Why is Australia suffering from a plague of mice?
AUSTRALIANS SHARE their country with some of the most dangerous predators on Earth. But over the past six months a usually timrous beastie has caused havoc: the tiny house mouse. The government is struggling to contain one of the worst plagues of mice in decades, with the rodents particularly numerous in the south-east of the country. They are ravaging crops, infesting houses and even biting people in their beds. The Economist Today Hand-picked stories, in your inbox A daily email with the best of our journalism Sign up  The blame could be laid at the feet of the British. Two centuries ago Australia was one of the last continental landmasses to be free of Mus musculus, the house mouse. When in 1788 the first fleet of British ships arrived in New South Wales to found a penal colony, they brought both convicts and rodents. The mice quickly spread, causing a headache for farmers trying to cultivate the land. Over the past century, outbreaks of mice have been occurring more frequently, hitting grain-growing regions of southern and eastern Australia hard. One of the worst recorded plagues caused A$64.5m ($43m) worth of damage in 1993, equivalent to A$84m today. The weather is to blame for the latest outbreak. Studies show a tendency for plagues of mice to follow prolonged or severe droughts. One possible explanation is that once conditions improve, the short reproduction cycle of mice means their numbers can rebound much faster than those of their predators. After two years of drought and devastating bushfires, heavy rains this year have allowed grain growers to plant the largest area of winter crop ever recorded, with farmers sowing more than 23m hectares of land. The bumper harvest has created ideal conditions for a mass mouse-breeding season. (The rain also caused Traralgon, a town in Victoria, a south-eastern state, to be blanketed by spiders webs as the arachnids climbed to higher ground.) The result is one of the costliest plagues the country has ever seen. An estimated A$100m in damage has been done to crops and grain stores so far. New South Wales is seeking the federal governments approval to use bromadiolone, a poison that is normally banned. The bait, which can be laced over large tracts of land by drone or tractor, could wipe out the mice en-masse. Critics worry that the poison could find its way into the food chain and harm other animals too. Australias deputy prime minister railed in Parliament against animal-rights activists on June 16th, but admitted that the trouble with that bait [bromadiolone] is that it also does have secondary influences on native birds and other animals, pets around the house and indeed livestock. The Commonwealth Scientific and Industrial Research Organisation, a government research agency, is unsure whether the plague is likely to last for one or two years. If it is the former, the mouse population will drop sharply when the rodents stop breeding over the winter months of June to August and will not recover. So far such a decline has not happened. If the latter, numbers will plateau over the winter and pick up again in the spring when they breed again. In the end, it will probably not be pesticides but a combination of overpopulation, the spread of disease and dwindling food supplies that bring this plague to a halt. Farmers will have to wait until spring to find out whether the rodents will return next year.