A land of flooding plagues: Australia’s history of mice and rat irruptions

By Angus Dalton 26 April 2021
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The current mouse plague echoes a relationship between the La Niña weather cycle and Australia’s native rodents that dates back hundreds of thousands of years.

One hundred and sixty years before hordes of mice began terrorising parts of regional Australia in late 2020, a gang of men camping at Cooper Creek slept with their supplies of flour, dried meat, stirrups, saddles and shoes dangling in the air above them. Anything that wasn’t slung from tree branches with ropes was destined for destruction by the voracious swarms of rats that raided camp each night.

The scurvy-prone campers were awaiting the return of their expedition leaders, Robert O’Hara Burke and William John Wills, who had ventured north towards the Gulf of Carpentaria four months earlier in December 1860. Monsoonal rains had the dual effect of slowing the famed explorers’ return to camp and triggering an explosion in populations of the native long-haired rat (Rattus villosissimus), also known as the ‘plague rat’. 

While Indigenous groups such as the Diyari, Yandruwandha and Arabana peoples celebrated and capitalised on the abundant food source, the rats made life hell for colonists. 

“While a vast array of native animals were passive victims of the European invasion of the continent, the long-haired rats fought back,” writes Professor Tim Bonyhady in The Enchantment of the Long-haired Rat. “Possibly no other mammal – not even the dingo – responded so aggressively to Europeans.”

It’s Tim’s belief that the prodigious numbers of the native rats contributed to the death of Burke and Wills. After their men abandoned camp, missing the explorers’ return by mere hours, subsequent search and rescue efforts were stymied.

Expedition member William Brahe was asked why there was no trace to be found of Burke and Wills during an inquiry into the doomed venture. He explained simply: “From the number of rats.”

The sheer number of scampering claws had erased the explorer’s footprints. 

Related: Why we need a better appreciation of our native rodents

From native rats to invasive mice

While fires, floods and COVID-19 border closures have made disrupted travel plans the norm over the last 18 months, during Easter mammal ecologist Dr Alexandra Carthey found a new reason for reconsidered holiday plans: a horrendous mouse plague. 

“We were actually planning to go out to the Macquarie Marshes and stay on a farm, but we were thinking, ‘Oh, if there’s mice everywhere these places are probably gonna have mice in them too’,” says Alexandra.

While the species that plagued Burke and Wills were native rats, the recent swarms of darting dark bodies illuminated by farmer’s torches belong to the house mouse (Mus musculus), whose ancestors were stowaways on the aforementioned rat-terrorised colonists’ ships.

Mouse plagues are usually preceded by years of drought, explains Alexandra. 

“Then you get a drought-breaking rain, and suddenly there’s lots of rainfall. Conditions become really good because everything sprouts and grows, and mice have the biology and social structure that allows them to suddenly ramp up their breeding, extend their breeding season, have bigger litters, and take advantage of those conditions way faster than any other animal can.

“Predator numbers don’t respond as quickly and any diseases don’t take hold quickly enough to slow down the spread. And as soon as they give birth they can become pregnant again. There’s no pause. They just produce on repeat.

“We’re talking about going from less than 50 mice per hectare to thousands of mice per hectare. Often it happens before anyone’s aware they’re increasing.”

The meteorological mouse

Both the historical population booms or “irruptions” of the long-haired rat and modern mouse plagues are phenomenon intimately tied to the La Niña weather cycle. 

“The long-haired rat is among those species which is most responsive to climatic fluctuations,” says Tim. “Extreme wet periods which we now identify with La Niña are a trigger – especially if these periods extend for more than a year.”

Twenty years after the native rats dispatched of Burke and Wills, they returned with a vengeance off the back of a likely La Niña year in 1879. A correspondent in South Australia’s Northern Argus newspaper reported the plague with grave militaristic flair:

“There are rats of every size, rank and denomination, from the grey-headed old general, the leader of a battalion, to the young recruit… They invade every place, respecting no thing and no person.”

In 1940, another La Niña year, the longest and most intense rat plague occurred. It was reported that rats pried open “seven-pound tea tins” on shop shelves, chewed the wool off sheep’s backs, and consumed the trousers of Longreach’s Catholic priest.

As populations of the long-haired rat have declined, the human-dependent house mouse has assumed the position of Australia’s most troublesome plague mammal. Mice are far more grain-driven and human dependent than the long-haired rat, resulting in infestations that concentrate around farms and homes. 

We can expect mouse plagues of the current scale around once every seven years. The last notable plague occurred in 2011 during one of the strongest La Niña cycles on record. That infestation inflicted $200 million of crop damage in autumn alone. 

Now, we’re seeing the cycle repeat again. As the rain falls, ground cover sprouts, farmers’ yields flourish, and the mice start swarming. 

Related: Here are 7 clever Aussie native rodents

Holding out for a cold snap

Dubbo grain farmer Tom Harvey is at the epicentre of the current mouse plague. Videos filmed from around Dubbo began to go viral in January this year. They show thousand-strong swarms of mice dashing across roads and zig-zagging the ground under the torches of residents who braved seeing the nocturnal hordes firsthand.

“It’s virtually impossible to stop them,” says Tom. “We’ve been baiting regularly around our hay sheds. There’s thousands of dead mice but it’s rendered ineffective anyway. There’s just so many coming in behind them.” 

Despite innovations such as bait-bombing drones, Tom’s yet to see any measure that has significantly reduced mouse numbers. One farmer he knows dropped 300 kilograms of bait over one night because “the ground was just moving with mice”. The next morning they used a front loader to gather up the casualties.

“Within 10 minutes they had a pile of dead mice as big as a ute,” says Tom. “And again – that’s not scratching the surface.”

The economic costs of the plague are devastating. Tom has booked up tens of thousands of dollars’ worth of bait for his farm’s winter sowing program – although he’s doubtful it’ll be enough to make a dint in mice populations – and is concerned that hundreds of thousands worth of mice-riddled hay stored away has been rendered worthless. 

Related: The rakali, a native water rat, found feasting on cane toads in the Kimberley

“They burrow into the hay and live in it. They chaff it up and it falls out of the stack and they chew the polypropylene strings that hold the bales together. And they also shit and piss in it which contaminates it and makes the hay unsaleable. It’ll have to be dragged out and perhaps burned.

“It’s quite disappointing when you have such a barren three years and you have a good one and you think right, make hay while the sun shines, and put a bit away for later, only to see it being destroyed before your eyes. I don’t know how you prepare for that. People can’t even stop them getting into their house. How do you stop them getting into a big open shed? I don’t know the solution to that one. Unless you had a moat.”

It’s only the cold snap of winter that will cause mouse populations to permanently crash. A mild autumn has kept the plague brewing and prompted concern for the mental wellbeing of people affected by the months-long plague. 

“I’m standing in the shed here now, where I’ve got a bulk bag of seed and mice have got into it overnight and just torn holes through the bottom of it, grain’s spilling, just making a mess,” says Tom. “They piss and shit all over it and it stinks. Everything you touch has mice piss on it. It’s just horrible. That’s mice for you. Bring on the cold weather.”