TOXINS 101 – THE DETAILS

By Phil Bell (for the Predator-free Rakiura Governance Group)

Following on from the article in this month’s SIN (October), here are more details on each of the toxins outlined (with a table of pros and cons at the bottom). The majority of the information below has been adapted from the ‘Pesticide Information Review’ for each toxin – these are available via DOC on request if you would like even more detail.

 

Cyanide

Registered for what target species on Rakiura: possums

Registered for use: in bait stations; in bait bags; hand laying in paste form. Operators require a Controlled Substances License to use it in all formulations.

Trade names: Feratox™;Cyanara50 Cyanide Paste; Cyanide Paste for Possum Destruction; Trappers Cyanide Paste

 

Track record

Cyanide has been used in New Zealand for killing possums since the 1940’s.

 

How does it work?

Cyanide disrupts energy metabolism by preventing the use of oxygen in the production of energy, leading to respiratory failure and death.

 

Fate in the environment

Cyanide (in all formulations) is potentially hazardous until broken down and unrecognisable, likely to take several months depending on environmental conditions. Cyanide is rapidly lost from soil through volatilization, chemical reactions with other compounds, and biodegradation by microorganisms. Leaching of cyanide into water will occur. Cyanide is highly soluble in water.

 

Effects on non-target species

Cyanide appears to be toxic to a range of vertebrates and invertebrates. There is limited data on the toxicity of cyanide to reptiles and amphibians, though it would appear that cold-blooded animals such as frogs are less susceptible.

 

There are no LD50 values (the individual dose required to kill 50 percent of a population of test animals within a set timeframe) available for native New Zealand species. Native species found dead after pesticide operations using cyanide include kiwi, kea, tomtit, robin, silvereye, tui, weka and short-tailed bats. Most of the non-target deaths have been reported after handlaying of cyanide paste.

 

Accidental dog and cattle deaths have also been reported during pest control operations using cyanide. Cyanide is not accumulated or stored in any mammal studied, and cyanide biomagnifications (accumulation) in food webs has not been reported.

 

Time to death

Cyanide is seen as one of the most humane pesticides available for possum control, with death occurring within a matter of minutes (averaging 10-20). However, the rapid onset of toxicosis means that possums can become bait shy if they receive a sub-lethal dose of cyanide.

 

Brodifacoum

Registered for what target species on Rakiura: rats and possums

Registered for use: aerial application and hand broadcast (for island or ‘behind fence’ eradications); in bait stations

Trade names: Pestoff™

 

Track record

Brodifacoum has been used successfully in predator eradication programmes around the world, including the majority of island (e.g. Taukihepa/Big South Cape) and ‘behind a fence’ projects (e.g. Maungatautiri) in New Zealand. Brodifacoum in bait stations has also been used on the mainland to control brushtail possums and rodents, including on private farms. Many households throughout the country use brodifacoum to kill rodents that become a nuisance pest in their homes.

 

How does it work?

Brodifacoum is a second generation anticoagulant (meaning it is more potent and has a much longer action). It acts by interfering with the synthesis of vitamin K-dependent clotting factors. This increases the clotting time of blood and leads to death from haemorrhaging.

 

Fate in the environment

Soil type, temperature, and the presence of soil micro-organisms capable of degrading brodifacoum will all influence the degradation time, with baits potentially remaining toxic for months in certain conditions. As baits disintergrate, brodifacoum is absorbed into the soil where it is slowly degraded over weeks to months.

 

Brodifacoum has a very low solubility in water. The only residues of brodifacoum to have been detected in water bodies following pest control operations in New Zealand comes from a single sample of stream water collected 24 hours after bait application and within 20cm of baits in the streambed.

 

Effects on non-target species

Most terrestrial vertebrate species are likely to be susceptible to brodifacoum if enough is eaten either through primary (eating the bait directly) or secondary (eating poisoned animals or carcasses) poisoning. There is only limited information, principally for birds, on the toxicity of brodifacoum to native New Zealand species. All NZ trials to date have failed to show an effect on invertebrates that have fed on brodifacoum baits. Native non-target individual deaths, and residues, have been reported in a wide range of species after the use of brodifacoum.

 

The benefits of using brodifacoum to eradicate rats from offshore islands are also apparent. In all cases, any short term losses of individuals of native species are offset by the longer term benefits of removing the rats (e.g. successful breeding and juvenile recruitment).

 

Feral and domestic non-target deaths (cats, pigs, deer, and sheep) have been reported following brodifacoum use. Surveys of feral animals have shown that contamination (in both sub-lethally and lethally poisoned animals) has occurred where there has been sustained use of brodifacoum. Brodifacoum is not readily metabolised and is stored in the liver of sub-lethally exposed animals, where it can remain for many months.

 

There is an effective antidote (Vitamin K) for domestic animals poisoned with brodifacoum. However, long term treatment is required.

 

Time to death

The mean time to death of brushtail possums is 20.1 days and Norway rats is 7.2 days after ingestion of a lethal dose. This long latent period (time from consumption to death) means that bait shyness is unlikely to develop in animals receiving a sub-lethal dose.

