Meet the ferals: House Mouse

Warning: this post contains images of dead mice…lots of dead mice.

Primary school children handling mice on open day, Wangaratta, 1970. Source: State Library Victoria

The European world referred to Australia as either New Holland or Terra Australis across the 17th to 19th centuries. When ships arrived from countries such as Britain or the Netherlands they transferred people, equipment and goods ashore. Somewhere amongst those items transferred from ship to shore were mice. The House Mouse, Mus musculus, may be one of the world’s smallest mammals, but its impact on the Australian landscape, ecology and agricultural industries has been immeasurably disastrous.

At the time that this blog post is published, regions within Eastern Australia are experiencing a severe mice plague. Many households experience the odd mouse or evidence of, but this plague is somewhat more than that. The scenes and incidents are what you would normally expect to see in a CGI-fuelled Hollywood disaster movie. This is not the first time that a mouse plague such as this has occurred in Australia and it won’t be the last. This small, seemingly harmless rodent has caused much immeasurable damage directly and indirectly with each outbreak. So what causes mice to become a plague? It’s a series of events combined with the ability for mice to multiply rapidly.

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The 2020/21 grain harvest season is expected to be the second highest on record across Australia. While this is indeed welcome news for farmers in this fickle farming country, it also meant an abundance of food for mice. The more food for mice, the more that are able to survive and the more that are able to breed and multiply. Last Summer was also rather mild throughout much of the country which enabled plentiful food and good breeding conditions which continued into Autumn and Winter.

To say that mice are efficient at breeding is a gross understatement. A female mouse can have up to 9 litters of 4-6 pups within a year. And that female can begin mating as early as 4 weeks’ old, but typically 6-8 weeks. While a mouse may only live for 12-18 months, a female can give birth to approximately 50 pups within that lifetime. And if we estimate that half of those pups are females that’s another 25 to contribute to the next generation of the mice army. If you repeat that equation enough you can imagine how a plague happens.

Many individual farmers put their financial losses at $AU125,000 or more. The fear at the moment is that conditions may be favourable enough for the mice to continue at this rate into next year. Aside from losses from last year’s crops farmers are unable or unwilling to sow what they should be this year – there’s no point planting seeds that mice will eat before it can grow.

It’s not just farmers who have been affected by the mice. A recent casualty of the plague is Wellington Prison in Western New South Wales. The mice have eaten through electrical cables causing a fire hazard. Their presence is also a health hazard due to their ability to attract mites and disease after they die and decay. The mice have been at the prison since late last year. But their numbers are now so great that they have forced the relocation of prisoners while building remediation works are carried out.

So what to do with the mice? What normally happens is that the mice eat food stocks until there’s not enough to spread amongst their population and there is mass starvation. To attempt to expedite the demise of the mice there is also the option of poison baits, which brings another set of problems. Farmers typically use Zinc Phosphide poison to poison mice throughout their paddocks. In May 2021 the New South Wales State Government applied to the Australian Pesticides and Veterinary Medicines Authority to allow use of the chemical Bromadiolone for agricultural use. The poison has been referred to as ‘Napalm for mice’ and is banned for agricultural use in most countries around the world. The application was declined due to the high potential for secondary poisoning. Birds of prey such as owl species who typically eat small mammals would also ingest the poison that remains in a mouse they may eat, subsequently killing the predator. Snakes and large reptiles such as Goannas are also potential victims for secondary poisoning. Secondary poisoning also occurs with Zinc Phosphide and other poisons, but they are considered faster acting and limit the travel area of affected mice.

There is even the possibility of humans becoming involved in the poison food chain. In another scene reminiscent of a bad horror movie, a popular fishing target has been found to have been eating some of the mice!

The great plague of 1917

This is not the first time that Australia has experienced a mouse apocalypse. There have been dozens of outbreaks of plagues since the arrival of Mus musculus. One of the more notable occurred in 1917 throughout the states of Queensland, New South Wales, Victoria, South Australia and Western Australia.

The below photographic series outlines the response at the time, using what was known as the ‘double-fence’ system. The mice were corralled down a fenced channel into a pit where they would be poisoned or gassed…in great numbers. Photos have been sourced from the State Library Victoria’s online database.

The mice plague was daily news. An article published in The Argus newspaper on 31st March, 1917 was headlined ‘MICE PLAGUE: MILLIONS SLAUGHTERED – NUMBERS NOT DECREASING’.

Mice are now being slain in millions as the result of the campaign for the protection of the wheat stacks. The biggest catch for one night at a single stack so far has been 30,000 mice at the Lascelles Station. At the Marnoo Station the figures for the first three nights were 8,000, 10,000, 12,000. It looked as if the numbers caught were going to rise indefinitely, but after the third day the haul settled down to about 10,000 a night. Banyena and other stations on the Lubeck-Marnoo line show results similar to those at Marnoo.

So far the system adopted of fencing in the stacks, with traps at intervals, has worked well judging by the number of mice caught, but it seems to make little difference in their numbers. At Wahring, bisulphide of carbon proved effective in driving the mice out of the stack, but most of them were only dazed and recovered afterwards, though some died.

In the Goulburn Valley and the North-East the wheat is being rapidly moved away. In the Western district, to the south of the Adelaide line, the mice trouble is not serious, and the same is true of the district north of Bendigo as far as Kerang and Cohuna.

In the districts most affected, such as the Wimmera and parts of the Mallee, the whole country is overrun with mice. The more adventurous mice are coming down to the seaboard. Sometimes when railway trucks full of wheat arrive at Geelong the mice jump out in dozens. Up in the Mallee the mice are now invading the houses, and doing much damage. At Donald a travelling representative of the Wheat Commission found in the morning that a mouse had made a nest under his pillow, and it is said that the mice also eat holes in the bed clothes. A conference of inspectors and others is to be held on Monday to consider the results of the measures taken to protect the wheat stacks and the further action to be taken.

Although the 1917 plague happened over 100 years ago the mice problem is no different. It requires labour, resources and a lot of money to be thrown at the problem. Collectively the farmers of New South Wales estimate that the current plague has cost over $1 Billion already. That state’s government has already committed $150 million to combat the mice.

For a small, seemingly cute, innocent mammal that weighs 20 grams the mouse is a perfect example of what can be achieved when a species has such a weight of numbers. Nobody knows how many mice first set foot on Australia, but this tiny rodent has caused immeasurable damage and will continue to do so.

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