Mice plague eastern Australia in record numbers


Jou before Christmas last year, Julie Leven and her husband Des took their campervan to visit their son in northern New South Wales, Australia. Returning at night to their home in Gilgandra, about 430 kilometers northwest of Sydney, they saw masses of white spots moving across the dark surface of the road. The spots, they soon realized, were mice.

Once home, the Leven saw a scene of rodent devastation. Mice had invaded their house in such numbers that it was unlivable. The creatures had made their way into the pantry and ruined any food they could fit. Their feces and pungent urine were spread throughout the house, through the upholstery and bedding. Rodents had even eaten the insulation around the engine wiring in two tractors and ruined their harvested hay bales.

Mouse plague in eastern Australia has caused extensive crop damage. Here, mice chewed the strings that held the baled hay.


“It was just unreal,” says Julie Leven. The house was in such a bad state that the couple had to sleep in their campervan and are still cleaning up the mess six months later.

Eastern Australia is in the throes of one of the worst mouse plagues in living memory. The mice would have bitten people in hospital beds, the population of a prison had to be moved after mice gnawed at the facility’s electrical infrastructure, furry pests were reported running around classroom, and farmers fear that crop damage will exceed 1 billion Australian dollars.

Australia is no stranger to epidemics of this introduced species. “We have data going back to [the] The 1900s basically show that there are mouse invasions somewhere in Australia every four or five years, and in any particular area it could be every seven or ten years, ”says Peter Brown, a ecologist who studies vertebrate pests at the Australian National Science Agency, CSIRO, in Canberra. But Brown says he’s heard from farmers that this is possibly the worst mouse plague they’ve ever seen.

The main cause is believed to be the abundance of food after a lush, humid summer. After several years of intense drought, culminating with the devastating bushfires of 2019-2020, eastern Australia experienced high rainfall levels for much of 2020, especially in agricultural areas. Mice are opportunistic eaters – they eat almost anything – but grain crops offer the most tempting bounty.

The models use precipitation as one of the main predictors of the number of mice, says Brown. “Our models are basically based on our rainfall as a surrogate for the food supply, because while it’s a very good year for growing, it’s a very good year for all kinds of other things that mice eat,” explains he does.

Population surveys from September to November 2020 – spring in the southern hemisphere – already indicated a rapid increase in activity levels in mice. This is monitored by both trapping – specifically, examining captured female mice for scars left from previous pregnancies, an indication of their fertility – and mouse chew cards, small squares of paper. soaked in canola oil that researchers distribute for mice to munch on. The greater the amount of chewing when the cards are collected, the greater the activity of the mouse at that location.

“In the spring of last year and throughout the summer we were keeping an eye on what was going on in different areas, and the north and west of New South Wales was definitely starting out as a hot spot. activity, ”said Brown.

The number of mice skyrocketed throughout the summer and caused so much damage to crops that the state government committed A $ 150 million to help farmers cope with the attack.

Ecological effects of the invasion

The house mouse (Mus musculus) probably came to Australia with British colonizers in the late 1700s and quickly found its way into local ecosystems, causing problems for native species. “One of the threats to rodents native to Australia is actually competition with introduced rodents,” says Emily Roycroft, evolutionary biologist at Australian National University in Canberra. “When things have similar diets and have similar body sizes, it’s usually quite difficult for the ecosystem to support more than one similar species in this way,” Roycroft explains.

There isn’t a lot of data on how the new plague is affecting native animal populations, but for the record, environmentalists who study native species primarily find house mice in their traps, Roycroft says. “It would make sense that when an invasive species reaches such a high density, our native species would be pushed out a bit. “

Conversely, the overabundance of mice could help some of Australia’s predators. There are reports that Murray’s cod in the Murray River is puffy mouse, and sightings of birds of prey during an invasion of mice in the 1970s suggested that their numbers exploded thanks to the abundance of food.

The popularity of rodents with predators has raised concerns that the widespread use of rodenticides may cause accidental poisoning of native species, especially birds. The New South Wales Environmental Protection Authority recently issued a Warning on the inappropriate use of poison-coated grain – for example, its distribution too close to the native bush – after an investigation into bird deaths in central New South Wales found that some of the deaths were caused by mouse bait.

The main rodenticide used in agriculture is zinc phosphide, which is used to coat the grains left out for mice. It turns into deadly phosphine gas in the acidic environment of the stomach, but this dissipates quickly and is unlikely to cause secondary poisoning. However, this method does not always work, especially if there are a lot of other foods for the mice to eat or if they are not eating enough grain to get a lethal dose of the poison.

A mouse in the wheat stubble


The New South Wales government recently tried to get farmers’ approval to use another poison called bromadiolone, a blood thinner similar to those found in household rat poison. However, this request was rejected by the state pesticide authority due to concerns about secondary poisoning of animals that could eat the poisoned mice.

Research is underway on a control method that would use CRISPR-Cas9 to modify the genomes of male mice which would then be released into the wild to reduce mouse populations. The technique, called X-shredder, involves designing a CRISPR-Cas9 complex that, when activated, targets certain repeated sections of DNA on the X chromosome and cuts those sections. But if the CRISPR-Cas9 complex is activated during sperm formation, it only does so in the sperm, says Paul Thomas, a biochemist and head of the University of Adelaide’s genome editing lab. “We are trying to shred the X chromosome in the male germ line so efficiently that we are looking to alter spermatogenesis so that only sperm carrying the Y chromosome pass to the next generation,” he says. The result would be that all descendants would be males.

Evidence of a similar approach in laboratory studies in mosquitoes suggests that, if effective, these alterations in sex ratios could cause the population to collapse within 10 generations. The X-shredder can be passed down for a few generations, but is designed with a safety switch to ensure that it does not persist in an individual lineage beyond this point.

However, this research is still in its early stages, and during this time the plague of mice continues, although there are signs that it may lose momentum. Historical data suggests that most plagues end within a year, Brown says, with a rare event that can last for two years if mouse populations can survive the winter.

“We were out the other week, doing a lot of trapping, and we were catching thousands of mice, and we only caught one pregnant female,” he says. Low reproductive levels mean the population is aging, he explains, while overcrowding and food shortages in winter also lead to epidemics. But while modeling methods get a lot better at predicting plagues, they aren’t as good at predicting when plagues will end.

“Historically, the numbers have normally collapsed in May, June or July,” Brown said. The scientist at the end of last month. “May is gone, June is almost gone, and given that the numbers have been so high I would expect the numbers to collapse soon, but it’s hard to say when.”

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