 

Diphacinone

Registered for what target species on Rakiura: rats

Registered for use: in bait stations

Trade names: Pestoff 50D™; RatAbate™; D-Block for the Control of Rats

 

Track record

Overseas use report successful rodent eradications on small islands up to 72 ha. More recent attempts to eradicate rats on small islands in Hawaii and Japan by aerial application of baits containing diphacinone have had mixed success (for multiple reasons). Successful rat control was recorded at some DOC mainland islands using bait station networks containing diphacinone.

 

How does it work?

Diphacinone is a first generation anticoagulant (meaning it is less potent than Brodifacoum). Like other anticoagulants, diphacinone inhibits the formation of vitamin K- dependent clotting factors in the blood, leading to multiple haemorrhages causing death.

 

As with all first generation anticoagulants, diphacinone is readily metabolised and rodents are far more susceptible to poisoning through multiple feeds than a single dose, meaning it is best used as a toxin within bait station operations.

 

Fate in the environment

Diphacinone is practically insoluble in water and slow to break down in the environment (taking weeks to months). It binds tightly to soil and is not readily leached or likely to be taken up by plants.

 

Effects on non-target species

The only information available for NZ native species relates to an estimated 25% mortality of short tailed bats in 2009 following the use of diphacinone paste in biodegradable plastic bags. Birds appear relatively tolerant of diphacinone, although birds of prey may be more vulnerable than others, particularly if they are exposed to regularly repeated doses.

 

Mammals are highly variable. Although dogs and cats will likely survive secondary doses received from scavenging single rat carcasses, they may be at risk from baits and multiple doses of contaminated food. Aquatic species are unlikely to be exposed to diphacinone if used correctly within bait stations. Invertebrates appear to be highly resistant to anticoagulant poisoning.

 

Residues in terrestrial living animals and carcasses can be expected and secondary poisoning has been demonstrated. Elimination time in the livers of moderately dosed deer and pigs were about 12 and 28 days respectively.

 

Time to death

Diphacinone baits at 3 mg/kg will kill rodents in 5–8 days. It is most effective in multiple doses where rodents can feed freely for 10 days or more without running out of bait.  The latent period means that bait shyness is unlikely to develop in animals receiving a sub-lethal dose.

 

Sodium monofluoroacetate (1080)

Registered for what target species on Rakiura: rats, possums, and feral cats

Registered for use: aerial application; hand broadcast; in bait stations; in bait bags; in paste form. Operators require a Controlled Substances License to use it in all formulations.

Trade names: Pestoff Exterminator Paste™; 1080 Pellets; 1080 Rodent Pellets; 1080 Feral Cat Bait; Pestoff Professional 1080 Possum Paste; No Possums 1080 Gel Bait

 

Track record

1080 is the most widely used poison for rat and possum control in New Zealand for situations where numbers need to be reduced rapidly over large areas. It is the main tool being used by DOC in the current ‘Battle for our Birds’ rat and possum control programme, in response to the widespread beech mast event. Of note for the Rakiura context, vertebrate pesticides containing 1080 are also registered for the control of feral cats.

 

Manufactured 1080 is chemically identical to the toxic compound found in some plants, including in black tea leaves (grown in India and Sri Lanka).

 

How does it work?

Monofluoroacetate is converted within animals to fluorocitrate, which inhibits the tricarboxylic acid cycle. This results in accumulation of citrate in the tissues and plasma, energy deprivation, and death. In other words, 1080 results in accumulation of citrate in the tissues and plasma, energy deprivation, and death.

 

Fate in the environment

1080 in baits may take several weeks to degrade depending on the environmental conditions but it breaks down into harmless substances. It does not accumulate in the soil.

 

Sodium monofluoroacetate is highly water soluble so leaching out of soil will occur. Water samples have been collected from streams following numerous pest control operations using 1080, and 96.6% of these samples contained no residues of 1080. Where residues were found most of these had less than 1 µg l1 1080.

 

Effects on non-target species

1080 is likely to be toxic to most native animals. There is wide variation in sensitivity between taxonomic groups with mammals more sensitive than birds and invertebrates (on a weight for weight basis). The risks 1080 operations pose to aquatic species is considered very low. There are records of a range of individual native birds found dead after aerial poisoning operations and many of these individuals have contained residues of 1080; however the populations have all survived and benefited.

 

Dogs are especially vulnerable and highly likely to die if they eat 1080 baits or scavenge animals killed by 1080. Feral deer population mortality from aerial poisoning operations targeting possums is highly variable and does not appear to be consistently influenced by toxic loading, sowing rate, prefeeding, or bait type. Most estimates of deer kill range between 30 and 60%. 1080 has a relatively short half-life in sub-lethally dosed animals and it is metabolised and eliminated from living animals within days – it does not accumulate in the food chain. However, 1080 can persist in carcasses for months.

 

Time to death

Most deaths of pest species occur 8 – 48 hours after ingestion of a lethal dose. The short latent period means that bait shyness can develop in animals receiving a sub-lethal dose.

 

4-aminopropiophenone (PAPP)

Registered for what target species on Rakiura: feral cats

Registered for use: in bait stations. Operators require a Controlled Substances License to use it.

