6.3 Alternative poisons

There are six poisons currently registered in New Zealand for possum control: 1080 (sodium monofluoroacetate), cyanide, cholecalciferol (Vitamin D₃), phosphorus, pindone and brodifacoum. Some are also registered for the control of other pests. All six poisons are used in concentrations in baits that are deadly poisonous and should be treated as such. Of these, 1080 is the only poison registered for aerial application. There are advantages and disadvantages associated with each poison and these are summarised in Table 5.

Cyanide, phosphorus, cholecalciferol and 1080 are all used in pastes for ground control of possums. Cyanide kills possums in 10-20 minutes by disrupting oxygen metabolism, leading to respiratory failure and death. Cyanide is cheap and has been used in New Zealand for several decades, but, its instability is a potential risk, and it is considered too hazardous for use in pest control in a number of countries. A significant drawback of cyanide has been the aversion that some possums have to its smell, and the fact that some possums become cyanide-shy if they do not receive a fatal dose. One study showed that the proportion of cyanide-shy possums in four populations ranged from 12% to 54%. Advances in the formulation of cyanide paste may overcome some or most of the cyanide shyness. Cyanide is now also used as a pellet (Feratox®), developed as a safer alternative to paste, that is less likely to lead to bait shyness. There is little known about the non-target impacts of cyanide, although cyanide baits have been known to kill native birds including kiwi. Phosphorus was first used for killing rabbits in the 1920s. Deregistration on the grounds of inhumaneness has been considered, although there is currently some debate about the basis for concluding that the mode of action of phosphorus is inhumane.

Table 5
Comparison of effectiveness and risks of different poisons

Poison	Advantages	Disadvantages
1080 paste cereal pellet carrot baits gel	moderately rapid effects (4-12 hours) very effective low environmental persistence only poison registered for aerial application effective for multiple pest control very cheap	high risk to dogs secondary poisoning risks currently no antidote
Cyanide paste pellets (Feratox®)	cheap rapid action (possums die in 10-20 minutes) low environmental persistence low secondary poisoning risk effective for fur recovery	hazardous in paste form risk to humans if ingested cyanide aversion reduces effectiveness antidote must be used immediately
Phosphorus paste	effective	considered to be inhumane causes secondary poisoning in birds and dogs no antidote
Cholecalciferol paste (Feracol®) cereal bait (Campaign®)	effective low risk of secondary poisoning no poison licence required	expensive compared with 1080 or cyanide long time to death risk to dogs secondary poisoning risks unknown
Brodifacoum cereal pellets (Talon® or PESTOFF®)	effective against low density numbers effective against bait or poison-shy possums antidote available no poison licence required	long time to death (1-2 weeks) very persistent in the environment high secondary poisoning risk widespread contamination of other wildlife possible expensive compared with 1080 or cyanide risks not acceptable for DOC areas where feral pigs, deer or other animals, taken for human consumption, are present
Pindone cereal bait	low secondary poison risk antidote available	need large amount before death occurs possums take 2-3 weeks to die low effectiveness moderate persistence in environment

The SPCA opposes its use. There is little research data on its effectiveness, fate in the environment, or its persistence in carcasses. Phosphorus can cause secondary poisoning in birds and dogs.

Possums are particularly sensitive to cholecalciferol (Campaign®), which causes heart failure. A relatively new poison, used only in bait stations since 1995, cholecalciferol is more expensive than 1080 or cyanide. Not enough is known yet about its potential to cause secondary poisoning. Early tests show that cholecalciferol is less toxic to birds than 1080 or brodifacoum, although its non-target and environmental risks are not yet known.

Brodifacoum and pindone are both anticoagulant poisons, registered in New Zealand in 1991 and 1992 respectively. They act by disrupting blood clotting factors, causing extensive internal haemorrhaging, with death following after two to six weeks for possums. They are incorporated in cereal baits, and are currently used only in bait stations on the mainland. Pindone use has declined because of the large amounts of bait (over two kilograms) that possums can eat without being killed, and is considered more effective at controlling rats and rabbits than possums. Brodifacoum and pindone are particularly useful in areas where possum populations are at low density, such as after aerial 1080 operations.

The major disadvantage of brodifacoum is the persistence of brodifacoum residues after both primary and secondary poisoning. Trials with captive pigs have shown that brodifacoum accumulates in the liver and, to a lesser extent, in muscle. Research has shown that possums retain brodifacoum in the liver and, to a lesser extent, the muscles, for eight months after treatment with near-lethal doses⁵³. While large amounts of brodifacoum-contaminated meat would need to be eaten to cause death, it is important to remember that a much lower sub-lethal dose will produce significant clotting abnormalities and some haemorrhaging. Predatory and scavenging birds such as morepork, weka, southern blackbacked gull and the Australasian harrier are at risk from secondary poisoning. DOC no longer uses brodifacoum widely on the mainland because of concerns about residues accumulating in non-target species.

Since the costs of developing a new poison could be as high as several million dollars, there is a considerable incentive for the smarter use of existing poison products to reduce risks to non-target species and to minimise unwanted environmental effects.

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