​​Swarms of Mice Descend on Australian Farms Again

Mice have taken over the streets in parts of Australia, swarming in large groups that could cause damage to crops, as they have done in the past.

The mice have been seen "swarming all over the road," New South Wales Farmers President Xavier Martin told the Australian Associated Press.

"The worst of it was probably at least half a kilometer [0.3 miles] long," Martin said, having spotted the pests near Coonabarabran, a small town about 270 miles northwest of Sydney. "I came into some big patches where there were hundreds of mice running backwards and forwards in different directions looking for food."

The mice are also being seen elsewhere, Martin said—between Queensland to the north, Adelaide to the west and Victoria to the south. "We've got mice at the moment damaging our sorghum crop and no doubt damaging other farmers' summer crops and fodder," he said. "They're getting into hay sheds, and we've just got to hope we can get on top of them and not let them develop like they did two years ago."

Mice plagues have occurred periodically in Australia since the rodents first arrived on the island with the arrival of British colonists in the late 1700s, according to National Geographic. The only other country to experience such plagues is China.

These plagues are defined as swarms of at least 800 to 1,000 mice per hectare, according to Australia's national science agency, the Commonwealth Scientific and Industrial Research Organization (CSIRO).

"We've got data going back to [the] 1900s essentially showing that there are mouse plagues somewhere in Australia every four or five years, and within any particular area it could be every seven or 10 years," Peter Brown, an ecologist who studies vertebrate pests at CSIRO, told The Scientist in 2021.

The rodents then proceed to gorge themselves on crops, mostly wheat, barley, canola and fodder for animals, costing farmers huge amounts of money.

Swarms of mice have been a massive problem for Australia in previous years, notably during 2020 and 2021, when one of the largest plagues ever seen occurred. Millions of mice descended on farmland in New South Wales, causing hundreds of thousands of dollars of damage to farmers' crops and fodder with their insatiable appetites.

The swarm affected non-farmers, too, biting people in their sleep and drowning in household water tanks. The mice even caused extensive damage to cars and machinery, piling over one another in roiling masses of bodies and biting through wiring. Some people were catching between 500 and 600 mice per night in the traps they had set to stem the tide of rodents, according to Australia's Nine Network, and videos on social media showed the ground seemingly crawling with the creatures.

The 2021 plague was caused by a combination of factors, Australia's ABC News said at the time, including the end of a period of drought, recent heavy rains and ideal weather conditions for the rodents to breed. Predators that died in the drought were present in lower numbers, and the mice thrived, feeding on the crops of the extra-fertile harvest season.

"The prolonged rain and the bumper crop of food means that the rodents have lots to eat for a very long time," Steven Belmain, an ecologist at the University of Greenwich in the U.K., told Scientific American in 2021. "It wasn't a great surprise to many of the experts because they have had these outbreaks going back more than 100 years in Australia. They're periodic, and they are always sort of related to these bumper years of rainfall and wheat production."

Farmers and residents used traps and the rodenticide bromadiolone to attempt to thin the crowds of mice. Eventually, as the food ran out and the defensive measures started working, the swarms diminished.

The mice population has been steadily rising in 2023, according to Australia's 7News. CSIRO researchers have found increasing numbers of the rodents in New South Wales, Western Australia and South Australia.

CSIRO research officer Steve Henry has urged farmers to use chew cards to record mice numbers on their properties and to stay vigilant at baiting the mice to prevent the populations from booming to 2021 levels, 7News reported.

With the encroaching effects of climate change, mice plagues may become more common in Australia because of increased drought conditions and more extreme weather events like intense rainfall, which create conditions that are ideal for a population explosion.

"Are these outbreaks something that we just have to live with? Are they going to become more frequent? You could argue [that] with climate change Australia is going to become much more of a drought-stricken country," Belmain said.

"Or perhaps these rainfall events are going to just be coming through much more severely, which would then drive the outbreaks to become more frequent," he continued. "In some other parts of the world, we definitely know that climate change is going to have a bigger impact, particularly where you have extreme weather events such as cyclones and hurricanes."

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