Australia's Mouse Plague: What You Need to Know

As people sleep in parts of Australia, mice are hidden in their beds biting their fingers, chewing machinery wiring and hoses, destroying grain crops, and rotting in streets.

Some regions are in the grip of a mouse plague on a scale not seen since 2021.

Millions of rodents have been wreaking havoc for months.

And there's no simple solution in sight.

Here's what you need to know about Australia's mouse plague.

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Why is there a mouse plague?

Mice began breeding extensively after the summer's grain harvest in Western Australia.

Agronomists and farmers in WA began raising the alarm in February, but even then, they were surprised at just how many rodents they were seeing.

Mice can multiply quickly, they breed at six weeks old, can give birth to a litter of up to 10 offspring every 20 days, and can fall pregnant again as soon as they have given birth.

Conditions were perfect for vigorous breeding: there was plenty of food in paddocks after a record harvest and fewer sheep to clean the spilt grain and stomp on their nests.

And then some summer rain fell, providing mice with drinking water and milder temperatures.

Where are the mice?

Mice typically live everywhere humans do, but often in low numbers, so they're not always seen.

When they build to plague levels, there's no escaping them.

Some towns in Western Australia, such as Northampton and Morawa, have mice infestations in their public buildings and homes. Surrounding farms are crawling with swarms of mice.

Community leaders are worried the mice may lead to a disease outbreak, and the Department of Health has issued a public health alert, urging increased hygiene and care.

Farmers in South Australia, Western Australia, and parts of northern NSW are seeing unusually high numbers of mice; some estimates are more than 8,000 mice per hectare.

Citizen science website FeralScan shows reports of mice in every grain-growing state, from west of Brisbane to north of Sydney and close to the Adelaide CBD, along with extensive hotspots in WA.

Why is the standard bait not working?

The standard bait farmers use contains 25 grams per kilogram of a toxin called zinc phosphide.

To die from eating it, a mouse has to eat two or three grains of the poisoned wheat.

In some paddocks, farmers are baiting, but there is a lot of food for the mouse to choose from, so it may only eat one poisoned grain.

One grain means the mouse will just feel sick, it won't die, and farmers worry it may learn not to eat another poisoned grain.

There are also so many mice that while the bait may remove half of a population, the remaining just quickly breed and build up numbers again.

This week, the Australian Pesticides and Veterinary Medicines Authority issued an emergency permit to allow some farmers to spread a double-strength bait on their paddocks.

This is expected to bring the mouse numbers down quickly because it only has to eat one poisoned grain to die.

What's the environmental risk of the stronger farm bait?

CSIRO mouse expert Steve Henry has been studying this.

He says that zinc phosphide doesn't bioaccumulate, with most of the toxin used up by killing the mouse, and what's left dissipates into the environment.

So the chances of an animal eating poisoned mice and then getting sick are considered low.

There is a risk that birds, such as parrots, may eat the poisoned grains in the paddocks, and there is a chance they can be poisoned.

But so far, the CSIRO research is not showing bird deaths as a concern, but there's more work to be done in this space.

The APVMA has imposed strict conditions on the emergency permit to help protect 'non target species'.

What damage are the mice doing?

Grain farmers are busy planting this season's grain crop, but some have described the mice vacuuming up the seeds once they're in the ground.

Crops that do start growing are also vulnerable to baby mice chewing them off.

Some producers will have to re-seed their paddocks, a significant cost during a period of record-high fuel and fertiliser prices.

Mice are also damaging cars and farm machinery, eating wiring and hoses.

They have spread from paddocks into towns, where food-based businesses like supermarkets are inundated. Some have thrown out tens of thousands of dollars' worth of stock.

And they're taking a significant mental toll on people who have been trapping and baiting them for months.

Some communities rely on income from wildflower tourism in winter; it is yet to be seen if travellers change their plans.

How long will this plague last?

Mouse plagues typically end due to a combination of factors, including food shortages, disease and cold weather.

Once farmers spread the more effective baits on their paddocks, they should see a reduction in numbers within hours.

But Mr Henry warns the bait can only be used on farms, not in urban areas, and while it will have an impact, it's not a "silver bullet" for the problem.

Mice may stop moving en masse out of the paddocks into houses and buildings, but they'll still need to be removed from the areas they're inhabiting now.

Residents will need to keep trapping and baiting the vermin.