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New Zealand Journal of Zoology
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tnzz20
Alternatives for mammal pest control in New
Zealand in the context of concerns about 1080
toxicant (sodium fluoroacetate)
Bruce Warburton, Charles Eason, Penny Fisher, Nick Hancox, Brian Hopkins,
Graham Nugent, Shaun Ogilvie, Thomas A. A. Prowse, James Ross & Phil E.
Cowan
To cite this article: Bruce Warburton, Charles Eason, Penny Fisher, Nick Hancox, Brian
Hopkins, Graham Nugent, Shaun Ogilvie, Thomas A. A. Prowse, James Ross & Phil E.
Cowan (2021): Alternatives for mammal pest control in New Zealand in the context of
concerns about 1080 toxicant (sodium fluoroacetate), New Zealand Journal of Zoology, DOI:
10.1080/03014223.2021.1977345
To link to this article: https://doi.org/10.1080/03014223.2021.1977345
Published online: 04 Oct 2021.
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REVIEW ARTICLE
Alternatives for mammal pest control in New Zealand in
the context of concerns about 1080 toxicant (sodium
fluoroacetate)
Bruce Warburton
a
, Charles Eason
b
, Penny Fisher
a
, Nick Hancox
c
, Brian Hopkins
a
,
Graham Nugent
a
, Shaun Ogilvie
d
, Thomas A. A. Prowse
e
, James Ross
b
and
Phil E. Cowan
a
a
Manaaki Whenua –Landcare Research, Lincoln, New Zealand;
b
Lincoln University, Department of Pest
Management and Conservation, Lincoln, New Zealand;
c
OSPRI NZ Ltd, Wellington, New Zealand;
d
Eco
Research Associates Ltd, Christchurch, New Zealand;
e
University of Adelaide, Mathematical Sciences,
Adelaide, Australia
ABSTRACT
The ongoing use of 1080 toxin for the control of mammal pests in
New Zealand remains highly contentious. Several reviews over the
last 25 years identified information gaps and areas of concern, both
social and scientific. In this paper these areas of concern are
discussed and the extensive scientific and social research that has
been undertaken to clarify and address them is reviewed.
Although there has been a major national investment in research
aimed at finding an alternative to 1080, that has not yet been
fully achieved because of low or inconsistent efficacy and/or low
cost-effectiveness of alternatives, regulatory difficulties in
obtaining approval for aerial delivery of any alternative, and toxic
residue concerns. Finding an alternative that has similar efficacy
while satisfying the demands for species-selectivity, no residues,
and humaneness is a continuing challenge. The most promising
prospect appears to be through understanding the genome of
the target animals and opportunities for genetic manipulation,
either by developing species-specific designer lethal toxicants
based on genome mining, or by gene editing to develop non-
lethal technologies. Both will require considerable time and
funding for research, and considerable effort and engagement to
address social and regulatory hurdles.
ARTICLE HISTORY
Received 14 June 2021
Accepted 3 September 2021
HANDLING EDITOR
James Briskie
KEYWORDS
1080; alternative control
methods; bovine
tuberculosis; conservation;
sodium fluoroacetate;
vertebrate pest control
Introduction
Aerial distribution of poison baits containing the toxicant sodium fluoroacetate (1080)
has been widely used in New Zealand for the large-scale control of introduced small-
mammal pests for almost 70 years. This globally unique practice continues, despite some-
times fierce opposition from some sections of society, simply because aerial 1080 baiting
is currently considered to be by far the most affordable approach to addressing the major
threats to conservation and agricultural production (particularly bovine tuberculosis)
posed by small-mammal pests at landscape scales.
© 2021 The Royal Society of New Zealand
CONTACT Bruce Warburton warburtonb@landcareresearch.co.nz
NEW ZEALAND JOURNAL OF ZOOLOGY
https://doi.org/10.1080/03014223.2021.1977345
The technique was first developed in the 1950s in response to increasing concerns
about the spread and impacts on conservation lands, plantation forests, and agriculture
of a range of introduced mammals, including several deer species, goats, pigs, brushtail
possums, mustelids, rabbits, wallabies, and ship and Norway rats (Gibbs et al. 1945; Kean
and Pracy 1949; Holloway 1950). These concerns drove pest managers to seek control
methods that could be used over large areas and be more cost-effective than the com-
monly used ground-based techniques of trapping and shooting.
Initial trials of toxicants such as strychnine, arsenic, and phosphorous, mainly for
rabbit control, were unsuccessful, and none were suitable for aerial application
because of problems with formulation and environmental contamination, and risks to
non-target species (McIntosh 1948). However, successful trials in Tasmania using
sodium fluoroacetate (1080) to control rabbits persuaded New Zealand’s Department
of Agriculture to evaluate this toxicant (Cairney and Forrester 1954; Lazarus 1956). Fol-
lowing the successful use in New Zealand of 1080 to control rabbits (Wodzicki and
Taylor 1957; McIntosh 1958), it was trialled on a range of other species, including tahr
(Douglas 1967), red and fallow deer (Daniel 1966), and possums (Batcheler et al.
1967). Baits containing 1080 were first registered for aerial application in 1964 (Green
2004) under the Agricultural Chemicals Act. Later, 1080 was specified as a controlled pes-
ticide under the Pesticides Act 1979. The first aerial 1080 baiting operation specifically
targeting ship rats was carried out in 1989 (Innes et al. 1995).
Concerns and opposition to aerial 1080 baiting in New Zealand soon followed its
widespread adoption. Publicity surrounding the deaths of introduced and indigenous
birds after aerial 1080 operations in the 1970s prompted a ministerial review of the prop-
erties, effectiveness, and regulatory controls of 1080, and potential alternatives (Batcheler
1978). That review supported the continued use of 1080 but identified the need for
improvements in standardising 1080 stock solutions, the adoption of best-practice
manuals, bait quality monitoring, and methods for assessing risks to non-target
species. The review also identified research needed to fill a range of knowledge gaps.
Many of these recommendations were implemented (PCE 1994). Use of 1080 contin-
ued and increased, but opposition remained. This resulted in the Parliamentary Commis-
sioner for the Environment investigating in 1994 the need for 1080, its use, benefits and
costs, and alternatives (PCE 1994). The Commissioner’s conclusion supported the
ongoing use of 1080. Then, in 2006, the Animal Health Board (now OSPRI) and the
Department of Conservation applied to the Environmental Risk Management Authority
(ERMA –now the Environmental Protection Authority, EPA) for a formal reassessment
of 1080. ERMA decided that 1080 use should continue to be permitted.
1
Subsequently,
opposition escalated to a point where some members of Parliament called for a morator-
ium on the use of 1080. This triggered a re-investigation by the PCE, which concluded
that ‘not only should the use of 1080 continue …but that we should use more of it’
(PCE 2011).
These fact-based reviews have done little to reduce the long-standing opposition to
1080, and also highlight the limited success in developing alternative tools. There are cur-
rently renewed calls, including from politicians, for alternatives, most notably from the
Ban1080 political party.
2
There is a risk, if the basis of the opposition is not well under-
stood, that time and money will be wasted developing, testing, and registering alterna-
tives to 1080 that themselves become targets for opposition. This paper therefore
2B. WARBURTON ET AL.
outlines the major areas of concern raised by 1080 opponents, and then summarises the
changes that have been made to the use of 1080 to address these concerns. It then
explores the impact on conservation and bovine tuberculosis (TB) control if 1080 use
should cease without an equally effective replacement. Finally, it summarises what
additional toxicants for mammal pest control have been, or are being, investigated,
and to what extent they might address those concerns.
What is 1080 and how is it currently used?
Sodium fluoroacetate (1080), as currently used as a vertebrate toxic agent (VTA), is the
synthetic sodium salt of the natural toxin fluoroacetic acid, which occurs in some plants
(e.g. species of Gastrolobium and Dichapetalum; Batcheler 1978). Consumption of such
plants may be lethal. In Brazil, for example, about 100,000 cattle are killed per annum by
browsing such toxic plants, one species Palicourea marcgravii producing 1080 (Tokarnia
et al. 2002).
Sodium fluoroacetate was first identified as a potential vertebrate (rodent) pesticide in
the early 1940s by chemists at the Patuxent Research Centre, Maryland, USA, where it
had been given the laboratory serial number 1080 (Kalmbach 1945). Comprehensive
information about 1080 is provided by Green (2004) and the New Zealand Parliamentary
Commissioner for the Environment reports (PCE 1994,1998,2011). In New Zealand,
formulations of 1080 must be registered by the New Zealand Food Safety Authority,
Agricultural Compounds and Veterinary Medicines Group, and they are classified as
VTAs. For aerial control, 1080 is most frequently mixed into cereal baits for possum
and ship rat control, and surface coated onto carrot baits for rabbit control. Paste-
based formulations are also registered for possum and rabbit control.
New Zealand is the world’s largest user of 1080. This reflects the unique situation in
New Zealand, which has only two native mammal species (both are bats with restricted
distributions) but 35 introduced mammal species. Several of these are widespread and
have significant impacts on both native biodiversity and/or agricultural production
(King 2005). The extensive use of 1080 baits for mammal pest control in New Zealand
therefore poses less risk to native mammals than in other countries with more extensive
native mammal fauna, such as Australia. Research (reviewed in PCE 1994,2011, and
summarised in EPA reports) has shown that the use of 1080 in baits results in
minimal residue issues in carcases and water. Additionally, the welfare impacts of
1080 on pest mammal species are moderate compared, for example, to more recently
developed anticoagulant toxicants also widely used for pest mammal control (Beausoleil
et al. 2016). However, risk from 1080 residue in possum and rodent carcasses remains a
concern for unrestrained pet and working dogs.
In 2018, 55% of the total land area aerially treated with 1080 baits was for possum
control as part of the OSPRI goal of eradicating TB from livestock and wildlife by
2055. The aim is to protect domestic deer and cattle from this zoonotic disease, for
which possums are the primary wildlife host (Hutchings et al. 2013; Livingstone et al.
2015). Most of the remainder (42%) was aerially treated by DOC to reduce possum
and rat abundance. This was done to protect a range of native species, including the
North Island kōkako, whio (blue duck), kererū, kea, Hochstetter’s frog, ngirungiru
(South Island tomtit), kārearea (New Zealand falcon) and kiwi (several species, including
NEW ZEALAND JOURNAL OF ZOOLOGY 3
North Island brown kiwi and great spotted kiwi); at-risk populations of long-tailed bats;
and vulnerable plant species such as Hall’stōtara.
3
Historically, aerial 1080 baiting was an important rabbit control tool, but its use
declined temporarily to zero after rabbit haemorrhagic disease was released in 1997,
before increasing somewhat a decade or so later (Nugent, Twigg et al. 2012). Its use con-
tinues at a low level, and at a much smaller scale than for TB and conservation purposes.
In 2018 there were only three aerial 1080 operations against rabbits.
