Australia's mouse plague and how to avoid it

Mice are in plague proportions: in homes, storage sheds, cropfields, farm machinery, grain stores -anywhere a potential food source can be found.

The mouse emergency has reached such a critical level the NSW government has announced a $50 million campaign to subside baits and traps and the agricultural pest control zinc phosphide has been doubled in potency for grain producers.

The new permit increases the concentration of zinc phosphide active from 25 grams per kilogram to 50g/kg of mouse bait.

Concerns about a potential coming plague in Canberra have triggered a near-panic among consumers who have cleaned out the shelves of traps and baits in hardware stores, rural stockists and supermarkets. To find out more about why this plague is happening now and what to expect, Australian Community Media spoke to one of the nation's leading experts on the plague, CSIRO researcher and mouse guru Steve Henry, as he racked up thousands of kilometres throughout western NSW advising farmers and townsfolk.

Mouse plagues of this sort generally happen about every 10 years or so.

A moderate mouse outbreak occurred in 2016-17 but this is a magnitude several times larger than that.

"We don't fully understand why this season is as bad as it is," Mr Henry said.

"But what's happened this year is that climatic conditions have been really highly favourable for mouse breeding. There's been lots and lots of food in the system and we've got a really high level of survival of the juveniles.

"We had a really mild, moist summer, particularly in northern NSW and they have just kept on breeding right through the summer and into the autumn, and are breeding still."

Mice start breeding at six weeks. They can have a litter of six to 10 pups every 19-21 days after that.

But the real kicker is that as soon as they have that litter of pups, they fall pregnant again. So while they are rearing the first litter, they are gestating the second litter.

After three weeks of rearing pups, they kick them out of the nest and start raising the second litter and so there's no break in pup production.

"What I've been telling the farmers out here [in western NSW] is that if you think you've got 200 mice per hectare and 100 of those are going to have 6-10 pups, in three weeks you're going to have 600 mice per hectare, and in 12 weeks you're going to have twice as many as that again," Mr Henry said. "So the rate of breeding becomes exponential."

What is being seen in the ACT, Mr Henry says, is a localised "build-up in the system"; a scaled-down version of what's happening out west.

"There are always mice in the system wherever humans are," Mr Henry said.

"So where you get conditions that are favourable for breeding, you get these localised build-up of numbers. So what's happening in Canberra is that they [the mice] are getting to numbers where they are starting to be detected more easily and we're getting a lot more reports now, as it cools down, because as it gets colder, the mice are moving into houses and sheds in an endeavour to find food and shelter, and get out of the weather.

"Yes, the ACT probably has more mice now that normal but they are not moving in [from NSW]; they are building up in number in specific areas and becoming more noticeable."

Farmers are permitted to use different products, such as zinc phosphide, that are not suitable for use around homes.

For small numbers, old-fashioned mouse traps work well provided they are checked regularly, in locations where they can't be accessed by children or animals, and any bait - such as peanut butter or cheese - is refreshed every few days.

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Domestic, off-the-shelf rodent baits are known as a "second generation" anti-coagulants.

"It's important that these [baits] are used appropriately," Mr Henry said.

He said dogs and cats which eat mice poisoned by baits run the risk of also being poisoned. So if householders use bait, it's important they pick up the mice carcasses and dispose of them.

"Prevention is also important; block up all the crevices and gaps around your house - where the pipes come through the walls and up into the roof space - with steel wool. Put door seals on your doors; remove all the food sources from around your house such as any uneaten food in a pet bowl, or any bird seed from the aviary. Also keep your grass mown and move any piles of wood or timber away from your house," he said.

It ends - and can happen very suddenly, as occurred around this time in 2017 - when nature takes a very ugly and morbid course. So don't plan to eat while reading this.

"What happens is that the population reaches a point where they are starting to deplete food," Mr Henry said.

"When they reach such huge numbers, the mice are interacting with each other and they are getting highly stressed. Mice are carrying diseases all the time and that interaction and stress facilitates the spread of disease. Because they are running out of food, they start to turn on the sick and weak ones and eat them. Mice always prey on babies but when there's a lot of mice in the system, they become a food source. When the mother leaves the nest to find food, other mice then move in and eat the babies. And it's at that point that the population crashes away dramatically."

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