Signposting poisonous plants

These two shrubs growing in the Stirling Range in south-west Western Australia contain the poison 1080 (or something chemically very similar) but the sign is coincidental. They both also have leaves arising in pairs, opposite one another, along the stem. That may or may not be coincidental, but it's a useful fact.

Let's start with what 1080. Not only the year when the King of England, William I, refuses to accept Pope Gregory VII as his 'overlord', it's the product number than turned into a common shorthand for a poison we use to kill animals we don't want. The active ingredient of 1080 is sodium fluoroacetate, and in Australia it has been used to kill wild dogs, feral pigs, foxes and rabbits since, well since about when I was born in 1960.

Curiously, I think you'll agree, fluoroacetate (as potassium fluoroacetate) is also found in about 35 plant species native to Australia. These plants have often been helpfully given common names ending in Poison Bush or sometimes just simply Poison. These two are Prickly Poison, Gastrolobium spinosum, and Heart Leaf Poison, Gatrolobium bilobum, I believe,and yes, they have 'opposite leaves'.

I think all the flouroacetate containing plants are legumes. That is, they produce pods like a pea. Most are in the family Fabaceae (the pea flowered ones) but there are some poisonous wattles in the family Mimosaceae (such as Acacia georginae). All or most occur naturally in Western Australia or Northern Territory.

Another of the pea family poisonous plants is the Stirling Range Poison (Gastrolobium leakeana, also sometimes known as Nemcia leakeana), which grows a little higher up on the slopes than this sign.

So Brian (Bully) Bilney, manager and guide at the nearby Stirling Range Retreat (and featuring in the first picture), says that his friend, a local Aboriginal man, says it's easy to pick the poisonous plants in this region. If they opposite leaves, he says, don't eat them.

Simple enough, but is it true and what might it mean? I doubt every plant with opposite leaves is poisonous but then it's probably no hardship to avoid eating them anyway. As to whether there are plants without opposite leaves but also with fluoroacetate, the wattle I mentioned is at least one example. My advice, if you really want it, would be eat lettuce and broccoli mostly and to avoid any native plants unless you are with a local guide.

As to why there should be an association between opposite leaves and this chemical, I imagine that's coincidence. It is possible of course the lineage with opposite leaves evolved from plants with the poison, or vice versa. And there may be some connection chemically, an unintended consequence if you like. But this is all speculation.

Back to 1080, the poison not the year. Animals vary in their sensitivity with dogs and foxes the most susceptible. At the other end of the spectrum are reptiles and fish, which seem to be very resistant.

In between are us and native animals. Apparently it takes a lot to kill a human, but I'd avoid all plants with opposite leaves anyway - particularly their flowers and young shoots. According to my Queensland Government source, 'Australia’s native mammals, birds and reptiles have developed much higher tolerance to 1080 than introduced animals, due to their evolution with naturally occurring 1080 in some native plants'.

In fact there is much local variation in this tolerance. Western Australian bush munchers, such as the local Tammar Wallaby, are apparently partially immune to fluoroacetate while their eastern Australian cousins, not brought up in poison bush country, are not.

The emu, a seed eater, has high resistance when browsing in the south-west. The effect is less pronounced for carnivores, because they are less directly exposed to the toxin (having to eat herbivores first).

And the brushtail possum in south-western Australia is 150 times less susceptible to flouroacetate poisoning than those munching my garden plants in the east. And you can trust this research because the report comes from New Zealand, where they understand what kills possums.

So 1080 is still used to control pest animals, because it tends to kill them more than local and home fauna. As you'll see repeated repeatedly, flouroacetate also doesn't persist long in the environment, whether deposited as a bait or leaching from a native plant.

Images: from my recent trip to Western Australia, except the possum hat which is from the New Zealand Possum Marino site.