4
Concerns about the use of 1080
General perspectives
There have been numerous surveys of the attitudes of the New Zealand public to existing
and proposed forms of vertebrate pest control (Sheppard and Urquhart 1991; Fitzgerald
et al 1996; Wilkinson and Fitzgerald 1997,2006; Fraser 2001; Farnworth et al. 2014; Kan-
nemeyer 2017; Kaine et al. 2020; MacDonald, Balanovic et al. 2020). The most commonly
raised concerns related to the use of 1080 are: (1) its potential environmental and human
health risk, particularly the perception that 1080 enters waterways and poses risks to
human health, and that it persists in the environment; (2) the limited control over
where baits land when applied aerially; (3) its lack of species selectivity; and (4) its
animal welfare impacts on both target and non-target species. Neither the perceived
risk of contaminated water nor environmental persistence are supported by research
and ongoing monitoring (Eason et al. 2011).
In their review of public submissions to the 2007 ERMA reassessment of 1080, Green
and Rohan (2012) reported that 62% of submitters expressed a general antipathy to 1080
and toxicants per se for vertebrate pest management, but the dominant concern related to
by-kill and secondary poisoning, especially of native species, but also game animals and
dogs. The welfare impact (humaneness) of 1080 was also a concern. In relation to the
application method, ground baiting of 1080 by hand laying or use of bait stations was
considered marginally more (Green and Rohan 2012) or no more (Kaine et al. 2020)
acceptable than aerial application.
To address the most frequent concerns expressed about 1080 in the ERMA review, any
alternative would therefore ideally need to:
(1) be much more selective (e.g. affect only rodents and possums) and ideally be species-
specific (e.g. kill only species of Rattus)
(2) have no greater welfare impacts than 1080
(3) pose no greater environmental persistence than 1080
(4) pose no greater risk to human health than 1080.
Given recent developments in genetic technology for future pest control (see section
‘Genetic technologies for mammal pest control’), a further condition may need to be
considered:
(5) be an alternative to toxicants, such as a vaccine or genetically modified organism, but
that is socio-politically acceptable.
4B. WARBURTON ET AL.
Māori perspectives
Māori, as do Pākehā, hold a wide range of opinions about the use and risks of 1080. Their
views about the aerial application of 1080 range from strong support to strong opposition
(Horn and Kilvington 2003). Consequently, some Māori hold a view that alternatives to
1080 are needed. Māori remain concerned and acutely aware that invasive species are
continuing to damage and threaten native and endemic species. ‘We are losing our
endemic species at unacceptably high rates, and a failure to act falls short of our respon-
sibilities to our ancestors, and future generations’(Ngata 2018).
In the ERMA reassessment of the use of 1080, 28 written submissions were made by
individuals or groups that identified as Māori (Ogilvie et al. 2010). Although many Māori
submitters recognised that the use of 1080 baiting was essential to control animal pests,
there was also concern that 1080 aerial application could have a negative impact on mauri
(life force), kaitiakitanga (guardianship), wairua (spirit), and tikanga (customs) within
baited areas. There was also concern about wāhi tapu (sacred sites) and mahinga kai
(food-gathering) areas, which often have deep cultural and historical significance for
iwi (Barlow 1991). The death of non-target, and often tāonga, species was also of concern.
About a third of the submitters expressed a need for continued investigation into suit-
able alternatives to 1080. The use of ground baiting was considered to allow greater
control over the use of 1080. Even with the use of GPS systems to increase the precision
of aerial application there was still concern about baits entering waterways, properties
bordering the control area, and wāhi tapu sites (Ogilvie et al. 2010). Māori respondents
felt that future research could realistically lead to more environmentally and culturally
acceptable alternatives to 1080, potentially eliminating many of the concerns held by
Māori regarding the polluting effect of 1080 in the environment (Ogilvie et al. 2010).
There was also a strong desire that iwi, as stakeholders, participate in the design, plan-
ning, and implementation of effective and appropriate pre- and post-control monitoring
plans to enhance successful co-management (Blackie et al. 2014). Such inclusion would
encourage Māori participation and responsibility for pest control, thus fulfilling kaitiaki-
tanga responsibilities.
Submitters also commented on the importance of considering locally appropriate pro-
cesses in the application of 1080. More expensive methods, for example, could provide
employment opportunities, such as the use of trapping, baiting with cyanide (both of
which also provide potential recovery of possum fur as a resource), and ground
baiting with other toxicants, including 1080. Greater control over the method of 1080
application would allow Māori to retain kaitiakitanga over the environment, comfortably
and safely carry out customary practices, while at the same time reducing vertebrate
pests. This, in turn, would go some way towards helping to alleviate concerns about
the way 1080 is used and would enable greater control over its use in the environment.
The statement released by prominent New Zealand Māori environmentalists and acti-
vists regarding the use of 1080 (Ngata 2018) indicated continued support for the appro-
priate use of 1080, informed by scientific evidence, while research continues to
investigate alternative forms of pest control. Ngata and co-signatories suggested that
the concept of a total ban on 1080 is not an option for Māori, as this would undermine
kaitiaki decision-making about how best to care for the whenua, as guaranteed by the
Treaty of Waitangi. For example, some iwi, hapūor whānau may see 1080 as the best
NEW ZEALAND JOURNAL OF ZOOLOGY 5
tool for the desired outcome in certain situations, and their right to make such a decision
should not be constrained.
What if New Zealand stopped using aerial 1080 for large-scale pest
control?
Currently, the only alternatives to aerial 1080 baiting for possum and rat control are
shooting, trapping, and poisoning using either hand-laid baiting or aerial delivery of
an alternative toxicant. Although possums and rats have been eliminated from small
areas of up to about 3000 ha using these methods, the very high costs and potential
residue risks of using second-generation anticoagulants preclude their use at very large
scales, especially in over-forested mountainous areas that are not readily accessible.
Self-resetting traps (possibly with remote monitoring) may prove to be useful for
possum and rat control if the capital cost of the devices can be reduced, and their
reliability and efficacy improved.
The only toxicants licensed in New Zealand for aerial delivery for pest control are 1080
and two anticoagulants, pindone and brodifacoum. Aerial delivery of pindone bait is
used exclusively for rabbit control. Aerial application of brodifacoum bait has been
used successfully to eradicate rodents from offshore islands, and rodents and possums
from New Zealand mainland fenced sanctuaries (Russell and Broome 2016). Although
it is also effective for sustained control of possums and rodents, repeated field use of bro-
difacoum can result in transfer of residues through the food chain, mainly because of its
persistence in prey animals and carcases (Bowie and Ross 2006; Fisher et al. 2010). For
that reason it is not used for repeated aerial application in New Zealand. Pindone and
brodifacoum are unlikely to be acceptable alternatives to 1080 for aerial poisoning, not
only because of concerns about their environmental persistence, but also because they
have relatively high animal welfare impacts. In the absence of an alternative to aerial
1080 baiting, the impacts on bovine TB eradication and native species protection
would be significant.
Impact on bovine TB eradication
By 2010, 40% (c. 10.5 million ha) of New Zealand was considered likely to contain TB-
infected wildlife. That area has since been reduced to 7 million ha. In 2017/18 OSPRI’s
aerial 1080 operations covered c. 357,000 ha (100% habitat) and ground control oper-
ations covered c. 1,128,000 ha. However, in ground control operations only possum
habitat is treated (often as low as 5–10% of total area) so the actual area treated with
ground control will be less than 1,128,00 ha for that year. Aerial control is the preferred
–and sometimes the only practicable –method for maintaining possum densities below
TB transmission thresholds, where terrain, ground cover, and access severely hamper or
preclude cost-effective ground-based control.
OSPRI has identified the loss of aerial 1080 as a major threat to the achievement of
their National TB Management Plan for TB (NPMP) objectives. In response they under-
took a scenario modelling exercise to identify the magnitude of the impact on the NPMP
from loss of aerial 1080 baiting (OSPRI 2019). Impact scenarios were modelled for each
OSPRI region (North Island, northern South Island and southern South Island) for the
6B. WARBURTON ET AL.
loss of aerial 1080 coming into effect at each of four dates: 2019, 2024, 2029 and 2034. For
planned aerial operations that were cancelled after the loss date, aerial 1080 was either
replaced with ground control, where feasible, leading to delayed local elimination of
TB at greater per hectare cost, or control was abandoned where it was considered
there was no cost-effective replacement for aerial control, based on access difficulties
and high worker safety hazards. Abandoned areas were to be surrounded by a sustained
control buffer zone, with ground-based possum control carried out in perpetuity to
prevent expansion of wildlife infection within the abandoned areas and to protect
herds from infection. Modelling identified the total area where local elimination of TB
would still be achieved by the target date and under forecast costs according to
current Tuberculosis Management Area (TMA) plans and control costs. For sustained
buffer control zones and areas where aerial control could be replaced with ground
control, costs were recalculated.
Modelling clearly showed that the earlier aerial 1080 baiting was lost as a control tool,
the greater the impact on NPMP outcomes. Loss after 2034 was inconsequential, as all
planned aerial 1080 operations would have been completed by then. However, loss
from 2019 or 2024 would have very significant impacts, both in failure to eradicate TB
from possums across large areas and from greatly increased costs to eradicate from
areas where aerial control was able to be replaced by ground control. There would
also be high costs to maintain sustained control in perpetuity in buffer areas around
the abandoned TB risk areas (Table 1).
Loss of aerial 1080 baiting any time up to 2024 would almost certainly trigger a pre-
emptive Ministerial review of the NPMP under S100D (2) of the Biosecurity Act. Loss of
aerial 1080 baiting from 2029 would leave remnant wildlife TB in the forested high
country of the central and southern Alps west of the main divide, where it could be
Table 1. Impact on TB eradication success and control costs and land areas from loss of aerial 1080
control at selected dates.
(a) North Island
Date of
aerial
1080
loss
Area eradicated to
indicated date
according to
current plan (ha)
Risk area
abandoned
(ha)
Sustained
control in
perpetuity
(ha)
Delayed
eradication
achieved by
ground control
(ha)
Total cost for
eradication ($
to 2030)
Annual cost
for sustained
control ($)
2019 907,000 607,000 270,000 30,000 64,780,000 19,641,000
2024 1,220,000 398,000 178,000 7,323 71,215,000 12,921,000
2029 1,565,000 143,000 79,000 0 84,987,000 5,759,000
2034 1,788,000 0 0 0 90,000,000 0
Northern South Island
Date of
aerial
1080
loss
Area eradicated to
date according to
current plan (ha)
Risk area
abandoned
(ha)
Sustained
control in
perpetuity
(ha)
Delayed
eradication
achieved by
ground control
(ha)
Total cost for
eradication ($
to 2030)
Annual cost
for sustained
control ($)
2019 2,304,000 404,000 196,000 281,000 183,454,000 10,816,000
2024 2,692,000 173,000 86,000 253,000 132,087,000 4,597,000
2029 3,076,000 76,000 33,000 0 83,029,000 1,755,000
2034 3,185,000 0 0 0 85,000,000 0
Notes: Apart from 2034, total cost for eradication is the cost for eradication only where it is considered feasible, not for all
New Zealand. Minimal aerial control is done in the southern South Island, so it was not included in the analysis.
NEW ZEALAND JOURNAL OF ZOOLOGY 7
contained. However, in the North Island, the corresponding remnant wildlife infection in
and around the northern Tararua, Kaimanawa, and Kaweka Ranges would be very costly
and difficult to contain.
