Scientists warn against using 'super poison' to stop mice plague

Local extinctions of wildlife could follow under a NSW government plan to use a highly toxic super poison to control mouse plagues, scientists warn.

While farmers in regional parts of NSW are at breaking point after eight months of rodent infestation, five leading researchers argue the state's proposal to use toxic poison bromadiolone is not the answer.

Edith Cowan University’s Dr Robert Davis, a critic of the scheme, has first-hand experience with mouse plagues, and believes while the poison will kill some mice, a change in weather is the only thing likely to end the traumatic situation.

A mouse plague has been devastating parts of NSW. Source: Brody Roche via Storyful / Facebook
A mouse plague has been devastating parts of NSW. Source: Brody Roche via Storyful / Facebook

“We've never really successfully controlled amounts of plague, so unfortunately I do have the view that anything that we do is probably unlikely to make a difference on a broad scale,” he told Yahoo News Australia.

Where the poison will be effective he says is clear, and that's in killing wildlife.

“We’re just really concerned we’re going to be starting a local extinction cascade, where we could be wiping out many species of birds and reptiles as a result of this activity,” Dr Davis said.

As mice become sick, they are likely to slow, making them targets for predatory wildlife who then ingest both the pest and the poison.

The more mice they eat, the more bromadiolone they ingest.

A dead barn owl on the pavement.
Predatory birds including owls have been shown to be particularly vulnerable to second generation poisons. Source: Kathy Sproat

“Our work is quite clearly shown that there are likely to be very severe secondary impacts on wildlife,” Dr Davis said.

“That extends from birds of prey, including owls and hawks and kestrels, and our recent work has shown clearly that it can impact things like goannas, snakes and even bob-tailed lizards.

“That's probably just the tip of the iceberg, we’re also finding exposure in mammal species. This is really not a good idea.”

Scientist urges government to use alternative mouse bait

While bromadiolone is often used inside Indonesian palm oil plantations, broadacre use of the chemical in Australia, as is being proposed by NSW Agriculture Minister Adam Marshall, is less common.

His government has acquired 5000 litres, which is enough to treat approximately 95 tonnes of grain.

Ariel vision of a palm oil plantation.
Second generation anticoagulants are commonly used in Indonesian palm oil plantations. Source: Getty

Dr Davis believes there are clear alternatives which are effective in killing mice, but less likely to move up the food-chain.

“Being in a mouse plague is awful and we understand that action is required,” he said.

“The science is in, and in all regards that first generation of rodenticides are effective, and they are appropriate in Australia.”

Ultimately he would like to see investment into long-term control measures including bio-controls and gene-editing.

"That's probably where we need to be thinking rather than broad-scale poisoning," he said.

Horrifying details of wildlife poisoning revealed

His colleague at Edith Cowan University, adjunct lecturer Dr Michael Lohr, has studied the effects that second generation rodenticides have on bookbook owls in Western Australia.

Left - a boobook. Right - a tawny frogmouth.
Research in Western Australia has documented the effects of second generation rodenticides. Source: Getty

Once the poison gets into their system, it blocks the recycling of vitamin K in the liver, and as the bird runs out of its reserves over the next five to eight days, it begins to haemorrhage.

“So internal and external bleeding, that's primarily the mechanism of action,” Dr Lohr said.

“Inside you'll see symptoms where it looks like all of the blood is in the wrong places and none of the blood is in the right places.

“With an animal that's been poisoned, you'll see really pale gums… they'll often have blood in faeces, they'll often have really massive bruising.

“Dissecting some of these animals, you may open them up and find no blood whatsoever, where they’ve bled out from a tiny little cut, or you open them up and it’s bled out internally, so they’re just absolutely full of blood.”

Concern about use near Indigenous communities

Under the government's plan, farmers in regional areas including Wagga Wagga, Walgett, and Moree will receive bromadiolone free of charge as part of a $50 million assistance package.

“We’re making this as easy for farmers as we possibly can," Minister Marshall said in a statement last week.

"No tedious rebate forms to fill out, just bring your grain to have the experts treat it free of charge.

“Free bait is better than any rebate for our farmers, who we continue to stand behind post drought, bushfires and floods.”

Dr Lohr is particularly concerned when second generation rodenticides are used near Indigenous communities who eat goanna as part of a bush tucker diet.

Recent research has found that the large lizards have a high tolerance for the bait, despite the poison continuing to build up in their bodies.

“There's a green dye from the baits that you can actually see in their droppings,” he said.

“That same species of goanna is a really common bush-tucker food for Aboriginal people across the continent.”

High concentrations of the poison accumulate in the liver and fat, parts of the goanna that are often sought after as food.

Non-lethal exposure to the poison in humans, Dr Lohr says is not benign, and this he believes is “really concerning”.

Agriculture Minister Adam Marshall was contacted for comment, but did not respond before deadline.

The author, Michael Dahlstrom, is a registered wildlife carer.

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