Conservation Matters: Toxins for pest control

Much conservation work involves removing pests.

Trapping is a major part of this, but the use of toxins is also common, although far more controversial.

Large-scale pest control, especially in remote, rugged, untrappable country, must be done with toxins. New Zealand leads the world in pest eradication on islands by toxins.

In a perfect world a toxin would be species-specific.

It would only kill the pest we wanted to remove, and it would do it in a quick and humane way. Many current toxins kill a range of species.

Possums and rats, for example, are both killed by brodifacoum, an anticoagulant.

This is probably acceptable, but this toxin will also kill birds, cats, dogs and invertebrates.

Cyanide is commonly used for possum control but will not kill rats. It will kill other species, however, humans included.

A rapid, humane kill is ideal. Some toxins, cyanide for example, act within minutes, but others take much longer to take effect.

Many anticoagulant poisons take days or weeks to work, as the animal must first use up its existing vitamin K (stored in the liver) before blood clotting stops.

Most toxins must be taken within a set period of time to be fatal.

The entire dose must be ingested; a smaller dose may cause the animal to be unwell but not die.

It will then remember this toxin and avoid it, making it no longer useful. Some toxins require multiple feeds over several day.

This can be quite labour-intensive. Poisons must also be palatable, or sub-lethal poisoning will occur.

Once the toxin has killed its intended pest it may then make the carcass another form of bait; this is known as secondary poisoning.

Brodifacoum, for example, will kill a rat, and if a stoat or cat then eats that rat, it too will be poisoned.

This can be good, especially as stoats are so tricky to kill, but it can also be dangerous.

All New Zealand hawks have brodifacoum in their livers (at a sub-lethal level) as a result of this.

Brodifacoum is also very long-lasting in the environment, taking years to break down and damaging multiple levels of the food chain.

Yet it is readily available over the counter, with no real advice given.

Other toxins break down quickly to non-toxic products. A possum killed by cyanide will die quickly from suffocation, but it will not be toxic to anything eating it.

Sodium fluoroacetate will also break down quickly in the environment.

This is a naturally-occurring compound found in tea and many Australian plants.

It is more commonly known as 1080. It will, however, give a secondary kill.

An ideal toxin would also be cheap, have an antidote in case of accidental poisoning, be safe to lay and be readily available.

Hopefully this makes you think about what you are using.