Impact on conservation outcomes
Byrom et al. (2016) reviewed information from 47 accounts of responses of native biota
to possum control. Of these, 60% quantified responses to aerial 1080; the remainder were
ground-based. Possum control benefited vegetation by increasing foliage and fruit pro-
duction, and reducing tree mortality. Controlling ship rats and possums together
improved bird populations, but rats recovered rapidly and long-term outcomes for
rat-vulnerable birds are unknown. Large-bodied invertebrates also benefited from exten-
sive pest control. A meta–analysis of 84 response measures from 35 of these 47 studies
demonstrated that both ground and aerial control of possums provided substantial col-
lateral benefits for native biota (Binny et al. 2020).
Monitoring by DOC before and after aerial 1080 control operations targeting
possums, rodents, and stoats has repeatedly shown consistent benefits for nesting
success in a range of native bird species,
5
including kiwi, kea, kākā, robin, blue duck,
rock wren, yellowhead, and rifleman.
6
As an example, native birds were found to have
doubled in number after more than 20 years of sustained predator control in the Land-
sborough valley in South Westland (O’Donnell and Hoare 2012). Predator control began
in the Landsborough valley in 1994 after the impact of predators on birdlife was
observed. Since then DOC has done valley-wide trapping and six aerial-1080 operations
timed with increasing rodent levels, with the most recent two, in 2014 and 2016, covering
the entire valley. Bird numbers have been indexed using 5 min bird counts each spring at
175 fixed points in the valley to provide an index of relative bird abundance. The index of
total native bird abundance doubled over the more than 20-year study. Seven species of
native birds showed a steady increase in numbers, four species remained stable (but
would have declined in the presence of predators), and two species declined for
reasons unrelated to predators.
7
Birds are not the only native taxa likely to suffer negative impacts if the use of 1080 to
control introduced mammal predators were halted. Both of New Zealand’s native bats are
preyed on by introduced mammals, particularly during rodent outbreaks triggered by
seed masting events (Edmonds et al 2017;PCE2017). Mortality rates of 20–30% of bat
populations have occurred during rat irruptions (Pryde et al. 2005;O’Donnell et al.
2011), and such impacts can be prevented by the control of mammalian predators
(Pryde et al 2005;O’Donnell 2010;O’Donnell et al. 2011). Monitoring of native bat popu-
lations during and after a recent aerial 1080 operation showed no population reduction
and enhanced survival for at least a year afterwards (Edmonds et al. 2017). For reptiles,
recovery from predation by introduced predatory mammals has been demonstrated by
population monitoring before and after 1080 control operations, and comparison of
populations outside and inside predator exclusion fenced areas. Examples include the
increased captures of various species of skink and gecko after rodent eradication
(Towns 1991; Newman 1994), and after predator control and fencing (Reardon et al.
2012), and improved survival of skinks released into the wild at times of low compared
to high mouse abundance (Norbury et al 2014). Information about the responses of
8B. WARBURTON ET AL.
native frogs to 1080 is very limited but does not suggest that aerial 1080 poses a high risk
(Perfect and Bell 2005).
Information about native invertebrates mostly suggests negative impacts from
increased predation if use of 1080 were halted without effective replacement. Although
abnormal behaviour and primary poisoning have been recorded in the field in a range
of invertebrate taxa (McIntyre 1987; Notman 1989; Eisler 1995; Spurr and Drew
1999), Spurr and Berben (2004) found no short-term negative effect of 1080 poisoning
on the abundance of cockroaches, slugs, and spiders occupying artificial wētārefuges.
A review of the effects of pest control by Byrom et al. (2016) concluded that Auckland
tree wētā, a large-bodied and canopy invertebrate, was the only species that has shown
benefits from 1080 aerial pest control application on the New Zealand mainland.
However, rodent eradications on islands may have significant benefits for invertebrates
(Powlesland et al. 2005; Ruscoe et al. 2013). The abundance of ground wētā(Hemiandrus
sp.), large spiders (Miturga sp.), stag beetles (Hemidorcus spp.) and scale insects (Coelos-
toma zealandica) increased after rat eradication (Atkinson and Towns 2001; Green 2002;
McClelland 2002; Towns 2002).
Other invertebrates showed positive responses to mouse eradication, including spiders
(Uliodon sp. and Meringa sp.), moth larvae (Grypotheca sp.), various beetles (Leiodidae,
Carabidae,Staphylinidae,Nitidulidae and Corticariidae spp.), Cook Strait giant wētā
(Deinacrida rugosa), and exotic land snail (Helix aspersa) (Newman 1994;McIntyre
2001; Ruscoe 2001). Native land snail communities also respond positively to intensive
management of ship and Norway rats using 1080 and other toxicants. In the larger-
shelled (≥4 mm maximum shell dimension) component of native land snail commu-
nities, strong effects of rat management regime were evident, with increased land-snail
abundances, species richness, and functional trait values (Barker 2016).
Clearly, if aerial 1080 poisoning were stopped without an equally efficacious alterna-
tive, many of the conservation gains of the kind reported by DOC and Byrom et al. (2016)
would quickly be lost as possums and predator numbers recovered, in a similar way to the
rapid increase in bovine TB infection in the 1980s when possum control was reduced pre-
maturely (Coleman and Livingstone 2000).
Options for reducing and/or replacing 1080 use
The PCE reviewed the use of 1080 extensively in 1994 and 2011, along with update
reports in 1998 and 2013.
8
In the most recent report, the PCE concluded:
Research to develop better poisons (and possibly biocontrol options) should absolutely con-
tinue. Alternatives, whether currently available or on the horizon, can complement the use
of 1080, but cannot replace it. The huge effort, expenditure and achievements to date in
bringing back many species and ecosystems from the brink would be wasted if the ability
to carry out aerial applications of 1080 was lost. (PCE 2011, 67)
In 2018 there were 29 aerial 1080 operations across New Zealand. Consultations with
Māori and hunting groups during planning resulted to changes in 11 out of the 29 oper-
ations. These modifications included boundary changes, timing of the operation,
addition of deer repellent, and changes from aerial to ground application at some
sites. About 230 water samples were tested following aerial 1080 operations. Only six
NEW ZEALAND JOURNAL OF ZOOLOGY 9
samples had detectable levels of 1080, and none of those exceeded the Ministry of
Health’s maximum acceptable value for 1080 in drinking water (2 ppb).
Since its adoption in the 1960s as a pest control method, aerial application of 1080
baits has been repeatedly refined, both to increase operational efficacy and to address
concerns about non-target and environmental risks (Fisher et al. 2011). By 2004 there
were significant improvements in bait preparation and deployment, with bait sowing
rates reduced from a high of 30–40 kg/ha to routinely <5 kg/ha (Morgan 1994). Bait frag-
mentation, a risk factor for non-target species, was reduced significantly by either screen-
ing out small carrot bait fragments or more precise cereal bait manufacture, primarily to
reduce the risk of non-target poisoning of birds. GPS guidance systems were used rou-
tinely to avoid large gaps in bait coverage and to ensure bait was not laid beyond control
operation boundaries. Since then, aerial 1080 baiting has evolved in several overlapping
directions: reduced use of carrot bait, better control of possums for bovine TB eradica-
tion, multispecies control targeting rats and stoats as well as possums, continued use in
rabbit control, and attempts at local elimination of possums and rats.
Since 2005, 1080 research has focused on three main operational improvements that
also aimed to address concerns about 1080 use: (i) rapid eradication of TB by local elim-
ination of possums (thereby reducing the frequency of repeat 1080 applications); (ii) new
bait sowing regimes to reduce the amount of 1080 used (lessening concerns about
environmental impacts and non-target risk, as well as reducing cost), and (iii) reduced
non-target impacts (reducing by-kill, particularly deer and kea).
Rapid eradication
The local elimination concept was prompted by the apparent achievement of 100% kills
by aerial 1080 baiting in some areas (Morgan et al. 2006). A key initial focus was on pre-
feeding with non-toxic bait, which was not always used previously (Veltman and West-
brooke 2011), but which is now standard practice for aerial 1080 baiting. Investigation of
various combinations of toxic sowing rate (1–5 kg/ha) and the number of pre-feeds (0, 1,
2) (Nugent et al. 2011) confirmed that pre-feeding was particularly important for rats
(with two pre-feeds more effective than one) and that sub-lethal poisoning of possums
was still largely a consequence of bait fragmentation. Efforts were made (Nugent,
Morgan et al. 2010) to further reduce fragmentation of cereal baits by reducing flight
speeds, increasing bait hardness, and increasing efforts to reduce fragmentation at
manufacture.
Different bait sowing regimes
A complementary solution focused on sowing bait in clusters or strips separated by large,
unbaited areas, so that bait density was high within the strip or cluster, but overall bait
density was reduced significantly to <2 kg/ha without reducing control efficacy (Nugent,
Warburton et al. 2012). From about 2009, a series of cluster- and strip-sowing trials
aimed to reduce the sowing rate further still, with one trial achieving a 100% reduction
in measures of possum and rat abundance with a sowing rate of just 0.17 kg/ha of toxic
bait (Nugent and Morriss 2013). In two subsequent trials in unforested mountain land,
strip-sowing of 1080 bait from fixed-wing aircraft resulted in 99.4% of 168 radio-collared
10 B. WARBURTON ET AL.
possums being killed with 1080 sowing rates of just 0.3–0.5 kg/ha (Morriss et al. 2015).
This led to successful field testing of aerially pre–feeding coupled with ground-based pla-
cement of toxic bait (to avoid any bait getting into waterways) in accessible areas
(Morgan et al. 2015).
Reducing non-target impacts
Deer are often killed incidentally during aerial 1080 baiting operations, angering many
hunters. In the 1990s estimates of percentage kill varied widely, from near zero to over
90% (Nugent et al. 2001; Morriss et al 2020). The resulting opposition by hunters to
aerial 1080 use prompted the successful development of a proprietary deer repellent
(Speedy 2005; Morriss 2007). The repellent is applied to bait as a surface coating after
manufacture. This is a difficult and expensive process, so deer-repellent bait has
tended to be used only where required by the landowner, or on public land where the
deer population is regarded by hunters as particularly valuable. Recently, Orillion and
Pest Control Research, two pest product manufacturing companies, have developed
new formulations of deer repellent that are incorporated into the cereal pellets during
manufacture. This is anticipated to provide lower–cost deer-repellent bait. However,
some conservationists resent any extra cost of using repellents because of the lost oppor-
tunity to do more control elsewhere (Driver 2019).
The unintended kill of native birds during aerial 1080 baiting (Spurr 2000; Veltman
et al. 2011) has also long been a driver of public opposition to 1080. Modern baiting prac-
tices (reduced bait fragmentation and low sowing rates, see above) and use of dyes and
taste masking agents (Cowan and Crowell 2017) have greatly reduced the impact on
common native birds (Morriss et al. 2016). However, some species continue to be vulner-
able, including some nationally threatened species such as kea (Nestor notabalis),
prompting tight constraints on where and when 1080 can be used in kea areas (Depart-
ment of Conservation 2017). Research aimed at identifying an effective repellent for kea
(and other birds) (Weser and Ross 2013; Van Klink and Crowell 2015; Cowan et al. 2016;
Crowell, Booth et al. 2016; Crowell, Martini et al. 2016) has had little success (Van Klink
and Crowell 2015), although a new conditioned aversion method shows promise
(Nichols et al. 2020).