Trade names: PredaSTOP™.

 

Track record

PAPP is a newly registered toxin, and as such it does not yet have the track record to illustrate its effectiveness. Trials in Australia have used it in feral cat eradications on small islands.

 

How does it work?

PAPP induces methaemoglobinemia by rapidly oxidising haemoglobin to methaemoglobin in red blood cells. Methaemoglobin blocks the blood’s ability to carry oxygen, resulting in the brain, heart and other muscles being starved of oxygen. This leads rapidly to unconsciousness and death due to heart failure. It is similar in effect to carbon monoxide poisoning

 

Fate in the Environment

There is limited information on its fate in the environment.  PAPP is soluble in water and will leach through soils.

 

Effects on Non-target Species

Weka are the only native species for which an LD50 has been estimated (~ 568 mg/kg). Overseas studies indicate that most species of bird appear to be relatively tolerant to PAPP. However, ducks are known to have a relatively low LD50 compared to other avian species. Reptiles may be vulnerable to PAPP. No non-target deaths were reported during field trials to gain registration for PAPP.

 

The metabolic pathways involved in the detoxification and excretion of PAPP vary between species. As a result, the toxicity of PAPP varies significantly between species, with mammalian carnivores being particularly sensitive.

 

PAPP and its metabolites are rapidly excreted so most of a sub-lethal dose will be cleared within 24 hours. There is no available information on residue levels in carcasses of poisoned animals.

 

If PAPP is used inappropriately, it could kill companion animals such as cats and dogs. An effective treatment for PAPP poisoning is available (methylene blue). However, the rapid of onset of symptoms in acute PAPP poisoning may limit the efficacy of treatment.

 

Time to death

The onset of symptoms and death is rapid after ingestion, within 246 minutes for feral cats.

 

Table 1. The pros and cons of each toxin described above

 

Toxin Pros Cons
Cyanide
  • Considered a fast acting, humane toxin for possums
  • Less risk to pet dogs
  • Short caution/withholding period (for hunting and consuming deer) of only 2 months required
  • Requires prefeeding to gain acceptance of the toxic bait
  • Fast acting toxin can result in bait shyness (which can last for 2 years in possums)
  • Significant non-target risks to wildlife (especially weka)
  • Risk of secondary poisoning to feral cats considered to be low
  • Bait is relatively expensive
Diphacinone
  • Has successfully eradicated rats from some islands
  • Readily metabolised so they have a shorter persistence time in living tissue (than second generation anticoagulants)
  • Antidote available (Vitamin K)
  • Requires rodents to consume more bait over a longer time period to receive a lethal dose (at least 5 consecutive nights)
  • Possums are relatively resistant to first generation anticoagulants
  • Less effective as secondary poisoning for feral cats (than second generation anticoagulants)
  • Significant non-target risks to wildlife and pets
  • Caution/withholding period (for hunting and consuming deer) of 8 months usually required
Brodifacoum
  • Proven track record in successful eradications around the world
  • Registered for use for rats and possums
  • Slow acting so can enable secondary poisoning of feral cats
  • Well known effects enable better mitigation planning to minimise non-target impacts
  • Registered for aerial application in areas enclosed within predator-proof fence
  • Antidote available for pets and humans
  • Policy restricts use of brodifacoum on DOC managed land – exemptions can be applied
  • DOC policy prohibits the use of brodifacoum targeting possums – exemptions can be applied
  • Significant non-target risks to wildlife and pets
  • Can remain in the liver of sub lethally poisoned animals for months (including deer)
  • Would require a 36 month caution period for deer hunting and consumption
Sodium monofluoroacetate (1080)
  • Less risk to non-targets and people as it degrades quickly and is metabolised rapidly
  • Registered for aerial application
  • Registered for use on rats, possums, and feral cats
  • Very well studied with a large knowledge base of impacts and effects developed
  • Well known effects enable better mitigation planning to minimise non-target impacts
  • Deer repellent available
  • Likely to achieve secondary poisoning of feral cats
  • Fast acting toxin can result in bait shyness (which can last for 3 years in possums)
  • Can be detected and avoided by some individual rats so it is not suitable for eradication (where no individuals can be left)
  • Significant non-target risks to pets (dogs are highly susceptible)
  • Deer repellent will not result in zero deer deaths
  • Requires prefeeding to gain acceptance of the toxic bait
  • Caution/withholding period (for hunting and consuming deer) of 9 months usually required
Para aminopropiophenone (PAPP)
  • Single feed toxin – only one bait needs to be eaten for a lethal dose
  • Reduced non-target impacts compared with other toxins
  • Highly toxic to mammals – carnivores are particularly susceptible, so ideal for feral cats
  • Veterinary treatment for PAPP poisoning available (Methylene blue)
  • Short caution/withholding period (for hunting and consuming deer) of only 1 month required
  • New toxin – never been used in eradication or long term control programmes
  • Requires prefeeding to gain acceptance of the toxic bait
  • Must be encased in raw minced meat
  • Bait station used must exclude non-target animals
  • Pets are at risk if they ingest the bait