Mouse plague in Eastern Australia in record numbers


JJust before Christmas last year, Julie Leven and her husband Des drove up their motorhome to visit their son in northern New South Wales, Australia. As they drove back to their home in Gilgandra, some 430 kilometers northwest of Sydney, at night, they saw masses of white spots moving across the darkened pavement. They soon discovered that the spots were mice.

When they reached their home, the Levens saw a scene of rodent devastation. Mice had invaded their house in so many that it was uninhabitable. The creatures had eaten their way into the pantry, spoiling all the food they could get. Her feces and pungent urine were spread from one end of the apartment to the other on soft furnishings and bedding. The rodents had even eaten the insulation around the motor cables of two tractors and ruined their harvested hay bales.

The mouse plague in eastern Australia has caused significant damage to crops. Here mice chewed the strings that held bales of hay together.

NSW FARMERS ASSOCIATION

“It was just unreal,” says Julie Leven. The house was in such a state that the two had to sleep in their mobile home and six months later they were still cleaning up the mess.

Eastern Australia is gripped by one of the worst mouse plagues in living memory. Mice have reportedly bitten people in hospital beds, residents of a prison had to be relocated after mice chewed through the facility’s electrical infrastructure, the furry pests are rumored to be roaming the classrooms, and farmers fear crops will damage more than AU $ 1 billion .

Australia is no stranger to outbreaks of this introduced species. “We have data back to back [the] 1900 essentially shows that there is mouse infestation in Australia every four or five years, and in a given area it could be every seven or ten years, ”says Peter Brown, an ecologist who studies vertebrate pests at the Australian National Science Agency, CSIRO. in Canberra. But Brown says he heard from farmers that this is probably the worst mouse plague they have ever experienced.

Food abundance after a lush, wet summer is believed to be the main cause. After several years of intense drought culminating in the devastating 2019-2020 bushfires, eastern Australia experienced heavy rainfall for much of 2020, particularly in agricultural areas. Mice are opportunistic eaters – they will eat almost anything – but grain offers the most tempting premium.

Models use rainfall as one of the key predictors of mouse numbers, Brown says. “Our models essentially rely on our rainfall as a substitute for food supplies, because if it’s a really good year for growing crops, it’s a really good year for all sorts of other things mice eat,” he explains.

Population surveys from September to November 2020 – spring in the southern hemisphere – already indicated a rapidly increasing mouse activity. This is monitored both with traps – specifically by examining captured female mice to look for scars from previous pregnancies that may indicate their fertility – and with mouse chewing cards, small squares of paper soaked in rapeseed oil that researchers nibble on for mice. The more chewed when the cards are collected, the higher the mouse activity at this point.

“Last spring and summer we kept an eye out for what was going on in different regions, and certainly northern and western New South Wales started as hotspots of activity,” says Brown.

The number of mice soared over the summer, causing such damage to crops that the state government allocated A $ 150 million to help farmers cope with the onslaught.

Environmental impact of the invasion

The house mouse (Mus musculus) probably came to Australia with British colonizers in the late 18th century and quickly invaded local ecosystems, causing problems for native species. “One of the threats to native rodents in Australia is actually competition from introduced rodents,” said Emily Roycroft, an evolutionary biologist at the Australian National University in Canberra. “When things are similar in diet and height, it is usually quite difficult for the ecosystem to support more than one species that are similar in that way,” says Roycroft.

There’s not much data on how the new plague is affecting native animal populations, but anecdotally, ecologists looking for native species mostly find house mice in their traps, Roycroft says. “It would make sense if an invasive species reached such a high density that our native species are displaced a little.”

Conversely, the mouse pond could help some of Australia’s predators. Murray cod has been reported to be bloated with mice in the Murray River, and observations of birds of prey during a mouse plague in the 1970s suggested that their numbers were booming thanks to the abundance of food.

The popularity of rodents among predators raises concerns that the widespread use of rodenticides causes accidental poisoning of native species, particularly birds. The New South Wales Environmental Protection Agency recently issued a warning of improper use of poison-coated grain – for example, because it was too close to native bushland – after an investigation into bird death in central New South Wales found some of the deaths by mouse bait.

The main rodenticide used in agriculture is zinc phosphide, which is used to coat grains that have been left out for mice to eat. It turns into deadly phosphine gas in the acidic environment of the stomach, but it dissolves quickly and is unlikely to cause secondary poisoning. This method doesn’t always work, however, especially if the mice are ingesting enough other foods or not eating enough grains to receive a lethal dose of the poison.

A mouse in a wheat stubble

CSIRO

The New South Wales government recently sought approval for farmers to use another poison called bromadiolone, which is a blood thinner similar to those found in household rat poisons. However, that request was denied by the state pesticide agency due to concerns about secondary poisoning of animals that the poisoned mice might eat.

Research is underway on a control method that would use CRISPR-Cas9 to manipulate the genomes of male mice, which would then be released into the wild to reduce mouse populations. The technique, known as X-Shredder, involves developing a CRISPR-Cas9 complex that, when activated, targets and cuts at specific repeated sections of DNA on the X chromosome. But when the CRISPR-Cas9 complex is activated during sperm formation, it only happens in sperm, says Paul Thomas, biochemist and head of the Genome Editing Laboratory at the University of Adelaide. “We’re trying to break down the X chromosome in the male germline, so we’re trying to optimize spermatogenesis so that only the sperm that carry the Y chromosome get into the next generation,” he says. The result would be that all offspring would be male.

Evidence from a similar approach in laboratory studies on mosquitoes suggests that these sex ratio changes, if effective, could lead to population breakdown within 10 generations. The X-Shredder may be passed down for a few generations, but it is equipped with a safety switch to ensure it does not stay in a single lineage beyond this point.

However, this research is still in its infancy, and now the mouse plague continues, although there are signs that it may be losing steam. Historical data suggests that most epidemics end within a year, Brown says, with the occasional rare event lasting two years if mouse populations can survive the winter.

“We went out last week and we made a lot of traps and we caught thousands of mice and we only caught one pregnant female,” he says. Low breeding rates mean the population is aging, he explains, while winter overcrowding and food shortages also lead to disease outbreaks. But while modeling methods are getting much better at predicting plagues, they’re not that good at predicting when those plagues will end.

“Historically, the numbers have usually collapsed in May, June or July,” Brown told The Scientist late last month. “May is over, June is almost over, and given the numbers that have been really high, I’d expect the numbers to crash soon, but it’s just hard to tell when.”

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