Multi-species control
The primary use of 1080 on conservation land was initially to prevent canopy collapse in
native forests caused by possum browsing, particularly in highly susceptible rātā–kāmahi
forests of the West Coast (Bellingham et al. 1999; Nugent, Whitford et al. 2010). With the
establishment of the Department of Conservation in 1987, the goal of possum manage-
ment broadened to more holistic protection of forest composition, and there is now
strong evidence that broad conservation benefits are being achieved (Byrom et al.
2016; Binny et al. 2020). By the 1990s rats were increasingly recognised as occasional
and sometimes major predators of native birds (Brown et al. 1993; Moorhouse et al.
2003). This led to the evaluation of aerial 1080 baiting as a rat control tool (Innes
et al. 1995).
NEW ZEALAND JOURNAL OF ZOOLOGY 11
Rats often reach very high densities with very small home range size (Hooker and
Innes 1995; Pryde et al. 2005; Harper and Rutherford 2016). Complete coverage is there-
fore far more important than for possums, and high rat densities require significantly
higher sowing rates (at least 200 toxic baits per hectare) than for possums, although
the smaller body size of rats means large baits are not required. The standard modern
specification for rat-focused aerial 1080 is therefore 6 g toxic baits with pre-feeding
(Brown et al. 2015; Elliott and Kemp 2016), with operations often repeated more fre-
quently than for possum control. During aerial control of rodents, it became apparent
that a high proportion of stoats were killed by secondary poisoning (Gillies and Pierce
1999; Murphy et al. 1999) and that possum-targeted operations also produced effective
reductions of rats and stoats (Brown and Ulrich 2005). Predator control therefore
became the primary focus of aerial 1080 baiting for conservation purposes, most
evident in the Battle for our Birds programme / Tiakina NgāManu (Veltman et al.
2011; Elliott and Kemp 2016).
Predator-free NZ
The adoption in 2016 of the goal of national freedom from key predators (possums, rats,
and mustelids) coincided with a renewed focus on the use of 1080 to locally eliminate
possums and rats (and then prevent or remove any re-invaders). A high-specification
‘1080-to-zero’approach has been championed by Zero Invasive Predators (ZIP 2017)
and involves (if required) a dual application of twice-pre-fed 1080 baiting at what are
now considered to be high sowing rates (i.e. 4 kg/ha). Initial trials showed promise,
with no possums or rats detected in a 394 ha area after a single high-spec sowing of
1080 bait in a south Westland study area (ZIP 2018), leading to a 12,000 ha trial in
2019. However, a subsequent trial of this approach in the Kaitake Range in Taranaki
failed to achieve even close to 100% kill (T. Sjoberg, pers. comm.). Parallel research
found that most possums surviving an initial 1080 baiting could not be killed by
sowing a similar toxic bait type soon after, but bait-averse rats could be killed if pre-
feeding was repeated (Nugent et al. 2019). Subsequent trials indicated that bait-averse
possums could be killed using a markedly different bait type (Nugent et al. 2020), so
this approach may be able to be implemented successfully at operational scales to
achieve 100% kills.
Alternative existing control methods
As already noted, shooting is not a feasible option for small mammal pests at large scales
and in less accessible areas. A frequent suggestion that aerial control by 1080 could be
replaced by combining the use of other toxicants and trapping is also impractical and
not cost-effective for large areas (i.e. >10,000 ha) especially in the more extensive and
remote, mountainous parts of New Zealand because of access, cost, and operator
safety reasons. Most ground-based control operations are less than 1000 ha while
aerial operations cover tens of thousands of hectares with costs for ground control up
to $60/ha compared to aerial control costs of $20–$30/ha (Brown et al 2015).
However, it must be noted that costs of ground control operations are highly variable.
12 B. WARBURTON ET AL.
In the much more accessible ‘developed’areas (i.e. most farmed areas and many ‘front
country’forest areas) aerial 1080 is already rarely used. OSPRI, for example, undertakes
ground control as a matter of course in almost all farmed and peri-urban areas and in
some of the most accessible unfarmed areas. In addition, large-scale conservation pro-
jects that do not involve aerial poisoning are underway across New Zealand, with the
New Zealand Predator–Free 2050 initiative supporting 10 large-scale, introduced pred-
ator eradication programmes relying heavily on trapping and ground-based toxic
baiting.
9
It also supports the development and testing of new ground-based control
tools, such as long-life lures and self-resetting and -rebaiting traps, and testing of a
remove and protect strategy.
10
Funding has also been provided for projects on social
research on novel pest control methods, welfare testing of traps, landscape-scale eradica-
tion of possums, thermal cameras, radio networks for trap status reporting, systems for
field data capture and sharing, and decision support systems.
11
Traps and ground-based control methods
Surveys to gauge public preferences for control options have generally favoured traps
over other methods (Fraser 2001,2006; Fitzgerald 2009). Traps (live capture and kill)
are currently used extensively as part of possum and ferret control programmes for
TB management (Warburton and Livingstone 2015), and control of stoats, ferrets,
feral cats, and ship and Norway rats for conservation purposes (Carter et al. 2016;
Tansell et al. 2016; Glen et al. 2019). Also, many trap-focused programmes are augmen-
ted by applications of toxicants, either sequentially or in parallel. The largest area over
which a vertebrate pest (stoats) has been managed in New Zealand solely with traps is
about 51,000 ha (Tansell et al. 2016). This operation is an exception, however, with
most, especially rodent control programmes, achieving mixed success in areas less
than 1000 ha (Gillies et al. 2012; Carter et al. 2016), orders of magnitude smaller than
the tens of thousands of hectares that can be covered by an aerial 1080 operation.
Aerial 1080 operations, even as large as 100,000 ha, can be carried out within 2–3
days, whereas trapping operations focused on possums can take several months to
achieve similar substantial population reduction (Montague 1997). For some pest man-
agement programmes, the time taken to achieve target pest reduction may not be a sig-
nificant constraint, but for others (e.g. reducing ship rat numbers in Spring before bird
breeding to mitigate predation), the desired pest reduction must be achieved rapidly (i.e.
within 4–6 weeks).
Historical comparisons of the cost-effectiveness of aerial- and ground-based (traps
and cyanide baits) possum control has shown that, in suitable situations, ground-
based hunters can achieve equivalent kills (c. 80%) to aerial 1080 control and at
similar costs per hectare (Morgan and Warburton 1987; Montague 1997). The results
of these trials supported the development of a possum control contractor industry (War-
burton and Hall 2016), and both DOC and OSPRI use ground-based contractors along
with aerial 1080 operations to achieve their management objectives. However, modern
pre-fed aerial baiting can often achieve near total kills (e.g. 99.6% of 243 radio-collared
possums killed in a 2016 operation covering 83,000 ha of forest; Nugent et al. 2017), a
level of control that ground-based contractors would seldom, if ever, deliver at a
similar cost to aerial.
NEW ZEALAND JOURNAL OF ZOOLOGY 13
Commercial harvesting
Commercial harvesting of possums has often been suggested as a potential alternative or
addition to official control. Jones et al (2012) used harvester interviews, trapping data, and
a combination of economic and spatial population models to see whether the seemingly con-
tradictory goals of economically sustainable harvest and biodiversity protection could be
accommodated in native podocarp-dominated forests in the North Island of New
Zealand. The optimal strategy for sustaining an income of NZ$30,000 from harvest required
trapping to stop when nightly capture rates fell to 25%, and then leaving possums to recover
for 3 years. This scenario, however, was considered unlikely to achieve conservation gains.
Harvesters could trap to the levels required to achieve conservation outcomes ifthey received
some payment by the management agencies that, in return, would benefitfrompossum
control, but at lower cost than current standard ground-control methods.
Recent trap and toxicant delivery devices
Throughout New Zealand many of the smaller-scale rodent control programmes carried
out by DOC, councils, community groups, PF2050 programmes, and sanctuaries use
various designs of kill traps.
12
Ship rat control programmes using single capture traps
(e.g. Victor snapback traps) have been able to maintain rats at acceptably low densities
during years when population productivity is low, but when a food-driven population
irruption occurs, additional control using aerial 1080 or toxicants in bait stations is
required to control such numbers (Brown et al 2015). Recently, gas-powered multi-kill
traps have been developed for possums and rats (Goodnature A12, A24, and AT220),
and the A24 has been used to reduce and maintain ship rats at low densities (Carter
et al. 2016). However, there have been mixed results reported on using these traps,
with some users reporting poor kills, trap malfunctions, and high initial and maintenance
costs (Gillies et al. 2012; Bogardus and Shiels 2020). Warburton and Gormley (2015)
showed through modelling that there was little advantage in using multi-capture traps
when pest densities are being maintained at low densities. So, although the concept of
a multi-kill trap might have some merit, the cost per hectare of their application is
still restricting their use to relatively small areas (<500 ha).
Grooming ‘traps’are a recent development, which use a mechanical means of deliver-
ing a toxicant onto the pest species body (Blackie et al. 2016). The animal is killed when it
ingests a lethal dose of toxicant while grooming the material from its fur. Traps of this
type are currently under field testing in Australia (Felixer; Moseby et al. 2020) and devel-
opment in New Zealand (e.g. Spitfire).
13
Recent developments in the application of artificial intelligence (AI) to pest control
have been spearheaded by the Cacophony project.
14
If this technology develops to the
stage of being operationally affordable, then it could enable traps to be more target-
specific and more capture efficient.
Other toxicants
Several toxicants, other than 1080, are registered for use in New Zealand against
possums, rabbits, rodents, and mustelids, but the databases supporting these are
14 B. WARBURTON ET AL.
much less comprehensive than that for 1080. In recent years new toxicants, zinc
phosphide, sodium nitrite, coumatetralyl and diphacinone combined (D + C), and
para-aminopropriophenone (PAPP), along with new formulations of cyanide,
have been registered (Eason et al. 2017,2020). However, none of these, including
the more recent formulations, were developed as replacements for aerial use of
1080, but rather to provide a wider range of options for pest controllers and com-
munity groups undertaking ground controlofpossums,rodents,mustelids,and
feral cats. Information about these alternative toxicants is briefly summarised in
Appendix 1.
Novel toxicants
Concerns that have been expressed about existing toxicants include:
.deaths of non-target animals –by-kill or secondary poisoning of native species, game
species, livestock, dogs
.short-term contamination of soils and waterways, and effects on aquatic life
.environmental persistence in soils, water, and animals
.ethical and welfare concerns
.human health risks
.economic impacts from potential residues.
In addition to meeting criteria of cost of manufacture and efficacy, any new or
modified toxins would need to collectively address these concerns to a greater extent
than existing compounds to justify their development costs, registration, and deploy-
ment. Below we discuss natural toxins and species-selective toxicants.
Natural toxins
In the hope that toxins derived from natural and more importantly indigenous
sources may moderate the general antipathy to toxicants, historical and con-
temporary mātauranga Māori (Māori knowledge) is being used to seek unique
alternatives to 1080. Researchers and representatives from Tūhoe and Lincoln Uni-
versity identified six native plant species recognised within mātauranga Māori for
their toxicity and medicinal properties (Pauling et al. 2009). Using a decision
matrix based on criteria that included the known environmental persistence and
humaneness of toxins and key cultural and environmental values, tutu (Coriaria
arborea)wasidentified as having the greatest potential. Further research showed
that tutin, the principal toxic component of tutu, could be a successful rodenticide
for Norway rats (Rattus norvegicus) at a dose rate of 55 mg/kg (Ogilvie et al.
