Farmers on South Australia’s Yorke Peninsula have had good reason to celebrate their efforts this year. A wet and productive growing season has ended with the largest winter grain harvest ever recorded in the state. But beneath the fertile soil, there’s trouble brewing: the same bumper crop that filled bank accounts and grain bins could also be the catalyst for one of the largest mice outbreaks for decades.
The common house mouse, Mus musculus domesticus, might be found in pantries and laboratories around the world, but it is only in Australia’s south-east grain belt that it has found the perfect environment to unleash its capacity for prolific breeding. The area, stretching from South Australia to south-east Queensland, provides the rodents with mild winters, no native competitors and a nearunlimited food supply.
Normally, mice live in the fields at virtually undetectable levels – around five to ten per hectare – but given the right conditions they erupt to levels of 1000 per hectare. Females will deliver a litter of five to ten young mice roughly every 19 days, and rear one lot of infants while gestating the next. Consequently, it can take a mere 21 weeks for one breeding pair and their offspring to produce 500 mice. All of this means that, during a plague, mice go from basically invisible to omnipresent almost overnight. A photo from 1917 shows a group of men in three-piece suits standing triumphantly near a pile of 500,000 dead mice killed over a four-night period in north-western Victoria.
In an average year, mice cost the grain belt about $36 million, mostly through lost crops, and damaged property and equipment. This figure increases with the more widespread plagues, which occur roughly every five to ten years. And then there are the extreme outbreaks: the 1992–93 season, when mice caused an estimated $108 million damage, is still used as a benchmark for what can be visited on the grain-growing districts. The impact is best summed up in a home video from the peak of the plague that shows a farmer throwing open the door to her piggery, unleashing a living wave of mice that rolls out for minutes on end.
Such an infestation brings with it not only a high economic cost but also social and emotional stress. When the population is at its highest, tracks linking thousands of active burrows crisscross the paddocks. The roads are so covered by mice that clear patches are easily distinguishable by changes in the tyre sound. People stuff steel wool in every crack and hole in the house and still have mice hopping over them in bed at night. The problem gets so bad that parents put the legs of their babies’ cots in buckets of water to stop mice climbing inside.
“There’s been at least a couple of occasions where for three, four, five, six months they just have been really bad through the yard, the sheds, the houses,” says Dylan Schulz, who has been farming near Port Victoria on Yorke Peninsula’s west coast for about 20 years. “You just drive around the yard and in the paddocks at night, and it’s ridiculous.”
The frenzy of activity usually ends in a matter of days, and locals still recall seeing paddocks littered with dead mice. Steve Henry, a CSIRO researcher who specialises in agricultural pests, says this rapid population crash is typical.