Timing and extent of crop damage by wild pigs (Sus scrofa Linnaeus) to corn and peanut fields
Introduction
Wild pigs (Sus scrofa Linnaeus) – known as wild boar in their native ranges - have one of the most extensive geographic distributions of any large mammal, occurring on all continents except Antarctica (Barrios-Garcia and Ballari, 2012). Within North America, wild pigs have existed on the landscape since the mid-1500s, although their distribution remained relatively stable for several centuries (Mayer and Brisbin, 2009). Beginning in the 1990s wild pig populations began increasing rapidly in size and distribution (Bevins et al., 2014; Snow et al., 2017; Mayer and Beasley, 2018), with the current estimate within U.S. exceeding 6.9 million individuals (Lewis et al., 2019). While range expansion has contributed to the rapid surge in population expansion within North America, illegal translocation of animals is widely regarded as a the most substantive driver of this increase (Gipson et al., 1998; Bevins et al., 2014, Tabak et al., 2017; Beasley et al., 2018). Similar population increases have been reported in other parts of their invasive and native ranges over the last few decades, although in Europe a combination of warmer winters and increased mast and maize availability have had the most critical influence on wild boar population growth (Saez-Royuela and Telleria, 1986; Geisser and Reyer, 2005; Massei et al., 2014).
Concomitant with increasing populations of wild pigs, over the last few decades the extent and distribution of damages associated with this species have increased markedly (Bevins et al., 2014). For example, wild boar damage to agriculture increased by 1600% from 1971 to 2004 in Luxembourg (Schley et al., 2008). Thus, the increase in range and population of wild pigs and wild boar should be seen as a global threat to agriculture and ecosystem health (Spear and Chown, 2009). Wild pigs overturn the soil while foraging (i.e. rooting), increasing erosion and altering the soil structure and subsequent nutrient cycle (Wirthner et al., 2011; Palacio et al., 2013), which can have cascading impacts on native communities and ecosystems (Barrios-Garcia and Ballari, 2012). Disturbances caused by rooting can have both direct and indirect effects on native plant communities, while their opportunistic predatory behavior poses a risk to invertebrate and vertebrate communities, including small mammals, ground nesting birds, snakes, insects, and salamanders (Massei and Genov, 2004, Wilcox and van Vuren, 2009). In particular, given their behavioral and dietary plasticity, wild pigs are able to exploit seasonally available resources such as cereal crops (Herrero et al., 2007; Schley and Roper, 2003). Indeed, studies in Spain have shown wild boar selectively target cornfields (Zea mays Linnaeus) over natural food sources when available (Herrero et al., 2007). Consequently, wild pig damage to agriculture is extensive throughout their native and invasive range. For example, wild pigs cause an estimated $61 million of damage to corn and $40 million of damage to peanuts (Arachis hypogaea Linnaeus) annually across 11 states in the U.S. alone (Anderson et al., 2016).
Wildlife damage to crops is typically variable in both space and time, with damage often concentrated during specific plant growth stages (Garrison and Lewis, 1987; Chiyo et al., 2005; Humberg et al., 2007; Bleier et al., 2017). For example, damage to a variety of crops in Africa and Asia by elephants (Loxodonta Africana Blumenbach and Elephas maximus Linnaeus, respectively) and pig species (Sus scrofa Linnaeus, Potamocherus larvatus Cuvier, and Phacochoerus africanus Gmelin) is often less severe during intermediate growth stages compared to either the seedling or mature stage (Gross et al., 2018). Similarly, wild boar primarily consume crop plants when nutritional value is maximal, such as immediately after planting or just prior to harvest, with a reduction in consumption and damage between these peaks (Gimenez-Anaya et al., 2008; Schley et al., 2008; Bleier et al., 2017). Wildlife damage to crops also is highly variable spatially, with both inter- and intra-field damage often correlated with habitat attributes associated with animal movements or preferred non-agricultural resources (e.g., forest edges, riparian areas; DeVault et al., 2007; Retamosa et al., 2008, Thurfjell et al., 2009). Although ecological generalists, wild pig movements are often driven by a combination of land cover and food abundance (Morelle et al., 2015). Indeed, Lombardini et al. (2017) found wild boar damage, generalized to a wide variety of crops in Sardinia, was most severe in proximity to forest and shrub edge. Similarly, Cai et al. (2008) found distance to forest and streams impacted the probability of wild boar damage to corn, potato (Ipomoea batatas Lamarck), and wheat (Triticum aestivum Linnaeus) in China.
Despite the extent of economic impacts of wild pigs to agriculture across their range, little quantitative research regarding timing of damage has been published. Similarly, although resource selection patterns of wild pigs have been reported (Spitz and Janeau, 1995; Thurfjell et al., 2009), the extent to which wild pig damage is correlated with preferred habitat attributes has largely been unexplored within their introduced range. Given the paucity of data on wild pig damage to agriculture within their introduced range, the objectives of this research were to identify the timing and extent of wild pig damage to corn and peanuts in central South Carolina, USA, and to model the influence of landscape habitat attributes and relative abundance of wild pigs on the presence and extent of damage. We hypothesized wild pig damage to these crops would follow caloric density of the crop fruit, and peak at planting and upon maturation for each crop. We also expected the presence and severity of wild pig damage would be positively associated with the proximity of fields to wetlands, forested areas, and other preferred habitat attributes, as well as pig abundance. Such data are needed to elucidate spatial and temporal dynamics associated with wild pig damage to crops to facilitate development of appropriate management strategies for minimizing impacts of wild pigs to agricultural producers.
Section snippets
Study area
We conducted crop damage surveys in Calhoun, Aiken, Bamberg, Colleton, and Hampton Counties, in the Southern Coastal Plain region of South Carolina, USA during the 2017 and 2018 growing seasons (Fig. 1). Calhoun County is in central South Carolina and bordered on the northeast by the Congaree River, adjacent to Congaree National Park. Aiken, Hampton and Bamberg Counties are located to the west-southwest of Calhoun County, and the five counties together have a total area of 6238 km2, of which
Damage counts
Over the two field seasons, we surveyed a total of 29 corn and 41 peanut fields. As farmers in South Carolina typically follow a three-year planting rotation, we placed cameras at different sites from 2017 to 2018. The area of surveyed fields across both years was 672.6 ha corn (mean = 21.44, SD = 19.73) and 734.7 ha peanuts (mean = 17.61, SD = 16.66) for a total of 1415.9 ha. Of the 29 surveyed corn fields, 18 (62%) had identifiable wild pig damage. Wild pigs, water, and small mammals were
Discussion
Wild pigs have one of the widest distributions of any terrestrial mammal on the planet, with the global distribution of both native and introduced populations spanning more than 100 countries (Barrios-Garcia and Ballari, 2012; Ballari and Barrios-Garcia, 2014; Herrero et al., 2007). As a result, agricultural damage by wild pigs is a global issue and thus the ability to determine factors influencing the timing and location of damage is crucial to mitigating conflicts between humans and wild pigs
CRediT authorship contribution statement
C.M. Boyce: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. K.C. VerCauteren: Conceptualization, Methodology, Resources, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. J.C. Beasley: Conceptualization, Methodology, Formal analysis, Resources, Writing - original draft, Writing - review & editing, Supervision, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Mention of commercial products or companies does not represent an endorsement by the U.S. Government. The findings and conclusions in this preliminary publication have not been formally disseminated by the US Department of Agriculture and should not be construed to represent any Agency determination or policy. This research was supported by the intramural research program of the U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Wildlife Services, as well as the US
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