2019). Research is continuing to assess if delivery of an effective lethal dose is
technically attainable in the field. Tutin, however, is a broad-spectrum, acute neu-
rotoxin and so may not provide advantages over 1080 and other toxicants regard-
ing humaneness and risk to non-target species, including dogs (Table 2; Beasley
et al. 2018).
NEW ZEALAND JOURNAL OF ZOOLOGY 15
Table 2. Assessment of the extent to which existing alternatives address concerns expressed about 1080.
Compound Species selectivity Humaneness Persistence in water Persistence in soil Secondary poison risk Human health risk Efficacy Cost
Anticoagulants (G1) = –+–=+––
Anticoagulants (G2) = –+––+=–
Zinc phosphide = = + + + = ––
Cholecalciferol + –+–++––
Sodium Nitrite + + = = + + ––
Diphacinone + Cholecalciferol = –+–=+––
Cyanide + + + + + = ––
PAPP + + = = + + ––
Notes: Scores are based on consensus views of the authors: (+) better than 1080; (–) worse than 1080; (=) similar to 1080;? unknown. G1 –first-generation anticoagulants; G2 –second-gen-
eration anticoagulants.
16 B. WARBURTON ET AL.
Species-selective toxicants
The development of species-selective toxicants directly addresses concerns about non-
target species risk and has high acceptability in public surveys. High-throughput screen-
ing and genome mining (technologies applied by the pharmaceutical industry to discover
targets for product development (e.g. Entzeroth et al. 2009; Belknap et al. 2020)) are
underway on pest species to identify targets that are suitable for the development of
species-selective toxicants
15
(White et al. 2019).
This approach relies on identifying proteins that control key physiological processes in
the target pest species. If these proteins are either unique to that pest, or sufficiently
different in the underlying gene sequence for their production and structure from the
equivalent proteins in non-target species, then interfering with their activity may cause
species-selective impairment or death. The mode of action of the rat-selective toxicant
norbormide demonstrates this concept (Clarke 1965; Russell 1965). Norbormide
induces a fatal vasoconstriction in Rattus species through its action on a receptor
located on the cell surface of rat smooth muscle cells (Bova et al. 2001). By contrast,
non-target species tested, including other rodent species, show an asymptomatic tempor-
ary vasodilation of blood vessels (Bova et al. 1996).
This suggests that the norbormide receptor is present in these other species but is acti-
vated very differently compared to Rattus species. Although the original norbormide
product failed commercially due to low efficacy, recent developmental work by
Manaaki Whenua –Landcare Research and Invasive Pest Control Limited has signifi-
cantly improved its efficacy, such that it is now at a level thought to be commercially
viable.
Progress with norbormide suggests similar unique or significantly differentiated
receptors and proteins may be identifiable in other pest species and used as targets for
novel species-selective toxicants developed through approaches like the pharmaceutical
drug discovery process. However, for most pest animals the role and interaction of
various key receptors and proteins is not fully explored, which makes target validation
much more difficult. Progress is being made with key pest species, and sequencing of
the genomes of brushtail possums, stoats, and ship rats has been completed (Genbank
numbers: Stoat GCA_009829155.1.; Ship rat GCA_011064425.1; Possum
GCA_011100635.1).
Even if developing a species-selective toxicant requires less rigour and testing than that
for a medical therapeutic drug, the process of researching and developing a new toxicant
will still likely take 10 years.
16
Research is underway globally on new compound screen-
ing technologies (e.g. the predictive in vivo fruit fly compound screening model –MBIE
grant Smart Idea C09X1710). If successful, such technologies could be used to accelerate
research for identifying new targets and toxicants for species-specific vertebrate pest
control.
The costs of research and development are likely to be a significant factor influencing
the search for new toxicants. Developing, testing, and approval for a successful drug for
humans is estimated to cost on average c. US$985.3 million (95% CI; $683.6 million to
$1.23 billion) (Wouters et al. 2020). The cost of developing a species-selective toxicant
should be significantly less, but the market for such toxicants is also significantly
smaller than that for many human drugs: the total global rodenticide market is estimated
NEW ZEALAND JOURNAL OF ZOOLOGY 17
to reach only US$5.9 billion by 2025.
17
This relatively small market, combined with the
increased measures being imposed on the registration of new toxicants by regulatory
authorities, is making the registration process longer and more costly. A new rodenticide
product registration in the EU has been estimated to cost c. €5 million and take about 5
years once the registration package has been submitted.
18
Rodenticide manufacturers are
reluctant, therefore, to develop new vertebrate toxicants and are fighting restrictions on
current products imposed by regulators responding to environmental and social con-
cerns. The market for pests specific to New Zealand, such as possums and mustelids,
is miniscule at an international scale and that may also create financial issues for their
commercial registration.
Novel non-toxicant approaches
Fertility control
A broad research programme was initiated in the 1990s to investigate fertility control of
possums as an alternative to lethal control, in part to address concerns about toxin use
(Cowan 2000;Ji2009). Two methods of controlling possum fertility were subsequently
investigated in detail: (1) immunological interference with fertility, and (2) disabling
the normal hormonal control of reproduction. Both approaches failed during the devel-
opment phase, due particularly to their inability to induce sufficiently long-lasting infer-
tility. Chemosterilants, such as gonadotrophin-releasing hormone (GnRH) vaccine
(GonaCon) and slow-release GnRH agonist implants (e.g. Deslorelin) are effective at
reducing possum fertility and have been deployed in local populations in Australia
(Lohr et al. 2009), but they are impractical to deliver at the scale required in New
Zealand (Cross et al 2011).
TB vaccine
Vaccination of possums against bovine TB has the potential to be an effective TB
controlmeasureandtoreducethenecessityforlethalcontrolforbovineTBmanage-
ment (Buddle et al. 2018). There has been substantial research investment in that
concept for possums using BCG (M. bovis bacilleCalmette-Guérin),aliveTB
vaccine that has been used in humans for over a century. Vaccination of possums
with BCG via subcutaneous, intranasal and oral routes induced a significant level
of protection against experimental M.bovis challenge. In a field trial with BCG
vaccine administered orally in a lipid matrix, there was a significant reduction in
the proportion of infected possums in vaccinates (1/51) compared to that for the
non-vaccinates (12/71), with a vaccine efficacy of 95% for prevention of TB (Tomp-
kins et al 2009). An aerially delivered vaccine bait reduced the prevalence of TB in
possums by 81% (Nugent et al. 2016). The major constraints on the use of BCG
vaccine in possums in New Zealand are the cost of the vaccine bait compared with
that of aerial 1080 bait (estimated $2–3 per bait, cf. $0.3 per 1080 bait), and the
need for 1–2-yearly repeated dosing to successfully break the TB cycle in possums.
Societal concerns about the deployment of a live vaccine that could transfer to
other animals or people would also need to be considered.
18 B. WARBURTON ET AL.
Genetic technologies for mammal pest control
Genetic technologies for pest control have the potential to be species-specific and have
acceptable welfare impacts, but they would undoubtedly face significant scientific, regu-
latory, and public scrutiny under New Zealand’s current approval process that governs
the development and release of genetically modified organisms.
19
In 2001 a Royal Com-
mission on Genetic Modification recommended that New Zealand adopt a cautious risk-
based approach to GM technologies (Royal Commission 2001), and there has been little
growth in the use of GM for industrial or agricultural purposes since that time (Dearden
et al. 2018). No genetically modified organisms (plant or animal) have been approved for
widespread release in New Zealand.
20
Although active debate over the relative merits of GM is ongoing, the concept of con-
trolling wild pest populations by introducing genetically modified individuals is not new,
having been first suggested almost a century ago (Klassen and Curtis 2005). Most pro-
posed genetic technologies are designed to influence the sex ratio, mortality, or fertility
of pests, and thereby suppress the target population. Recently there has been substantial
interest in the development of gene drives for the biological control of mammal pests. A
gene drive is a genetic element (i.e. a DNA sequence) that is much more likely to be trans-
ferred to offspring than usual, and could therefore spread rapidly through populations,
even if it has a negative impact on an organism’s survival or reproductive rate (Burt
and Crisanti 2018). Naturally occurring gene drives with inheritance-biasing properties
have already been identified and include homing endonuclease genes (Chevalier and
Stoddard 2001; Burt 2003), the t-complex in mice (Willison and Lyon 2000; Leitschuh
et al. 2018), and the Medea gene in flour beetles (Beeman et al. 1992). Importantly, syn-
thetic gene drives can now be engineered using the CRISPR/Cas9 genome-editing
system, which permits rapid, precise, and targeted genetic engineering in many organ-
isms, including mammals (Grunwald et al. 2019). Although naturally occurring gene
drive systems could potentially be used to spread deleterious genes through pest popu-
lations, recent research has focused on two synthetic gene-drive designs that might
have application to mammal population suppression: self-replicating homing gene
drives and shredding gene drives.
Self-replicating ‘homing’gene drives
Self-replicating CRISPR/Cas9 gene drives are DNA sequences that are integrated
within the genome of an organism and encode an endonuclease (Cas9) and a site-
specific guide RNA (gRNA). Self-replication (‘homing’) of the DNA construct
occurs when those two molecules act together to cut DNA at the matching target
site on the homologous chromosome, and then repair of the DNA break by homolo-
gous recombination results in replication of the gene-drive sequence on this partner
chromosome. If this homing mechanism is activated in the germline (i.e. the egg-
and sperm-producing cells), individuals with a single gene-drive allele should pass
on the drive to their offspring almost 100% of the time (Burt 2003; Sinkins and
Gould 2006). Assuming the homing mechanism is efficient, and the gene drive is posi-
tioned to disrupt a critical gene required for sex determination, embryonic develop-
ment or fertility, simulation studies indicate that this strategy could be used for
suppressing populations of a range of pest species (Burt 2003; Deredec et al. 2008;
NEW ZEALAND JOURNAL OF ZOOLOGY 19
Prowse et al. 2017; Wilkins et al. 2018). The development of homing drives has
received substantial attention as a means to control insect pests (e.g. Champer et al.
2017; Kyrou et al. 2018). Homing success rates in these studies are extremely high
(typically >90%). Although no homing drive for population suppression has yet
been demonstrated in vertebrates, Grunwald et al (2019) showed recently that
homing rates of up 72% can be achieved in mice.
‘Shredding’gene drives
The capacity of modern genome-editing technologies to make targeted breaks in DNA
could facilitate the development of gene drive systems that ‘shred’genes or entire
chromosomes. For example, a sex-distorting gene drive could be achieved by program-
ming a drive sequence with the machinery required to shred one of the X or Y sex
chromosomes. The biased inheritance of X-chromosome-shredding drives could be
achieved (without the need for a homing mechanism) by integrating the drive con-
struct within the Y chromosome, provided X-shredding activity could be limited to
the germline (Hamilton 1967; Deredec et al. 2011; Galizi et al. 2014). Galizi et al.
(2016) demonstrated that, in the mosquito malaria vector Anopheles gambiae, shred-
ding of the X chromosome in the male germline achieved a male bias in offspring
of between 86% and 95%, and concluded that such a system could be used to construct
a suppression drive. Although this technology has not yet been extended to vertebrates,
the CRISPR system can eliminate the mouse Y chromosome in embryonic stem cells,
and simulations indicate that a homing drive with Y-shredding capability could deplete
the pool of XY males and effect population eradication through mate limitation
(Prowse et al. 2019).
