suggested an 81% and 83% address which stakeholders are expected to reduction, respectively, indicating that the pay for translocating problem animals nor different monitoring method may lead to mention whether and for how long the conflict different conclusions. Indices of abundance lasted before it was resolved following the based on bait consumption tend to overestimate translocations of problem animals. If public population reduction because they do not interest in translocation to resolve human– include animals that do not feed on the bait. feral hog conflicts increases, stakeholders Cruz et al. (2005) established an extensive post- advocating this method should be informed eradication monitoring program on Santiago of the costs (including welfare costs), risks, Island (Ecuador) by distributing goat carcasses Too many hogs • Massei et al. 91 over the entire island and by monitoring for hog of hogs removed per hour, per trap night, or per disturbance 4 times, at 10- to 40-day intervals. area, and often they refer to combined costs of Mc Cann and Garcelon (2008) suggested that different methods. In addition, other costs, such post-population control monitoring should be as travel, administration, data analysis, and used also to direct removal activities toward report writing are seldom reported. In a review areas where signs of hog activity have been of feral hog eradication projects, McCann and observed. Garcelon (2008) found that costs varied from Besides quantifying the impact of population $165,000 to remove 144 hogs in 2 years from a control, monitoring also has the advantage 20 km2 area in California to $3.4 million over 15 that managers can see the long- and short-term years to remove >12,000 hogs from a 194 km2 consequences of control, for instance the de- island. crease in activity signs, such as soil disturbance, Comparisons of costs of different methods or the increase in species previously affected by are valid when these can be applied to the the presence of feral hogs. same location. For instance, on Santiago Ramsey et al. (2008) developed models to Island (Ecuador), Coblentz and Baber (1987) estimate the degree of confidence in the success employed a variety of methods aimed at feral of eradication program when monitoring hog eradication and concluded that trapping failed to detect any more hogs. These models and snaring were ineffective and costly, due allow researchers to determine the relationship to a combination of poor trapping and snaring between detection probability and searching success, costs of building, deploying and effort through aerial or ground hunting and checking traps, and to the malfunctioning of could be used to explain to managers the risk snares. Ground shooting was effective, but time- inherent in decisions that must be taken before consuming, and poisoning was comparatively declaring an eradication completed. Using a the most cost-effective as the cost of individual similar approach, Morrison et al. (2007) were hog removal by poisoning was estimated to be able to reduce the time for eradication and post- 11 times cheaper than shooting and 80 times eradication monitoring of hogs in Santa Cruz cheaper than trapping. Island from an initial estimate of 6 to 11 years In the Pinnacles National Monument, erecting to approximately 2 years. a 42-km-long fence to enclose a 57-km2 area cost $2 million (McCann and Garcelon 2008). Once Cost of mitigation fencing was completed, the eradication of hogs The costs of different control methods through hunting, trapping, and Judas hogs depend on density of animals, topography, cost $632,601 and 13,489 man hours, with an vegetation cover, local capacity (including estimated effort of 24.2 hours per hog removed, volunteers), resources, bureaucracy, and across all the techniques. When the researchers required environmental compliance. Stake- added the total number of hours spent on all holders' expectations concerning the time aspects of the project, such as field work, travel, to resolve a particular conflict also affect the and administration, the effort rose to 67.5 hours choice of methods, the intensity of application per hog removed. (such as number of traps and trap nights, In Hawaii Volcanoes National Park, the number of staff employed, etc.), and, ultimately, cost of wire fencing with single-strand barbed the cost, particularly if the mitigation of the wire at ground level was $18,000 to $26,700/ conflict requires a quick solution. If short-term km (Katahira et al. 1993). In the same area, the reduction of numbers is required, for instance cost of fencing in 2007 was estimated at $50,000 following a disease outbreak, the choice to $140,000/km when the cost of helicopter between population control methods depends required to transport material and personnel on which technique is more likely to provide to otherwise inaccessible areas was included quick reduction (Saunders 1993). (State of Hawaii 2007). In a different context, Costs of feral hog population control are using helicopters in New South Wales to shoot difficult to compare among studies, even when feral hogs reduced the local population by 95% the same method is applied, because they can in only 5 days at a modest cost of $11.35 per hog be expressed in different units, such as number (Saunders and Bryant 1988). 92 Human–Wildlife Interactions 5(1)
