Farmers on South Australia’s Yorke Peninsula are preparing for a plague
By Michael Dulaney
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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 near-unlimited 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.
“It’s probably a combination of disease and running out of food that leads them to die,” he says, noting that illness spreads quickly when such large populations are living closely in the burrows.
“It’s really quite dramatic. One day they’re everywhere and the next day they’re gone.”
Ominously, for those on the Yorke Peninsula keeping an eye on the fields for signs of mice, the agricultural calendar and rainfall for this season has nearly mirrored those of 1992–93 in some districts. What’s more, the difficulty for everyone looking to pre-emptively strike is that the causes of plagues are still not well understood.
“The problem is you don’t get a mouse plague after every good year – they’re just not that predictable,” Henry says.
Scientists had developed mathematical models that until the 1990s were able to predict mouse outbreaks about 75% of the time by using rain as a surrogate for food availability. But the introduction of no-till farming – where fields are only disturbed once a year, during sowing – and other intensive practices has thrown out the accuracy of the old models, and no one has been able to confirm those of the replacements.
Without this information, farmers only have a few weapons in their arsenal, and fighting a plague is presently akin to being in a constant state of chemical warfare. Sheds across the peninsula contain barrels full of bait labelled “Last Supper”, printed in army-style block lettering as if they were munitions caches. The barrels contain grains of wheat coated with zinc phosphide, the only effective and approved broadacre rodenticide.
Most farmers will spread the bait around paddocks just before seeding commences each year in an effort to suppress mice numbers long enough to get a crop established. Once eaten, the poison interacts with a mouse’s stomach acid and releases toxic gas. A kilogram of this bait has the theoretical potential to kill 22,000 mice per hectare when spread at the prescribed rate. But despite spreading more than enough lethal doses through the fields, the farmers say they can’t ever get on top of it.
Part of the fiendishness of the problem, according to Henry, is that mice will move back into the paddocks from the fringes soon after they’ve been cleared from an area, or a handful will survive in the environment, practically unnoticeable, until the conditions are right.
“It’s the same as most pest animals,” says Henry. “You can push them down to a really low level, but it’s impossible to eradicate them.”
One way this stalemate could be tipped in the farmers’ favour is if models to predict plagues could be updated using real-time, on-the-ground data. For the past few years, as seeding approaches, Henry has visited properties around the grain belt to build a mouse census and introduce farmers to MouseAlert, an online tool he has developed to track mouse populations. The idea is that farmers across Australia report activity on their properties, and this data is used to test new models. But getting busy farmers to use the online tool regularly has so far been the biggest challenge. Whether MouseAlert will clear up some of the mysteries in the battle against mice remains to be seen.
Grain growers have always been adept at dealing with uncertainty, however.
“We still haven’t got our head around when we seem to get a good result and when we don’t, and why that is,” says Schulz. “I think there’s something going on in nature we don’t understand.”