Advantages and disadvantages of gene drives for biocontrol
The primary technical advantage of gene drives for population control is that, assuming
all individuals within a population are susceptible to the construct, a single introduction
of gene–drive-carrying animals could theoretically spread the drive element to fixation
and effect rapid population eradication or suppression (Dearden et al. 2018). Animal
welfare concerns associated with traditional lethal control activities could also be alle-
viated by gene drives that act by distorting sex ratios or causing female sterility in the
target species, and therefore cause no pain or distress to the managed pest or any
non-target species. Importantly, however, no synthetic gene drive has yet been used
for the suppression of a wild population, and there are significant technical and social
hurdles still to be overcome.
In particular, the scalability of these technologies to large pest populations remains to
be demonstrated. In the case of homing drives, DNA mutations created when the homing
mechanism fails can produce resistant alleles, which cannot acquire the gene drive
(Gantz et al. 2015; Champer et al. 2018) and could allow populations to rebound after
an initial period of suppression (Eckhoffet al. 2016; Prowse et al. 2017). However,
homing-drive constructs that incorporate multiple guide RNAs that target adjacent
DNA recognition sites for cleavage could help reduce the probability of resistance-
allele formation (Eckhoffet al. 2016; Prowse et al. 2017; Champer et al. 2018).
Another significant hurdle to efficacy is the spatial structure exhibited by most animal
populations, resulting in isolated subpopulations that might simply never be exposed
20 B. WARBURTON ET AL.
to the drive (Prowse et al. 2017). Spatially explicit models also suggest that cycles of drive
invasion, subpopulation extirpation, and recolonisation could follow a single gene-drive
introduction event (Champer et al. 2019).
Alongside these general barriers to the development of suppression drives, species-
specific hurdles will also need to be overcome. Depending on the species, research
needs may include genome sequencing and the identification of candidate genes for
the disruption, optimisation, and validation of gene-drive constructs to target those
genes; genetic screening of wild populations to investigate metapopulation structure
and demonstrate universal susceptibility of target populations to the construct; demo-
graphic studies of wild populations and population modelling to predict the conse-
quences of a gene-drive release (Dearden et al. 2018).
Transgenic animals must also be mass-reared in a laboratory prior to release, which
might be feasible for species with high reproductive rates in captivity (such as mice
and rats) but will be challenging for many mammals (such as possums, which
produce only one to two offspring per year). This breeding step may be close to
impossible to achieve for stoats unless the current extreme difficulties of breeding
them in captivity can be overcome (McDonald and Lariviere 2002). For any species
targeted for management, there is clearly significant laboratory and field research
that needs to be undertaken before a gene-drive release could be considered
(Dearden et al 2018).
Political and social considerations
Given that there is currently little scientific or public consensus on the safety of, need
for or use of genetically modified crops (Hilbeck et al. 2015; Mitchell et al. 2018), the
barriers to obtaining the political and social licences to deploy self-sustaining genetic
biocontrols in natural ecosystems are likely to be high (Breed et al. 2019; Kirk et al.
2019). Public acceptance and a ‘social licence to operate’will be needed before any
gene drive introduction in the wild. This will require open communication between
all stakeholder groups and is likely to take many years. In the first instance, this dia-
logue might be facilitated by focusing on a specific pest and gene-drive strategy, so that
the costs, benefits and risks of that approach can be weighed transparently against
those of existing options for biological control (Dearden et al. 2018; Stronge et al.
2020).
Even if the development and use of genetic biocontrols gained general acceptance,
both scientists (Webber et al. 2015; Noble et al. 2018) and non-scientists (Kirk et al.
2019) place substantial importance on the risk of gene-drive-carrying organisms escaping
beyond the laboratory or the population targeted for management. These risks are real,
and could lead to political, economic or trade complications for countries that implement
gene-drive technologies before the development of a global regulatory framework to
govern their use. Technically, it might be possible to reduce these risks by designing
gene drives that are temporally or spatially restricted in their activity (Marshall and
Hay 2012; Akbari et al. 2013; Noble et al. 2016; Dhole et al. 2018). Nevertheless, the devel-
opment of gene-drive technologies for biological control needs to be undertaken within
both risk assessment (Webber et al. 2015; Breed et al. 2019) and risk mitigation frame-
works (Dearden et al. 2018).
NEW ZEALAND JOURNAL OF ZOOLOGY 21
Discussion
Since the advent of landscape-scale pest mammal control in the 1960s in New Zealand,
significant progress has been made in identifying, quantifying, and addressing the risks
posed by the use of various toxicants, and the views of the public related to control tools
and their applications. Clearly, public views and acceptability vary in relation to a wide
range of factors that include pest species, control method, species selectivity, and welfare
impacts. Control tools and their application vary in their acceptability, but none are uni-
versally acceptable, and we conclude that there is no immediately available alternative.
Future research and development therefore needs to continue to focus, in the short to
medium term, on addressing the negative aspects of current tools and their application
and, in the longer term, on developing new and modified tools that better address con-
cerns about current toxicants and traps. There is clearly little value in developing new
toxicants that have some, or all, of the negative characteristics of current toxicants,
because the concerns related to current toxicants are likely to be transferred to the
new ones when they become operational.
Improving the use of 1080
A huge amount of research has been done on all aspects of 1080 to minimise the extent to
which it is used and the risks associated with its use. Also, regulatory changes have been
made to improve its safe use, including mandatory incident investigation. The increased
conditions imposed by the EPA on aerial 1080 poisoning require that incidents where
livestock are exposed to 1080 baits be subject to a full investigation.
21
Despite the major refinements and regulatory control over how 1080 is used, its
ongoing use remains highly controversial, but its loss from the pest toolbox would
have significant environmental, social, and economic impacts in the short to medium
term. Most 1080 is used for aerially delivered pest control, but, while research has
driven huge reductions in amounts of toxic bait distributed per hectare, there has
been little progress to find an alternative toxicant for that purpose. Before such research
is considered, however, it would be appropriate for social researchers to establish the
extent to which concerns relate specifically to 1080 as opposed to aerial application.
Although opposition to 1080 appears to be irrespective of whether it is ground or aerially
applied (Kaine et al. 2020), attitudes to aerial application of a different toxin need to be
better investigated. Such an investigation could also usefully consider the impact of
widely available misinformation about 1080 (or future alternatives) in influencing
peoples’views (Hansford 2016). However, as people selectively filter misinformation
(and information) in accordance with their pre-existing attitudes and beliefs, the poten-
tial for strategies to counter misinformation to substantially change people’s views is
likely to be limited (Nisbet et al. 2015).
An Australian perspective
A recent Australian review (Invasive Species Council 2020) concluded that 1080 has been
essential for enabling the survival or recovery of many native threatened species and their
reintroduction to sites where introduced predators have been suppressed or eradicated.
22 B. WARBURTON ET AL.
The use of 1080 was also considered to have welfare benefits for native animals by freeing
them from the pressure of heavy predation or competition by introduced animals. The
review concluded that a ban on 1080 without an effective replacement would overall
result in greater suffering (as well as declines in native species), and recommended as
a high priority, research into effective replacements for 1080 that are more humane.
Other toxicants registered in New Zealand
None of the other toxicants currently registered in New Zealand as VTAs are likely to be
suitable replacements for 1080 for aerial delivery, and they were not developed specifi-
cally for that purpose. None are species-selective, although some, such as PAPP and
sodium nitrite, have a narrower spectrum of species toxicity than 1080. Also, none of
these toxicants have been shown to be as effective or as cost-effective as 1080, so any
shift to replace 1080 with one or more of these would have a significant impact on
control effectiveness and operational costs. These toxicants also vary in terms of their
welfare impacts, residue persistence, and secondary poison risks (Table 2). The recently
developed toxicants such as PAPP, sodium nitrite, and diphacinone plus cholecalciferol
(D + C), each offer benefits to one or more aspects that are of concern with 1080 or bro-
difacoum (e.g. PAPP is more selective to carnivores), but there has been a failure to ident-
ify a product that can address all the concerns of 1080 while retaining its cost-
effectiveness.
An indigenous toxicant
Tutin, because of its indigenous origins, might go some way towards addressing Māori
concerns about the use of 1080. However, like current toxicants it is not species selective,
and it appears unlikely to be significantly better than 1080 in terms of animal welfare. It
may also be less effective because of the relatively large lethal dose required and the effect
of that on bait palatability. Development of chemical-based toxicants has not provided,
and does not appear able to provide, species-selectivity, low welfare impacts, low residue
risk, and acceptable cost-effectiveness. This suggests that research effort to find accepta-
ble alternatives to 1080 should focus on pest species genetics, either through genome
mining to identify species-specific proteins for targeted toxicant development, or by
developing gene-drive technologies to limit pest reproduction.
Antidotes
Few of the toxicants used in New Zealand, other than the anticoagulants, PAPP and
sodium nitrite, have effective antidotes, and the use of antidotes for affected free-living
native species is likely to have very limited practical application. Research into antidotes
for 1080 has made very limited progress (Cook et al 2001; Eason, Murphy, Ogilvie et al
2010) and treatment remains largely focused on mitigating poisoning symptoms (Gonch-
arov et al 2006; Eason et al 2011; Williams and Parton 2019). However, new research has
been stimulated by the recognition of 1080 as a potential chemical weapon (DeLay Cox
et al. 2020). The methylene blue antidote for PAPP and sodium nitrite is also of limited
use for non-target animals as the toxicants act rapidly and it requires intravenous
NEW ZEALAND JOURNAL OF ZOOLOGY 23
injection, so that prompt veterinary attention is essential for effective treatment of acci-
dental poisoning. Antidotes may be useful, however, in cases of livestock, dog, and
human poisoning.
By-kill and repellents
Research is also addressing other major concerns about the risks of 1080 and other
broad-spectrum toxicant use. Repellent additives to toxic baits are being investigated
to reduce the risk of native bird deaths (Cowan et al. 2016) and deer (Morriss et al.
2020). However, finding a compound that deters birds from feeding on bait without
affecting consumption by target pests and reducing control efficacy is proving a signifi-
cant challenge, particularly for rodent pests (Crowell, Martini et al. 2016).
Welfare impacts
The humaneness of 1080 has been addressed by research into co-administration of
analgesics with 1080, and some improved animal welfare outcomes have been reported
for carnivore pests in Australia (Marks et al. 2009). Intake of an oral anaesthetic by
possums mitigated some of the welfare impacts of 1080 and zinc phosphide poisoning.
However, the oral dosage that produced the mitigation was at least 10 times greater
than doses recommended by veterinary labels for the compound in domestic dogs and
cats (O’Connor et al. 2007).
Thermogenic compounds have also been assessed for their ability to improve the
welfare of possums during 1080 poisoning by decreasing signs of illness or time to
unconsciousness and death. Caffeine produced significant increases in possum metabolic
rate within a suitable time frame and caused a significant reduction in the time to death of
1080-poisoned possums (Fisher et al. 2010). Given these initial positive outcomes, further
research into mitigating the welfare impacts of existing toxins would be justified.