In the United States, the cost of fixed hog of this review, we propose a framework to guide exclusion fencing was $8,200 to $21,300/km, decisions regarding control options to mitigate and electric fencing, often used to control the impact of feral hogs (Figure 2). impact by deer, cost about $2,000/km (Reidy In many parts of the world where feral hogs are et al. 2008). In the United Kingdom the cost of non-native, ecologists believe that the ultimate permanent deer fencing was $4,800 to $8,800/km aim of control should be eradication. This view, (Rural Development Service 2006). The cost of however, is not shared by all stakeholders (e.g., polywire-polytape electric fencing successfully recreational hunters). Complete eradication used to control feral hogs in Slovenia was $310 of feral hogs is difficult and expensive, but it to $380/km (Vidrih and Trdan 2008). has been achieved, largely on small islands. When evaluating different options for a Feral hogs have now been eradicated from at feral hog control program, managers must least 25 islands with areas from 5 to 600 km2 also acknowledge that the cost of hog removal (Kessler 2002, McCann and Garcelon 2005). increases substantially with time. For instance, Sites with newly established, geographically the cost of hunting hogs in the Namadgi isolated populations can be regarded as National Park, Australia, increased 5-fold from ecological islands. In these areas, efforts the first 6 months to the third year (Hone and should be focused toward eradication before Stone 1989). However, as lessons from previous the population range and numbers increase, eradication programs are learned, recent although disturbance could cause hogs to move eradications have been become substantially considerable distances (Leaper et al. 1999) and more cost-effective. For instance, the time may ultimately affect the success of a local taken to eradicate hogs from Santa Cruz Island eradication. For islands and geographically was half of that required on a neighbouring isolated populations, McCann and Garcelon island (Santa Rosa Island) of similar size and (2008) suggested that an intensive eradication 12 times faster than that on Santiago Island, program should be preferred to sustained Ecuador (Parkes et al. 2010). The success of the control for the following reasons: (1) only a high- Santa Cruz Island eradication program was intensity program can achieve eradication in a due to a combination of reasons: (1) extensive short period; (2) fewer hogs need to be culled stakeholder consultations prior to agreeing to because populations are not given the time to fund and proceed with the eradication; (2) a reproduce or to learn to avoid control; (3) the fixed-price funding model, where professional high cost of an intensive eradication program contractors were paid for completion of is likely to be less than that of sustained control eradication, regardless how long it took or of over a period of several years; and (4) a short, how much it cost them; and (3) use of modern well-managed program is likely to receive less technologies, such as GIS mapping of animals public scrutiny and opposition. In addition, the removed in different areas, to coordinate efforts longer an eradication project runs, the more and optimize control (Parkes et al. in 2010). it is exposed to factors that can undermine its When feral hog density becomes very low, success (Morrison 2007, Parker et al. in press). motivating staff is often a major challenge, and These factors include reinvasion of areas financial incentives might help to boost staff already cleared of feral hogs, reproduction that morale. Cruz et al. (2005) mentioned that social, causes the hog population to increase, public moral, and financial incentives were crucial in opposition, legal challenges arising in the course maintaining the motivation of hunters in the of the project, increased lack of staff motivation, last phase of hog eradication. and funders’ fatigue which may result in lack of sustained funding to complete the program. A decisional framework to Post-eradication monitoring also should be manage feral hogs included in any eradication program to confirm Current patterns in feral hog expansion of achievement of the objectives (Figure 2). range and numbers suggest that these trends If the hog population is on the mainland and will persist, and conflicts with human activities is surrounded by others, eradication might be will increase if long-term, effective population very difficult to achieve because, even if the control is not undertaken. Based on the results area is fenced, the risk of reinvasion persists. If Too many hogs • Massei et al. 93
Is eradication feasible in terms of logistics, costs, and social acceptance? NO YES
Is a reduction in population density
expected to mitigate the impact?
Can re-invasion NO YES be a voided? Evaluate alternative Can impact mitigation be sustained, methods to prevent impact. humane, socially acceptable and cost- efficient? YES NO Re-evaluate c ontrol s trategy.. YES NO
Can the success Can the effectiveness of Re-evaluate of er a d i ca t i on b e control be monitored ? control strategy. monitored ?
Risk of failure. YES NO Risk of failure. NO YES
Proceed with eradication. Proceed with control.
Monitor achievement of objectives.
