Shortcuts / 30 June 2021

The Mouse Plague

Mouse plague. They’re chilling words… For parts of Australia, it’s been the reality as thousands of people battle millions of mice in their homes, on their farms, and in their towns – everywhere they go. So in this edition of Squiz Shortcuts, we get acquainted with the house mouse, the scale of the current plague, and just what can be done to stop it.

What critters are we dealing with here…
The house mouse. It’s the rodent most Aussies would be most familiar with and the one that is responsible for this plague and others in our history. 

Where did they come from?
They were introduced to Australia when the First Fleet arrived in 1788, and the little stowaways went where humans went.

Where precisely?
South and eastern Australia, usually in the grain-growing regions. 

I don’t hear much about mouse plagues from overseas…
Good observation. China and Australia are the only 2 nations that battle mouse plagues. It’s down to favourable climates, food and shelter which sustain high mouse populations all the time. 

Lucky us… So why now?
Experts say plagues like this one we’re seeing now tend to follow a drought. So in practice, that means there’s a plague every 5 years, with a super big one every 10-15 years.

How can they survive a drought?
Tough little critters, aren’t they… The house mouse is able to live in Australia’s harsh conditions and then thrive when there’s lots of food and moisture. 

And when you say thrive, you mean plague?
Yes, and that’s down to their epic breeding cycle. It’s something else… 

Oh do go on…
They start breeding at 6-weeks old and give birth to a litter of 6 to 10 pups every 19 to 21 days after that. And after giving birth to one litter, females can immediately fall pregnant with the next. A single pair can produce 500 mice in a breeding season.

That is epic.
Told you.

When/where was Australia’s first mouse plague?
That was Walgett, NSW in 1871.

And the worst?
That title goes to the Toowoomba and Darling Downs region of Queensland in 1996. It caused an estimated $3 billion damage to crops. Then snake populations bred up as they fed on the mice. Deadly brown snakes and mice were both in plague proportions.

That’s possibly the worst thing I’ve ever heard.
Can. You. Imagine. 

So snakes like a mouse-flavoured snack?
Yes, as do other prey like birds, lizards, native carnivores such as quolls, and also introduced species like feral cats and foxes.

And with all those mice around, are people getting sick?
That’s not how mice operate. And sure, you don’t want them around because their urine and faeces pose hygiene and health problems. But of the diseases transmitted to humans by rodents, only a few are transmitted through the house mouse. Also, the fleas which house mice normally carry don’t tend to bite humans so that means when it comes to really bad things associated with rodents like the spread of the bubonic plague, it’s not a big risk. 

Well, that’s something.
Take your wins where you can… 

So this current plague. Where is it?
South Australia, Victoria, Queensland, the ACT and NSW. And it’s the last of those that has it the worst. 

What’s happened?
Since the end of 2020, mice have terrorised communities across the grain belt of western NSW, including Dubbo, Coonamble, Warren, Nyngan and Narrabri, and so many other areas. 

And it’s bad?
It’s considered to be the worst mouse plague the state has seen in decades.

What are the mice going after?
Crops are the main target. They target a wide variety – from cereals to canola, lentils and other pulse crops. And they are getting into them during all stages of crop growth – they are eating out freshly sown seed from the ground right up to climbing up a developed plant to eat the head. 

And I’ve seen pictures of them in silos?
Yep, crops that have been harvested and stored are definitely on their to-do list. And those farmers can’t then sell that grain because, well, mouse faeces is a problem. 

Where else are they?
People have found mice in their homes, ceilings, and rubbish. Mice have overrun schools and interrupted Naplan exams. They are in the stuffing of armchairs, and even biting people in hospital beds. 

So what’s the cost?
NSW Farmers say the mouse plague may wipe $1 billion off the value of winter crops. And that’s just the crop that they would have in the ground now. There’s the loss of what they had in storage, damage to farm machinery, homes and businesses, and what they’re spending on baits and mouse control.

That must be stressful…
There’s significant concern about the mental health of those affected thanks to the financial stress, and the stress of having mice around. All. The. Time. 

So people in those areas must be wondering when it’s going to end?
It would be hard to think of anything else… The breeding cycle is meant to slow in winter but there are concerns this plague could continue into a 2nd year, which would be unusual and devastating.

Why’s that?
This winter has been milder with lots of rain but it’s colder weather that’s needed to help bring on starvation or disease that typically ends a mouse plague. There was a cold snap a few weeks ago, which was good news for these communities, but more is needed.

So why can’t they simply be eradicated? There must be chemicals for that…
Yes, but it’s an expensive and tricky exercise. The main chemical being used at the moment is zinc phosphide.

How does it work exactly?
It turns wheat grain into poison that is scattered around paddocks. Just a single grain is enough to kill an adult mouse. 

And that’s allowed?
At the moment, the Australian Pesticides and Veterinary Medicines Authority has granted emergency permit applications to use it. But it’s knocked back the use of bromadiolone.

Which is…
It’s a chemical that’s been described as “napalm for mice”. The NSW Government made an urgent application for it to be registered for agricultural use, but last week it was knocked back on environmental safety grounds. The concern is for animals – particularly endangered and protected birds – that eat the mice. They could get sick and die.

So in the meantime, we’re sticking with zinc phosphide?
Yup. And the NSW Government has committed $100 million towards rebates for the purchase of the baits. There’s also a real push to get cracking to bring numbers down as much as possible before spring. 

A lot of unhappy campers…
Yup. Governments have been criticised for not acting more quickly to help farmers and regional communities before this crisis point.

So for now, it’s waiting for cold weather and relying on harsh chemicals?
Yes, for now. The CSIRO is looking at models to monitor populations to make predictions about future outbreaks, more effective baiting procedures, and also genetic control technologies. But those are in the future.

And before we finish, what’s the chance of this plague reaching the cities?
Mice only move 100m from their nest or burrow to forage and always return at the end of the night. We like the description from one expert who said mice only weigh about 13 grams and they’ve got very little legs. So it’s unlikely, even if the plague goes on past this winter. 

That’s a relief…
For city-slickers, at least. But for the thousands of Aussies living through it right now, they need it to end.

Squiz recommends:

Mouse Alert app

ABC journo Lucy Thackray’s Twitter account

Image Credit: AP

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