Residues
Specific issues with other toxicants are also being addressed. Bioaccumulation and sec-
ondary poisoning of non-target species have been of particular concern with the use
of the second-generation anticoagulant brodifacoum for possum and rodent control
(Eason et al. 1996; Eason, Fagerstone et al. 2010; Eason, Murphy, Hix, MacMorran
et al. 2010). In response to this concern, bait has been developed and registered contain-
ing the first-generation anticoagulant diphacinone and a low dose of cholecalciferol,
which enhances the haemorrhagic effect of diphacinone (Kerins et al. 2002; Eason
et al. 2020). This mixture has high efficacy for possum and rodent control, but is less per-
sistent and safer for non-target species than brodifacoum. The concept of addressing
specific concerns about the current toxicants in use, such as their welfare or non-
target risk, by the use of specific additives and/or synergistic combinations of existing
lower-risk toxicants, would seem to be a fruitful area for future research. Table 2 provides
an outline of these concerns relative to 1080 and would be a useful to guide future
research priorities.
24 B. WARBURTON ET AL.
Registration of toxicants
The concept of finding smarter and safer ways to use existing registered toxicants is
attractive because of the very high cost of development, testing, and registration of de
novo products, and the relationship between those costs and the likely market size and
return on investment. Novel pest control products for which New Zealand is the sole
market (such as possums and stoats) will be much more difficult to fund to market
than, for example, rodent control products, unless they are based on technologies that
are adaptable to multiple species. Nevertheless, the search for novel rat-selective com-
pounds is likely to be greatly assisted by the recent sequencing of the ship rat
genome.
22
Sequencing of the possum genome is completed and complements the
genetic information potentially useful for novel toxicant and delivery systems already
available, including a mixed heart and liver transcriptome (White et al. 2019), mitochon-
drial DNA sequence (Phillips et al. 2001), and sequences of a possum endogenous type D
retrovirus (Baillie and Wilkins 2001), a possum adenovirus (Thomson et al. 2002), and
two possum enteroviruses (Zheng 2007).
Multi-species control
While species-specific control methods might seem the ideal, New Zealand control oper-
ations often target multiple pests, particularly possums and rodents, or rodents and mus-
telids. Currently, possums and rodents are susceptible to some of the same baits and
toxicants, and mustelids are susceptible to secondary poisoning from pest carcase
feeding (Murphy et al. 1999; King et al. 2001). Thus, there is an ongoing need for multiple
toxicants or toxicant combinations that can target multiple pest species. At the same time
there is a need for research into additives to bait for aerial delivery that can render toxic
baits harmless to non-target species after a relevant time period. Generally, most aerially
delivered bait is eaten by target pests within 3 days (Nugent et al. 2011). But at cooler and
drier times of year, bait degradation and carcase decomposition are slowed, and residual
bait in carcase digestive tracts may retain toxicity for weeks rather than days and pose an
ongoing risk to non-target species.
Traps
Trapping is the other main method of pest control. From a welfare perspective, the
partial replacement of leg-hold traps by effective kill-traps is a positive outcome.
While advances have been made in trapping cost-efficiency through the use of
self-resetting traps for possums, rats and stoats, long-lasting lures and automatic
lure dispersers (Waters et al. 2017;Carteretal2019;Murphyetal.2019), recent
use of trail cameras has shown that many individual pests encounter traps much
more often than they interact with them (B. Warburton, unpublished data). The
key need to overcome this problem is for trap designs that do not require animals
to enter or interact with them directly. This could potentially be achieved using
AI devices that can recognise the target species and trigger the trap independently
of any interaction by the animal.
23
NEW ZEALAND JOURNAL OF ZOOLOGY 25
Biological control and biotechnology
Attempts in the 1990s to develop biological and biotechnological control for possums
and stoats were unsuccessful. No suitable classical biological control agents were ident-
ified for either species, despite intensive searching (Obendorf et al 1998; Cowan 2000;
McDonald and Lariviere 2001). Research to develop bait-delivered, non-transmissible,
possum-specific fertility control based on immune responses to zona pellucida proteins
was unable to develop a product that produced a sufficiently high and long-lasting infer-
tility (Cross et al. 2011). Attempts to develop a genetically modified, possum-specific
parasite expressing foreign proteins were also unsuccessful, due largely to issues of
unstable genetic modification and gene expression (Grant et al. 2006). Gene drives for
pest management are likely to face similar technological challenges and significant
debate within science and society about their ultimate use (MacDonald, Edwards et al.
2020). The previous research into biological control for possums and stoats was under-
pinned by extensive social research into the acceptability of the technology (Fitzgerald
et al 1996; Fitzgerald et al 2002,2005). Lessons from the concerns and views expressed
then are likely to be highly relevant to some of the issues that will be raised by gene-
drive technology.
Gene-drive technology in its currently proposed form also has some practical issues.
The need to breed modified animals for release may be a major constraint for the use of
the technology, given the possum’s low reproductive rate and the current difficulty of
breeding stoats in captivity (McDonald and Lariviere 2002).
Māori views
Ngata (2018), in their statement about 1080, strongly advocated for the inclusion of man-
awhenua in the decision making about pest control strategies on all the conservation
estate. They recognised the diverse range of topographies and ecologies across New
Zealand, and that for some tangata whenua 1080 is not a viable option, while for
others it is currently the only option. However, they argued that if 1080 use ceased
and the projected trajectory of species decline continued during the search for the
perfect alternative (for 1080), they would be failing in their duties as kaitieki. They
acknowledged that for many whānau, 1080 is a provocative issue, filled with mamae
(pain) and concern for whenua, waterways, and mokopuna, and ended by urging
whānau (family) to place fear and riri (anger) aside, and come together in the spirit of
kotahitanga (unity), and seek a pathway that upholds the inherent rights of Papatūānuku
(Mother Earth), and their mana as her kaitieki. Decision making on future research on
alternative forms of pest control should also, therefore, be inclusive of mana whenua.
Conclusion
In conclusion, although use of 1080 has enabled the protection of conservation and
primary production values for almost 60 years, its characteristics, and the perceptions
of it held by some parts of society, have perpetuated a strong and often vociferous oppo-
sition to its use, especially when aerially applied. Finding an alternative for aerial appli-
cation has been a challenge that, so far, has not been met because of regulatory difficulty
26 B. WARBURTON ET AL.
in getting approval for aerial delivery of an alternative, toxic residue concerns, and poor
or inconsistent efficacy of alternatives.
To find an alternative that has similar efficacy to aerial 1080 while satisfying criteria for
species-selectivity, no residues, and humaneness is an ongoing challenge. The most posi-
tive solution appears to be through understanding the genome of the target animals and
genetic manipulation, to either develop species-specific designer lethal toxicants based on
genome mining, or gene editing to develop non-lethal technologies. Both will require
considerable time and funding, and considerable effort to address and overcome
major social and regulatory hurdles. In the meantime, as emphasised by Horn and Kil-
vington (2003), efforts must continue to strengthen dialogue between agencies and
involved communities to find ways to address their concerns and to ensure the safe,
effective, and best-practice use of 1080 while the search for new control methods
progresses.
Notes
1. www.epa.govt.nz/database-search/hsno-application-register/view/HRE05002.
2. www.facebook.com/ban1080party/
3. www.doc.govt.nz/globalassets/documents/conservation/threats–and–impacts/tiakina–nga–
manu–2019/tnm–seedfall–threatened–species–2019.jpg; (accessed 30 March 2020).
4. www.epa.govt.nz/assets/RecordsAPI/EPA–annual–report–on–aerial–1080–operations–2018.
pdf (accessed 30 March 2020).
5. www.doc.govt.nz/nature/pests–and–threats/methods–of–control/1080/proof–that–1080–is
–saving–our–species/ (accessed 29 March 2020).
6. www.doc.govt.nz/our–work/tiakina–nga–manu/tiakina–nga–manu–monitoring–results/
(accessed 29 March 2020).
7. www.doc.govt.nz/our–work/tiakina–nga–manu/landsborough–valley–bird–numbers–
double/ (accessed 29 March 2020).
8. www.pce.parliament.nz (accessed 28 March 2020) (PCE 2013).
9. https://pf2050.co.nz/project/ (accessed 28 February 2021).
10. https://pf2050.co.nz/products–to–projects/ (accessed 28 February 2021).
11. https://pf2050.co.nz/funded–projects/ (accessed 31 March 2020).
12. https://predatorfreenz.org/resources/trapping-best-practice/ (accessed 28 February 2021).
13. www.stuff.co.nz/environment/117571706/five-new-trapping-tools-unveiled-to-achieve-
predator-free-2050-mission (accessed February 1 2021).
14. https://cacophony.org.nz/.
15. https://bioheritage.nz/research/possum-poisons.
16. http://pharma–docs.phrma.org/sites/default/files/pdf/rd_brochure_022307.pdf (accessed 29
March 2020).
17. Rodenticide Market –Global Forecast to 2022, www.marketsandmarkets.com (accessed 18
April 2020).
18. Exponent Consulting, www.exponent.com, pers. comm.
19. https://epa.govt.nz/industry–areas/new–organisms/ (accessed 28 February 2021).
20. https://epa.govt.nz/industry–areas/new–organisms/gm–field–tests/ (accessed 28 February
2021).
21. e.g. https://epa.govt.nz/assets/Uploads/Documents/Hazardous–Substances/Compliance–re
ports/enquiry–aerial–pest–control–operation–mapara–reserve–dec19–with–Annexe–links.
pdf (accessed 14 April 2020).
22. https://www.ncbi.nlm.nih.gov/genome/13128 (accessed March 2020).
23. https://cacophony.org.nz/technology.
NEW ZEALAND JOURNAL OF ZOOLOGY 27
Acknowledgements
We thank Ivana Giacon and Kaye Seymour of OSPRI for modelling the impacts of loss of 1080 for
TB Plan outcomes, GeoffKaine for comments on the draft manuscript and two anonymous refer-
ees for their feedback.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work was supported by Ministry of Business, Innovation and Employment: [Strategic Science
Investment Funding].
ORCID
Shaun Ogilvie http://orcid.org/0000-0003-1664-2588
Thomas A. A. Prowse http://orcid.org/0000-0002-4093-767X
James Ross http://orcid.org/0000-0001-7413-4704
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Appendix 1. Summary information about toxicants in use in New Zealand
as alternatives to 1080.
Cyanide is used in a paste formulation and in an encapsulated pellet form (Feratox). Paste is
usually laid on logs or similar sites. Feratox in bait stations is used for ground baiting for
possum and wallaby (Macropus spp.) control (Eason, Shapiro et al. 2010; Ross et al. 2011;
Shapiro et al. 2011). Cyanide disrupts energy metabolism by preventing the use of oxygen (cyto-
toxic hypoxia), resulting in rapid respiratory arrest and death (Osweiler et al. 1985; Gregory et al.
1998). Cyanide is considered the most humane toxicant (Gregory et al. 1998), because of the short
time to death of 15–20 min and the short duration of symptoms of poisoning.
Zinc phosphide is used in many other countries as a rodenticide (Marsh 1987; United States
Environmental Protection Agency 1998; Eason, Ross et al. 2012) and is favoured for its lack of per-
sistence and comparatively low risk of secondary poisoning. Zinc phosphide paste was approved
for use as a possum control method in New Zealand by the EPA in August 2011. To combat its
emetic action (not an issue for rats, which lack a vomiting reflex [Marsh 1987]), a micro-encap-
sulated form of zinc phosphide has been developed for use in paste, and may be developed for
solid cereal baits, initially for controlling possums and as a rodenticide (Shapiro et al. 2015).
