Millions of mice are wreaking havoc across rural NSW. Will the more recent disaster of floods help curb their onslaught, or will the devastation be experienced south in Victoria? By Justine Landis-Hanley.

The mouse plague in NSW

CSIRO mouse expert Steve Henry with a mouse.
CSIRO mouse expert Steve Henry with a mouse.
Credit: Sharon Watt / GRDC

First came the bushfires, then a global pandemic. Locust outbreaks and a once-in-a-hundred-year flood followed. And if that wasn’t biblical enough, there has been an escalating “mouse plague” in regional New South Wales and southern Queensland.

Since the end of 2020, swarms of mice have been destroying crops and feed, eating their way through pantries and supermarket shelves and soiling people’s homes. In startling videos posted to social media, hundreds of rodents can be seen scurrying across fields, hay bales and floors.

To get a sense of the situation and how it might be managed, The Saturday Paper talked to CSIRO researcher and resident mouse expert Steve Henry, whose work includes monitoring and predicting mouse population outbreaks and helping farmers develop more effective ways of controlling them.

The Saturday Paper Why is this mouse plague happening?

Steve Henry It’s something that happens typically after a run of dry years. Mice are present in farming systems all the time. When you get a break after a run of dry seasons, conditions get favourable for crops, and so farmers grow really good crops and that puts lots of food and shelter into the system. And, of course, those conditions are really favourable for mouse breeding as well … So if you get a high number of animals going on to continue to breed, then that rate of increase starts to get exponential really quickly.

Now the reason that rate of increase is so high is that mice start breeding when they’re six weeks old, and then they can have a litter of six to 10 babies every 19 to 21 days after that … So, while they’re feeding the first litter in the nest, they’re gestating the second litter, and then when the first litter is three weeks old, they boot all those guys out of the nest and have their second litter. And then they fall pregnant again.

TSP So mice are very efficient?

SH They’re remarkably efficient breeders. And if conditions facilitate that high level of juvenile survival, that’s how a lot of those animals are getting through the breeding age. Then you can imagine that the rate of increase is scary.

TSP This past year we’ve had fires, disease, locusts and now floods are ravaging NSW. And then there’s the mice. Should Australians be concerned that something is happening?

SH Mice are an intractable problem. And periodically we get these really big plagues. And, you know, the fact that they just seem to have coincided with a range of these other naturally occurring phenomena. That’s really unfortunate…

TSP The story has been everywhere in the media during the past two weeks. But when did people first start contacting you about the most recent suspected mouse plague?

SH We started to hear about high numbers of mice last October or November. And then numbers have steadily grown from that point. And it was one of those ones where it could have gone in either way, depending on the climatic conditions. But unfortunately, conditions have just been really favourable up through that northern part of the cropping zone.

We’re also starting to hear about higher numbers of mice through the central west of NSW. And just in the last day or two, there are starting to be stories out of northern Victoria about high numbers of mice there as well … At the moment, I wouldn’t call it widespread. But there are patches where there are quite high numbers of mice.

TSP When you say “high numbers of mice”, how many mice is that? For some people, one mouse is too many, but what would you, as a mouse expert, say is a high number?

SH So what we call a “mouse plague”, I’m reluctant to call it a plague because once you get over a certain number of mice, farmers need to be doing something about it anyway, whether it’s a plague or not, it doesn’t matter what you call it…

One of the things that we are trying to work on at the moment is to try to establish, well, just what is the economic impact of mice? And at what point is it worthwhile to farmers to start to control mice? But we would say that if you had over 200 mice per hectare, that would be cause for concern … [and] we would classify an outbreak as around 800 mice per hectare.

TSP And is that what we’re seeing now?

SH Certainly in parts of NSW, it’s through the roof.

TSP More than 800 mice per hectare?

SH When you see the ground moving, it’s millions of mice.

TSP There have been frequent stories from NSW of mice destroying crops, chewing through furniture, destroying electrical wires, even biting hospital patients. Aside from being inconvenient, what are the biggest risks for people about this outbreak?

SH I think the major concern at the moment for farmers is finding a way to protect that winter crop. Because we don’t want them to lose the winter crop as well as the summer crop … But in terms of the social impact of mice, people in rural communities are exhausted. Every day, they’re dealing with mice in their houses. And that’s psychologically wearing.

Some people are taking 100 mice out of their house by midnight. You can imagine if … every time you go to get clean sheets, you had to wash them before you put them on the bed because they’ve been fouled by mice. Every day you wake up to the smell of mice, go to bed with the smell of mice. And some of the rural communities are having trouble with the disposal of these mice because they’re starting to make the rubbish bins in the towns stink. And, like I say, people in the cities go, “Oh, my goodness, I’ve got a mouse in my house. This is a disaster.” Well, yeah, it’s tough going, but it’s really tough going out in the bush for these people.

TSP On to what you said about farmers who are getting more than 200 mice per hectare and need to handle the situation. Is there a humane way to “take care” of the situation?

SH That’s a really important angle to the story, I think. Because while these animals are pests, in the same way that [feral] cats and foxes and rabbits are pests, and they need to be controlled, we need to do that in the most humane way that we possibly can. And that involves using either the traps that are commercially available through the hardware stores, and around houses using those toxins that are commercially available, because they’re deemed to be humane.

Then for farmers, the only thing that they’re allowed to use is a product called zinc phosphide ... Because basically, it’s only mice that are out in these paddocks that eat the zinc phosphide and it kills the mice relatively quickly.

TSP Have the floods in NSW slowed the mice down at all?

SH It’s going to be really difficult to tell what effect this rainfall has had. And certainly, if it has flooded burrows to the extent that we’re hoping it has, then that could have really had a significant impact on those [mouse] populations.

TSP You mentioned there have been reports from northern Victoria about mice sightings there, and there have been headlines saying the mice might be migrating south. Should the state be bracing for something like what we’ve seen in NSW?

SH Okay, so we need to get away from the concept of mice migrating. Mice are present all the time and a lot of the time they’re there in undetectable numbers. What’s happening now is that those populations that are there are starting to get bigger and bigger and bigger because conditions are favourable.

They’ve continued to breed from early spring through the summer, and they are continuing to breed into the autumn now, so there’s a high level of survival. And so, all of these animals are starting to become more apparent in northern Victoria …

The other thing that I should have said about mouse plagues is that at the end of the mouse plague, the mice disappear almost overnight.

TSP Interesting. Why is that?

SH What happens is that, when mouse numbers get so high that they are all zooming around and bumping into each other and all interacting with each other, that high level of interaction facilitates the spread of disease.

But also, at that point, they are running out of food. And – this is the horror story part – as they run out of food, they start to turn on the sick and weak ones. They start eating them. And they also turn on the babies as well. Survival is the absolute driver … So, the population just crashes.

TSP It sounds very Lord of the Flies.

SH They are real survivors. We have these terrible, terrible droughts where you can’t imagine anything surviving, and then it starts raining at the end of the drought and – bing! – we’ve got a mouse outbreak. They are well adapted to survive in these systems.

This article was first published in the print edition of The Saturday Paper on March 27, 2021 as "Of mice and men".

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