Mice alert

The Monthly (Australia) - - THE NATION REVIEWED - MICHAEL DULANEY

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.

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