Additional research efforts and practical experience could enable the effective use of zinc phos-
phide as a conservation tool for bovine TB vector control in New Zealand (Eason et al. 2012;
Shapiro, MacMorran et al. 2016). In the future, zinc phosphide could be delivered in a resetting
toxicant-delivery system (Blackie et al. 2014,2016) and was being explored for rabbit (Oryctolagus
cuniculus) control (James Ross, Lincoln University, pers. comm.), in part because of secondary
poisoning concerns associated with aerial application of pindone baits for rabbit control and
the potential public opposition to aerial 1080 rabbit control.
Cholecalciferol (vitamin D3) was developed as a rodenticide in the 1980s (Marshall 1984;
Tobin et al. 1993) and first registered in New Zealand in the 1990s (Hix et al. 2012). It is syn-
thesised naturally in animals by the action of sunlight on its precursor, 7-dehydrocholesterol.
Natural dietary sources of vitamin D include fish, liver, fish oils, egg yolk, milk fat, and plants,
and background levels are detectable in the blood and tissues of all mammals (Fairweather
et al. 2013). To become toxicologically active, cholecalciferol must be converted to 25-hydroxycho-
lecalciferol (25OHD). The latter metabolite is the most biologically active form of vitamin D3
(Keiver et al. 1988) and can cause calcification and death from heart failure (Dorman & Beasley
1989). Time to death after a lethal dose, usually 3–7 days, is like that of rodents exposed to
lethal doses of anticoagulants (Marshall 1984; Eason et al. 1993). The single-dose LD
50
for chole-
calciferol in Norway rats (Rattus norvegicus) and house mice (Mus musculus) is very similar
(approx. 40 mg/kg). Possums are more susceptible than rodents, but other mammals vary con-
siderably in susceptibility. Cholecalciferol poses a low risk of secondary poisoning of dogs
(Canis familiaris) and has low toxicity to birds (Eason et al. 2000). However, primary poisoning
NEW ZEALAND JOURNAL OF ZOOLOGY 41
of non-target wildlife or pets is a risk if they have access to cholecalciferol-containing bait. Feeding
poisoned animals to cats, dogs, ferrets (Mustela putorius furo) and turtles (Testudines spp.) indi-
cates that the risk of secondary poisoning is low, despite the likely presence of elevated concen-
trations of 25-hydroxycholecalciferol in carcasses. Low doses of cholecalciferol have been added
to anticoagulant-containing baits to increase their effectiveness (Eason et al. 2020). Combining
aspirin with cholecalciferol has been shown in recent research to greatly improve the cost-effec-
tiveness and animal welfare impact of cholecalciferol alone (Morgan et al. 2012).
Para-aminopropiophenone (PAPP), originally studied as a treatment for cyanide and radi-
ation poisoning (Rose et al. 1947; Eason et al. 2014), is toxic to carnivores, although birds and
humans are less sensitive (Savarie et al. 1983; Fisher & O’Connor 2007; Murphy et al. 2007;
Eason, Fagerstone et al. 2010; Eason, Murphy, Hix, MacMorran 2010). This is primarily
because eutherian carnivores have metabolic pathways different from those in other orders of
animals (Wood et al. 1991). The registration of PAPP in New Zealand was assisted by research
in the U.S. (Savarie et al. 1983), the United Kingdom (Marrs et al. 1991), and Australia (Marks
et al. 2004; Cowled et al. 2008), and by DOC (Murphy et al. 2007). The toxic effects of PAPP
are related to its ability to reduce the oxygen-carrying capacity of red blood cells through the for-
mation of methaemoglobin. The onset of symptoms is rapid, and cats and foxes are usually uncon-
scious within 30–45 min (Marks et al. 2004), and the lack of oxygen to the brain and other vital
organs causes death by respiratory failure. Normally, methaemoglobin concentration in the
blood is below 1%. Clinical signs after a lethal dose first appear in 10–20 min for stoats and
around 35 min for feral cats; death usually follows within 2 h. Affected animals become lethargic
and sleepy before they die, which is why PAPP is classed as relatively humane. Methylene blue
administration will reverse the methaemoglobinaemia induced by PAPP, so it is considered a
viable antidote to PAPP exposure (Rose et al. 1947). PAPP has been developed in New Zealand
specifically for the control of stoats and feral cats (Felis catus) because of their sensitivity to
PAPP (Eason et al. 2014). PAPP paste was approved as a stoat and feral cat control agent in
New Zealand in 2011 (Eason, Murphy, Hix, Henderson et al. 2010; Eason, Murphy, Hix, MacMor-
ran 2010; Eason et al. 2014). Stoat and feral cat numbers can be rapidly reduced using meat baits
containing toxic doses of PAPP in bait stations (Dilks et al. 2011). However, there has been limited
practical experience with PAPP. In the future, PAPP may be developed in bait suitable for both
ground and aerial control (Dr Elaine Murphy, Dr Michelle Crowell, NZ Department of Conserva-
tion, pers. comm., 2019) and in a resetting toxicant-delivery system (Murphy et al. 2014).
Sodium nitrite, a meat preservative, was first investigated as a toxicant for feral pigs (Sus scrofa)
in Australia (Cowled et al. 2008). Of all the species tested, pigs are among the most sensitive, with
time to death after a lethal dose of 2–3 h, with few visual symptoms (Cowled et al. 2008). After an
extensive research programme, encapsulated sodium nitrite was registered in New Zealand for the
control of possums and feral pigs (Shapiro, Blackie et al. 2016; Shapiro, Eason et al. 2016; Shapiro,
MacMorran et al. 2016). It is a methaemoglobinaemia inducer, with a mode of action on red blood
cells like that of PAPP. Both rank consistently high on criteria of humaneness and the ready avail-
ability of an antidote (methylene blue). Sodium nitrite is the only toxicant available for feral pig
control in New Zealand. Pen and field trials with feral pigs have demonstrated high levels of
efficacy and welfare when the toxicant is applied at optimal doses in baits (Shapiro, Blackie
et al. 2016; Shapiro, Eason et al. 2016; Shapiro, MacMorran et al. 2016).
Norbormide is a selective rat toxicant developed in the 1960s. Its use was discontinued in the
1970s as anticoagulant toxicants became more popular, and because taste aversion limited its effec-
tiveness in the field (Telle 1967). After a lethal dose most animals die within 8–12 h. It appears to
be more humane than most other rodenticides because of the relatively short time to death and the
duration of symptoms of poisoning when compared with anticoagulant rodenticides and cholecal-
ciferol. Norbormide causes vasoconstriction (narrowing) of small arteries and vasodilation
(widening) of large arteries in rats (Roszkowski 1965), which causes a rapid fall in blood pressure.
Death probably results from circulatory disorders and heart failure (Cavalli et al. 2004; Riccheli
et al. 2005). Norbormide is highly toxic to members of the genus Rattus compared with other
mammals or birds (Roszkowski 1965). Prodrug forms of norbormide have been developed that
aim to delay the action of the toxicant and increase palatability by masking the taste (Rennison
42 B. WARBURTON ET AL.
et al. 2012,2013). Two New Zealand research teams are researching ways of improving the effec-
tiveness of norbormide and producing it in forms that are more palatable.
Pindone belongs to the indandione class of anticoagulants, which differ chemically from cou-
marin anticoagulants such as brodifacoum or warfarin. It was developed as a rodenticide in the
early 1940s. Its use to control rodents and possums in New Zealand has been largely replaced
by more potent second-generation anticoagulants such as brodifacoum. However, it remains
favoured and effective for rabbit control in New Zealand and Australia (Eason & Jolly 1993). A
single dose of approximately 18 mg/kg is sufficient to kill a rabbit, and the lethal 7-day repeat
dose LD
50
is 0.52 mg/kg/day (Hone & Mulligan 1982). Recent research has emphasised the
risks of secondary poisoning (Fisher et al. 2015), although this is less likely than for second-gen-
eration anticoagulants. The weaker potency of first-generation anticoagulants such as pindone is
related to its generally lower binding affinity compared with second-generation compounds
(Horak et al. 2018). As with all other anticoagulant compounds, clinical signs of pindone toxicosis
in animals will usually involve some manifestation of haemorrhage (Osweiler et al. 1985).
Diphacinone is a first-generation anticoagulant of the indandione class. It is more toxic than
warfarin and pindone to most rats and mice (Buckle & Smith 2015). In New Zealand it is registered
in cereal paste bait primarily for the field control of rodents, and it has also been incorporated into
fish-based bait for ferrets. In the United States it has been registered by the EPA for field use
against rodents for conservation purposes, providing an alternative to brodifacoum (Eason, Fager-
stone et al. 2010). Diphacinone, like other anticoagulants, inhibits the formation of vitamin
K-dependent blood-clotting factors. Clinical and post-mortem signs of toxicosis are as for other
anticoagulants. The persistence of diphacinone in the liver is like that of pindone, and both are
rapidly eliminated so do not bioaccumulate as do the second-generation anticoagulants (Horak
et al. 2018).
The superior potency of second-generation anticoagulants is related to their greater affinity for
vitamin K-epoxide reductase, and their associated greater potential to affect non-target wildlife
compared to first-generation anticoagulants is a consequence of their subsequent accumulation
and persistence in the liver and kidneys after absorption (Horak et al. 2018).
Brodifacoum (Hadler and Shadbolt 1975)differs from the first-generation anticoagulants in
that it is more potent and can induce death in several animal species after only a single dose. It
also has an unusually long persistence compared with first-generation anticoagulants (Fisher
et al. 2003; Crowell et al. 2013; Horak et al. 2018) because it is not fully metabolised and excreted
before death. It has been used successfully in rodent eradication programmes on offshore islands to
protect populations of endangered indigenous birds (Taylor and Thomas 1989,1993; Courchamp
et al. 2003; Towns and Broome 2003), and in bait stations to control possums and rodents in main-
land New Zealand (Eason, Murphy, Ogilvie et al. 2010). In New Zealand the problems associated
with its persistence have been compounded by concerns about its inhumane effects on larger ver-
tebrate pests such as possums (Littin et al. 2004). DOC no longer aerially distributes baits contain-
ing brodifacoum for routine mammal control on the mainland, restricting that use to ‘one-off’
eradication operations on offshore islands (Veitch et al. 2011).
Diphacinone at 0.005% combined with cholecalciferol at 0.06% produces a slow-acting bait
that is effective at killing possums and rodents and has a favourable risk profile compared with
second-generation anticoagulant rodenticides, such as brodifacoum. This new bait was registered
in New Zealand in 2019. The new cereal bait has the potential to play a key role in controlling
possums and rodents alongside other tools for conservation and TB vector management, and to
reduce over-use of brodifacoum and its bioaccumulation in non-target wildlife. This new bait
will be used for controlling possums and rats and will not require a professional licence for use.
Targeted delivery using Spitfire devices for rats and possums is under development (Eason
et al. 2017) to limit risk to non-target species, and solid extruded cereal baits containing diphaci-
none and cholecalciferol are in the pipeline.
NEW ZEALAND JOURNAL OF ZOOLOGY 43
