Managing feral pigs to mitigate kauri dieback disease in the face of ecological and epidemiological uncertainty

Figures & data

Figure 1. Hypothesised role of feral pigs (Sus scrofa) in the spread of kauri dieback disease in New Zealand. Arrows depict theoretical mechanisms by which various feral pig behaviours could contribute to Phytophthora agathidicida infections in kauri (Agathis australis). Note these are not established pathways, but only potential linkages, some of which could be tested through experimental studies.

Table 1. Summary of research needs for investigating the role of feral pigs (Sus scrofa) in the spread of Phytophthora agathidicida, the causative agent of kauri dieback disease in New Zealand.

Research need	Rationale	Potential approaches
Identify kauri dieback disease status and/or P. agathidicida infection status	Effective before-and-after surveillance of kauri is necessary to measure the success of any mitigation efforts. Accurate spatial and temporal comparisons within and between kauri forests are crucial for assessing spread of the disease and initiating prompt mitigation.	Identify/develop aerial or ground survey methods capable of documenting visible disease symptoms in kauri trees/forests (beyond the scope of this review). Identify/develop screening protocols capable of identifying whether an individual kauri tree is infected (beyond the scope of this review).
Confidently detect P. agathidicida presence in animal-associated soil and faecal samples	Determine if feral pigs are transporting P. agathidicida infected soil, internally and/or externally. Better understand the distribution of P. agathidicida in the soil environment, including pig wallows, areas used by livestock, and hydrological features.	Identify/develop appropriately sensitive tests capable of reliably detecting P. agathidicida from small soil samples, particularly samples associated with animal tissue (beyond the scope of this review).
Correlate pig abundance and kauri dieback disease prevalence in kauri forests	Strong positive correlations are indirect evidence that pigs play a role in pathogen spread (this evidence is not causative). The results could indicate potential sites where further research is needed, or mitigation is warranted.	Design a study comparing pig presence/absence, relative abundance, or activity levels (e.g. rooting activity, wallows) with visible symptoms of kauri dieback disease to re-evaluate the relationships explored by Auckland Council (2011). Test soil-associated samples from areas of high ferak pig activity, such as wallows, for the presence of P. agathidicida.
Determine how often and under what conditions feral pigs are associated with P. agathidicida infected soil	To date there have been no P. agathidicida detections from feral pigs in New Zealand. Any detections would confirm that feral pigs play a role in the spread of this pathogen and may provide insights into possible modes of spread.	Screen hunter-killed feral pigs harvested from in and around kauri forests to expand on the conclusions made by Krull et al. (2013) and Bassett et al. (2017). Sampling might consist of soil collected from snouts, trotters, and/or stomachs.
Basic feral pig ecology, especially habitat selection and movement ecology	If feral pigs are playing a role in pathogen spread, this spread would be related to feral pig ecology, which is largely unknown.	GPS collar deployments to collect data on feral pig distribution, density, and movement in, and adjacent to, kauri habitat.
Assess the effectiveness, practicality, and cost of exclusion fencing to prevent new P. agathidicida infections in uninfected kauri trees	By comparing differences between fenced (feral pigs absent) and unfenced (feral pigs present) areas, a precautionary management strategy could be tested that prioritises areas not yet infected.	Adapt the study by Perroy et al. (2021) on rapid ‘Ōhi‘a death in Hawai‘i for kauri dieback disease in New Zealand. Such a study would require monitoring both feral pigs and the disease or infection status of kauri trees.

Table

Vertebrate toxic agent	Advantages	Disadvantages	Potential for use in NZ
Sodium nitrite (NaNO2, used in Bait-Rite Paste®)	− Currently the only registered toxin for feral pigs in New Zealand − Can be used in bait stations to limit non-target access − Has an antidote, methylene blue − No licence is required to purchase and use it	− Efficacy in the field poorly understood	High
1080	− Can be effective against pigs − Can be applied to grains, cereal baits, or meal − Comparatively cheap	− Non-target impacts (specifically, high risk of primary and secondary poisoning of dogs) − Hunter concerns about residues in carcasses − Users must be licensed	Low
Cyanide	− Pen efficacy established against pigs − Fast acting and the most humane of toxin options − Feratox® formulation available	− Preliminary field results (Australia) not successful − Possibility of pigs developing bait shyness − Requires user licence	High
Cholecalciferol	− Delayed action may reduce risk of bait shyness − Lower risk of secondary poisoning non-targets	− Acute toxicity to pigs not known − Humaneness unknown − Requires user licence	High
Anticoagulants (especially Warfarin)	− Delayed action may reduce risk of bait shyness − Effective against pigs in Australia	− Animal welfare concerns − Concerns about residues in carcasses and environment	Low
Zinc phosphide	− Residues/secondary risk probably comparatively low	− Unknown efficacy against pigs − Animal welfare unknown − Licence requirements unknown	Low-medium
Phosphorus	− Effective in carcass baits (efficacy of other bait types unknown)	− Unacceptable animal welfare − Secondary risks and residues not well researched − Risks to human handlers − Requires user licence	Low

Auckland Council. 2011. An investigation into the distribution and spread of kauri dieback disease within the Waitakere Ranges. Draft Report. 2011. Auckland Council. Auckland, New Zealand.

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Krull CR, Waipara NW, Choquenot D, Burns BR, Gormley AM, Stanley MC. 2013. Absence of evidence is not evidence of absence: Feral pigs as vectors of soil-borne pathogens. Austral Ecology. 38:534–542. doi:10.1111/j.1442-9993.2012.02444.x.

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Bassett IE, Horner IJ, Hough EG, Wolber FM, Egeter B, Stanley MC, Krull CR. 2017. Ingestion of infected roots by feral pigs provides a minor vector pathway for kauri dieback disease Phytophthora agathidicida. Forestry: An International Journal of Forest Research. 90:640–648. doi:10.1093/forestry/cpx019.

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Perroy RL, Sullivan T, Benitez D, Hughes RF, Keith LM, Brill E, Kissinger K, Duda D. 2021. Spatial patterns of ‘Ōhi‘a mortality associated with rapid ‘Ōhi‘a death and ungulate presence. Forests. 12:1035–1017. doi:10.3390/f12081035.

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