Since 1917, mice plagues have occurred almost every five to 10 years in Australia, continuing to pose a significant threat to local ecosystems, agricultural production, and public health. The latest mice plague is spiraling out of control and could become Australia's worst disaster in decades. Since July 2020, the affected area has spread from New South Wales to southern Queensland and Victoria.
New South Wales’ farmland, homes, grain silos, and even agricultural equipment are all suffered threats. Rats ate everything they could get their hands on, such as crops, grain, electrical insulation, and wooden furniture. They also invaded schools and hospitals, where at least three inpatients and a 15-month-old toddler were bitten by mice.
Mice can typically carry and transmit dozens of pathogens, potentially adding to the health risks of the COVID-19 pandemic. The ubiquitous furry rodents could cause a $1 billion (730 million USD) worth of damage to local agriculture. On June 22nd a mice infestation forced the evacuation of a local prison in New South Wales.
Drought is the main reason. After three consecutive years of drought and hill fires, new South Wales has had a wet and mild year in 2020. According to the Australian Bureau of Meteorology, rainfall in New South Wales was 14 percent above the annual average last year, exceeding the total for 2018 and 2019 combined. Last year, March-May was also the coolest and wettest fall since 2012, with precipitation concentrated in the interior of the state. Rainfall from July to August is more than 10 percent of the local average. The central and southern regions of New South Wales are dominated by farmland. Grain crops such as winter barley and oats are mainly grown in the area, while autumn and winter are the seasons for sowing and rapid growth of crops. According to the Australian Department of Agriculture, with such favorable temperature and precipitation conditions, the state's winter crop area in 2020-2021 will more than double that of 2019-2020 and yield will more than triple, the second-highest in a decade.
However, the mouse came with the harvest. Some farmers have reported to the NSW Farmers' Grains Committee that their summer crops have been eaten or contaminated by rats. According to the survey, 30% of farmers spent 20 ~ 150 thousand Australian dollars to buy rodent control equipment to ensure the winter crop. This places a great financial burden on them. Australia's worst mice infestation was in 1993-1994 when it caused $96 million in damage.
The relationship between climate conditions and rat outbreaks began to be studied in Australia as early as 1977. The researchers found that in Australia's main agricultural regions, New South Wales and Victoria, mice plagues usually occur after more than two years of drought followed by one or two winters and springs with good rainfall. Similar to the pattern shown in historical records, this round of mice plague also occurred in a year of abundant rain between 2017 and 2020, following a series of droughts and wildfires.
Australia's three-year drought began to ease in the second half of 2020 following the formation of La Nina.
Stimulated by rainfall, crop acreage and yields soared. This provides plenty of food for rats that survive droughts and wildfires. A pair of mice can produce as many as 500 offspring in a season, far more than their predators can consume. By contrast, rat predators, which include wild cats, dogs, foxes, owls, snakes and others, have seen their populations reduced by previous droughts and wildfires and require much longer recovery times than rats.
Experts point out that a mild and wet enough summer in 2021 gave plants a longer growing season and longer food for mice. That means there is a good chance that more mice will survive the winter and become a new breeding force next spring. Perhaps the current mice plague is just the beginning, and next year could be worse, but it all depends on what happens in the next few months.
In May, New South Wales approved a $50 million plan for mice infestation areas, which will be used for researching new rodenticide, providing free rat poison for farmers, and offering a $1000 and $500 decoy compensation to small businesses and families respectively.
These compensation policies come too late to residents who have been living under the rat plague for the better part of a year. For mice, such countermeasures are not real solutions and may even cause other environmental impacts.
Zinc phosphide is the main ingredient in the widely distributed mice poison. The advantage of this chemical is that it breaks down naturally and does not cause the accumulation of toxins in the environment. But drops, especially from helicopters, still kill animals other than rats, dealing a second blow to wildlife populations that have not yet recovered from drought and fires.
In New South Wales, there have been reports of groups of birds dying from rat poison in the past few weeks, as well as several endangered native parrot species. On the other hand, it is not clear how effective poison-based extermination will be, especially in the face of an unprecedented rodent disaster.