Figure 2. Decision tree to evaluate control options to decrease the impact of overabundant populations of feral hogs on human interests. reinvasion cannot be avoided, managers should densities that minimize human–feral hog reevaluate the control strategy to determine conflicts. Sustained control includes methods whether the benefits of achieving temporary to provide short-term solutions (e.g., to reduce eradication justify the costs and whether other crop damage or disease outbreak) and long- options, such as temporary fencing to protect term management to mitigate or prevent the valuable crops, rather than control of feral hog occurrence of conflicts for several years. If a numbers could be successfully employed to reduction of feral-hog density is expected to reduce the conflict. mitigate the conflict, different control methods Where feral hogs have long been established, should be evaluated to determine their and particularly where the species is native, their feasibility, sustainability, costs, humaneness distribution and numbers make eradication and social acceptance (Figure 2). In case any very unlikely. Invariably, these areas have long- of these issues is expected to be controversial, established hunting traditions and recreational for instance if a strong public opposition arises hunters that oppose eradication. In some toward some of the proposed methods or if instances, the meat derived from hunting can adequate funding is not available to implement indeed contribute to the local supply of protein a population reduction program, the control or generate income from hunting tourism strategy should be reevaluated. and the meat export (Ramsay 1994, Milner- The review indicated that only poisoning Gulland et al. 2002). Although intuitively and shooting can quickly reduce the size of a perpetual freedom from a pest species has a population. However, poisoning is illegal in very high value, in some instances the benefits many countries and is unlikely to become a of retaining this species could partly offset the common practice, at least in Europe and in costs. This is because future benefits may have the United States. On the large scale, shooting lower economic value than those achieved that is carried out by recreational hunters immediately (Bomford and O’Brien 1995). does not appear to control feral hog numbers, Where eradication is unfeasible or is opposed probably because (1) a conflict of interests by local groups, sustained control must be due to hunters being more likely to support employed to keep feral hog populations at sustainable harvest than drastic reductions 94 Human–Wildlife Interactions 5(1) in feral hog numbers, and (2) poor planning, and fencing. In Australia and New Zealand based on inaccurate estimates of local densities shooting from helicopters often is integrated and lack of knowledge of the effects of different with poisoning to provide immediate solutions levels of hunting pressure on population size. to human–feral hogs conflicts. Conversely, when professional hunters are If methods to reduce immigration are not involved or when feral hog meat derived from available or practical to implement, control hunting provides a significant part of people’s should be directed toward both decreasing diet, shooting may substantially reduce feral reproduction and increasing mortality. As hog numbers (Geisser and Reyer 2004, Parker oral contraceptives are not available for hogs, et al. in press). Based on these considerations, the most cost-effective methods are shooting, Geisser and Reyer (2004) recommended the trapping, or using toxicants. For isolated development and introduction of new harvest populations or in suburban areas where these models among local hunting teams to maximize methods might be illegal or simply impossible population control. We suggest these models to carry out due to concerns about human safety could include: (1) integrating hunting with or the impact on nontarget species, fertility other methods, such as trapping or fencing; (2) control could offer a valid alternative to lethal use of reliable methods to estimate feral hog control options. density before and after control; (3) monitoring With few exceptions, very little research of the impact of different hunting pressures has been conducted to determine what on population size and impact; and (4) proportion of a feral hog population should coordinating efforts with other hunting groups be targeted to decrease population size, and other stakeholders to agree to participatory despite the requirement in several countries management of feral hog populations. for management plans to be submitted to the Hogs that survive control campaigns may authorities before hunting can commence. play a crucial role in rebuilding the population Clearly, this is an area that warrants further or maintaining diseases. Thus, it is important research, in particular to quantify the effects of in such instances that alternative methods of different control methods on population size control are also applied to target survivors. and to identify optimal integrated management For instance, Choquenot et al. (1993) observed approaches. that