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21 May 2021 Expert commentary

Parts of Queensland and New South Wales have seen a rise in mouse numbers due to highly favourable climatic conditions, giving mice optimal conditions to breed.

CSIRO’s mouse research aims to provide better solutions for controlling an increasing prevalence of mice in Australian grain-growing regions, supporting Australian businesses, families and communities. 

In May 2021, new research investment by the Grains Research and Development Corporation (GRDC), led by CSIRO, has enabled evidence-based increases to zinc phosphide (ZnP) mouse baits that will help grain growers battling above-average mouse numbers in eastern Australia. 

CSIRO researcher and leading mouse expert, Mr Steve Henry, comments on the mouse plague:  

Why are mouse numbers reaching such high levels?  

A mouse plague of this sort happens about every 10 years.  

Mice are currently responding to seasonal conditions. There’s been good rainfall after several years of drought and the bumper grain crops grown over spring and summer provide excess food for mice. Shelter and food sources combined create perfect conditions for mice to thrive and survive.  

The mild and moist summer particularly in northern NSW helped mice breed through the summer months through into autumn. Usually, mice breed in early spring and usually finish breeding in autumn. Breeding tails off through winter.  

Mice can breed from about six weeks old and can reproduce every 19 to 21 days. Their litters can have as many as 10 pups. As soon as they have a litter, they become pregnant again. While they are rearing a litter, they are gestating the next. After three weeks of rearing pups, they kick them out of the nest and start raising the second litter. There is no break in pup production. 

Farming practices have also changed over the years. Water conservation and environmentally sustainable methods, such as minimum or zero tillage have resulted in significant increase in both available shelter and alternative food sources for mice in fields.  

Are mice migrating into urban areas?  

Mice are not migratory animals, but they are present across Australia. Mice can move 100m from their nest or burrow to forage, but they will return at the end of the night.  

Mice live everywhere humans do, however, and most of the time go undetected. Recent reports of increased mouse activity in urban areas are due to a localised build up in the system. The following reasons explain why there is more mouse activity than usual in city areas: 

  • Existing populations are increasing because they have access to good food and shelter.  
  • The cooler weather encourages mice to find shelter inside homes, making them more likely to be seen.  
  • Mouse numbers are in abundance following the breeding season and juvenile mice disperse to find other places to live.  

How can primary producers control mice on the land?  

Farmers should use zinc phosphide-coated wheat bait, which is the only registered in-crop rodenticide for the management of mice damage in broad-scale agriculture in Australia. The Australian Pesticides and Veterinary Medicines Authority (APVMA) granted an emergency use permit on 7 May, underpinned by new CSIRO research funded by the Grains Research and Development Corporation.  

The emergency permit allows for grain baits to be covered in double-strength zinc phosphide. The bait will still be applied on-farm at one kilogram per hectare but will have twice as much ZnP on each grain, increasing the likelihood of a mouse consuming a lethal dose in a single feed. 

In addition to the approved rodenticide, farmers are also recommended to: 

  • Try to reduce alternate food sources for mice and introduce livestock to eat excess food in the fields.  
  • Bait regularly and coordinate baiting with other farmers to avoid reinvasion. Bait on as broad a scale as possible.  
  • Lay bait when food source in the paddocks is at the lowest level, this will give mice the best chance to find the bait.  
  • Apply bait while sowing crops.  

How can mice be controlled in urban settings?  

People in urban areas should patch up holes in their home, cracks in the walls, roof spaces and where pipes come through the wall. Pack holes with steel wool or space invader. Mice can squeeze into very small spaces, but they won’t eat through steel wool. Put seals on doors.  

Deny mice access to food sources. Clean up left-over pet food in bowls, bird aviaries and chicken runs.  

Keep grass mown and clean up around the garden. Remove anything that mice can shelter in. Move piles of wood and timber away from your house.  

Use snap traps, which takes away the need for chemicals.  

If using bait outside, pick up and dispose of any dead mice. Unlike agricultural baits, domestic poisons cause secondary poisoning which can harm domesticated and native animals.  

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