Mouse plagues: the terror of Australia

In 1993, south Australia was assaulted with a plague of mice. Sounds ridiculous doesn't it? But when half a billion field mice rise up in the spring to destroy your crops and eat your livestock alive, then no-one's laughing... And this is exactly what happened.

The common field mouse, Apodemus sylvaticus, is the culprit of the regular mouse plagues in Australia. The mouse, originating in Asia, was spread around the world as a stow-a-way on-board merchant trading ships.

The 1993 assault was Australia's largest recorded mouse plague and an estimated 500, 000 tons of wheat was destroyed by the rodents over the plague's 6 month duration, which is enough to feed the entire state of Utah (USA) for 4 years! The mental trauma of the plague and the loss in the livelihood of farmers was enough to drive at least 6 people to suicide and many farmers chose to abandon their livelihood and leave the at-risk areas in the south, for good.

Surprisingly, mouse plagues are fairly common in Australia and on average, happen about once a decade. (Australia suffered its most recent mouse plague last year, although it was nowhere near as large as the one in 1993). They typically erupt after a period of very wet weather that causes a 'bumper' harvest, which provides enough food for the mice to allow their populations to grow to enormous sizes. This, coupled with the fact that field mice living in Australia have very few natural predators, suffer from only a handful of the diseases that are endemic to their European cousins and have a very rampant sex life, means that their populations can grow to ridiculous sizes in a very short space of time. In fact, it has been estimated than one mouse and all of her offspring can produce 3, 000 new mice in a single year: a single pair can have babies every 3 weeks, producing an estimated 500 young in 5 months; babies that can then breed themselves at just 5 weeks old!

A pile of field mice that were poisoned by Australian farmers during the mouse plague of 1917. Although mice plagues are rare elsewhere in the world now, evidence suggests that have blighted humans throughout our history. Aristotle wrote about them 2, 351 years ago and they have even been mentioned in the Bible, in the First Book of Samuel.

As previously said, such large populations of mice are extremely destructive and consequently, farmers go to huge lengths to eradicate them in order to protect their crops and livestock. These techniques range from many simple (but not very effective) 'old-school' traps such as drowning mice in troughs full of water and leaving tarpaulins tied to the ground. During the day, field mice will hide beneath these tarpaulins, a fact that farmers have traditionally taken advantage of, using many inhumane methods like crushing and fire to kill the pests. Poisons are another, much more effective way to kill mice and are now laced over grain during mouse plagues. Poisons are usually sufficient to end mouse plagues, turning previously swarming land into killing fields. In 1993, 35 million mice were killed in just 1 month after poison was used (which reportedly hardly even dented their numbers) and one farmer claimed to be removing 70, 000 dead mice a day from his property.

Even without poisoning the plagues would eventually burn themselves out anyway, as such incredibly high numbers of field mice means that diseases spread throughout their populations like wildfire and they will eventually use up all of their available resources, meaning that they will starve to death. However, this won't be before they cause billions of dollars worth of loss to the Australian economy, so their government must still invest in expensive poisons (which also kill other native wildlife so are a last-resort weapon) in order to control the situation.

Despite being a fairly common occurrence in Australia, mice plagues cause huge amounts of damage and are very hard to control for in advance, mainly due to their unpredictability. They are incredibly damaging to the country's economy, to individual farmers' livelihoods and to local peoples' mental health! So all-in-all, are a real menace. Fortunately though, mouse plagues can be controlled and after human intervention their populations crash, with only an estimated 2 mice out of every 1, 000 surviving the cull...