poisoning and trapping preferentially removed sows, so males had to be targeted Acknowledgments in residual populations. Control of feral hogs This study was funded by a grant from also requires managers to alter techniques in the Darwin Initiative to manage human– response to changing animal densities, animal feral livestock conflicts in the Centre Hills, behavior, and environmental conditions. For Montserrat. We are grateful to S. Sanders, J. instance, trapping does not always remove Millet, G. Gray, R. Smith, and R. Quy for useful older, more experienced hogs, ground shooting discussions on various aspects of the review may preferentially remove solitary boars, and and to J. Allan for comments on the first draft. trapping may preferentially remove females (Choquenot et al. 1993, Saunders et al 1993, Literature cited Mitchell 1998). Anderson, S. J., and C. P. Stone. 1993. Snaring The vast majority of successful eradication to control feral hogs (Sus scrofa) in a remote programs employed an integrated management Hawaiian rain forest. Biological Conservation approach where several control options were 63:195–202. carried out at the same time or in sequence. Andrzejewski, R., and W. Jezierski. 1978. Man- This ensured that animals that could not be agement of the wild boar population and its targeted by 1 technique could still be removed effect on commercial land. Acta theriologica by adopting complementary control methods. 23:309–333. Conversely, current approaches to feral hog Barrett, R. H. 1978. The feral hog on the Dye Creek population control across Europe and the Ranch, California. Hilgardia 46:283–355. United States typically involve only shooting, Beringer, J., L. P. Hansen, J. A. Demand, and J. occasionally coupled with diversionary feeding Sartwell. 2002. Efficacy of translocation to con- Too many hogs • Massei et al. 95
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from hunting mortality in an intensively hunted Wilson, C. J. 2005. Feral wild boar in England: sta- wild boar population. Journal of Wildlife Man- tus, impact and management. Defra, London, agement 72:1532–1539. UK. Twigg, L. E., T. J. Lowe, G. R. Martin, and M. Ev- erett. 2005. Feral hogs in north-western Aus- tralia: basic biology, bait consumption, and Giovanna Massei is a senior research the efficacy of 1080 baits. Wildlife Research scientist at the Food and Environment Research 32:281–296. Agency in York, United Vassant, J. 1994. Les techniques de prévention Kingdom (UK). She des dégâts de sanglier. Office National de la received her B.Sc. degree in Natural Sciences from Chasse, Paris, France. Bulletin Mensuel ONC the Univeristy of Flor- 191:90–93. ence (Italy) and her Ph.D. degree in wild boar ecol- Vassant, J., and B. Boisaubert. 1984. Bilan des ogy from the University expérimentations enterprises en Haut-Marne of Aberdeen (UK). Her pour réduire les dégâts de sangliers (Sus scro- research interests focus on nonlethal methods to fa) à l’encontre des cultures agricoles. Pages manage wildlife, including fertility control, condi- 187–199 in Symposium International sur le tioned aversion, ecology, and population control of wild boar and feral hogs. She published 1 book on sanglier, 24-26 April 1984, Toulouse, France. the natural history of wild boars, 50 papers in peer- Vassant, J., J. M. Jullien, and S. Brandt. 1987. reviewed journals, and circa 60 popular scientific Réduction des dégâts de sangliers sur blé articles on wildlife management and behavioral ecology. et avoine en été. Etude de l’efficacité de l’épandage de maïs grain en forêt. Bulletin Sugoto Roy is an ecologist focusing on the Mensuel ONC 113:23–34. ecology and management of non-native vertebrates at Vidrih, M., and S. Trdan. 2008. Evaluation of dif- the Food and Environment ferent designs of temporary electric fence sys- Research Agency, UK. He received both his B.S. degree tems for the protection of maize against wild in biology and M.S. degree in boar (Sus scrofa L., Mammalia, Suidae). Acta wildlife management from Im- Agriculturae Slovenica 91:343–349. perial College, London Univer- sity, and his Ph.D. degree from Vilardell, A., X. Capalleras, J. Budó, F. Molist, Bristol University, where he did and P. Pons. 2008. Test of the efficacy of two research on the management of the introduced small Indian chemical repellents in the control of Hermann’s mongoose in the Republic of Mauritius. He has tortoise nest predation. European Journal of worked on the management of non-native species, in particular carnivores, such as the American mink, Wildlife Research 54:745–748. small Indian mongoose, and feral cats that were Waithman, J. D., R. A. Sweitzer, D. Van Vuren, introduced to island ecosystems. J. D. Drew, A. J. Brinkhaus, I. A. Gardner, and W. M. Boyce. 1999. Range expansion, popu- Richard Bunting completed a B.Sc. degree in zoology from Durham lation sizes, and management of feral hogs University, UK, in 2008. Since in California. Journal of Wildlife Management then, he has worked on wild 63:298–308. boar fertility control and feral hog management in England Walker, M. 2008. In Berlin's war, some side with and south Texas. He studied the hogs. Wall Street Journal,