• Samantha Schofield

The Mouse Plague: anyone have a really big mouse trap?

Updated: Jun 14, 2021

Since March, the mice plague of 2021 has been causing environmental and economic devastation. Mice have been running rampant around Australia, as if COVID wasn’t enough! So, could we have stopped this happening or are mice unstoppable and evil? For me, I knew mice were bad news ever since Tim Curry voiced the mouse king in Barbie in the Nutcracker…

Mice vs humans

The beginning of the relationship between mouse plagues and Aussies actually began with the native Australian rat (Rattus villosissimus) and some cranky English colonists. But, the real struggle began with the introduction of Mus musculus aka the house mouse. The house mouse was an unintentional introduced pest and arrived with another pest, the first fleet. Usually, the population levels of mice in Australia are low and cause minimal problems. But, mouse plagues are nothing new in Australia. Every 10 years or so, the stars align and conditions become perfect for the mice to breed like crazy.

Why is it so bad?

The reason why it’s so bad at the moment is the result of several factors. Firstly, mice are prolific breeders. Female mice can become pregnant from 6 weeks of age and can produce offspring every 19-21 days with the litter size being 3-12 pups. Mice are also able to breed throughout the entire year. This cocktail of breeding potential can cause rapid increase in the population. Secondly, seasonal conditions play an important role in the breeding rate of the mice. It begins with above average autumn rainfall. Australia has had years of drought and this year has saw rainfall and the subsequent growth of “bumper crops” (which means unusually good harvest). These bumper crops grown over spring and summer allow for excess food for mice. Differences in farm practises, including becoming more environmentally sustainable, have also meant increased shelter and alternative food sources for the mice. The mice are living the high life.

What is the effect on the environment

Mice are causing the biggest harm by chomping through a lot of crops. Usually, they don’t even eat the whole crop. Instead, just taking a bite and ruining the other 90% along with it. When you consider this with the whole plague of mice, you start to see devastation. The Guardian reported on the worst mice plague of 1993 costing up to $96 million in damages. But this isn’t just including crops. Mice chew through not just food, but farm equipment and electrical equipment too, which can cause fires and ruin farming infrastructure. The mice also have the potential to cause contamination and disease, in both humans and animals. When seeking out water, they can fall into water tanks, contaminating the water supply. They also carry salmonella and can infect livestock with pathogens and cause nasty illnesses.

The increase in the mice population also has a flow on effect on the surrounding ecosystem and other animals. As well devouring farm crops, the increased mice population is eating the food source of native species. The increase of mice also means a subsequent increase in predators of small rodents. Basically, this completely throws off the balance of the food web.

So, can we just buy a heap of mouse traps?

The CSIRO have developed this cute lil mouse tracker app that is used my farmers so this plague can be tracked. The farmers use a thing called a Mouse Chew Card that is soaked in oil and left outside. Farmers and the public then know how bad the mice are in that area, depending on how much of the card has been munched away. In the case of actually killing the mice, the farmers use zinc phosphide (ZnP) coated wheat bait. The Australian Pesticides and Veterinary Medicines Authority (APVMA) actually granted an emergency permit to double down on the amount of poison that is able to be used on these baits as of May 7th. This double strength ZnP-coated bait will increase the chance of a mouse being lethally poisoned from just one feed. This is important as research has demonstrated that mice who consume the toxic grain, will develop an aversion and will not consume further toxic grain. Smarty pants. Typically, the winter months help to end these plagues, with the majority of past plagues ending in July. This is because low temperatures stress the population as well as food being scarce. With a high population of mice, infection spreads fast and can take only days to weeks to completely crash the population and end the mice plague. But, researchers from the CSIRO have warned not to rely on the winter to end the plague and stress the importance of the ZnP-coated baits.

Move over ScoMo, the mouse king is here

So, will the urban areas be invaded by little mice soldiers? No. Mice do not migrate and whilst the media can put some rats with arrows pointing to the cities, that is not how ecology works. Mice are small animals and don’t have the capacity to move great distances. But, if there are mice already present in urban areas, those particular populations could be increasing due to conditions being favourable. Increased sightings of rats and mice in urban areas is expected this time of the year due to the cold causing mice to seek shelter. But, no crazy mice ocean will be sweeping through Sydney.

Sources and further reading