Mouse Plague Threatens Agricultural Regions in Western and South Australia
A surge in mouse populations is affecting agricultural regions in Western Australia and South Australia, with reports of numbers reaching levels defined as a plague. The situation is developing as farmers begin seeding, raising concerns about crop damage.
Current Situation
In Western Australia, areas around Geraldton (400 kilometers north of Perth) and the southern wheatbelt are reporting high levels of mouse activity. The Commonwealth Scientific and Industrial Research Organisation (CSIRO) has received reports of 3,000 to 4,000 mouse burrows per hectare in some locations.
CSIRO researcher Steve Henry stated that a population density of more than 800 mice per hectare is classified as a plague. Some farmer reports estimate 8,000 mice per hectare in parts of the northern grain belt.
On the Adelaide Plains and Yorke Peninsula in South Australia, monitoring indicates up to 400 mice per hectare. Grain Producers SA CEO Brad Perry advised farmers to check for signs of activity, noting that even two or three active burrows per 1,000 square meters is a warning sign before sowing. South Australia's unseasonably warm weather may slow breeding only when temperatures cool.
According to the CSIRO forecast, mouse activity in Victoria, New South Wales, and Queensland remains low.
Background and Biology
Mice arrived in Australia with the First Fleet in 1788 and have spread nationwide. The first documented mouse plague in Australia occurred in 1872 in Saddleworth, South Australia.
Biologically, female mice can begin breeding at six weeks of age and can produce 6 to 10 offspring every 19 to 21 days, allowing for rapid population growth.
Historical plagues often follow several years of lower-than-average rainfall followed by rain, which increases food availability. A study indicates that mouse plagues occur in Australia every 4 to 5 years.
The 2020-21 mouse plague in eastern Australia caused an estimated A$1 billion in damage over 11 months, destroying spring crops and farm machinery, and affecting communities across South Australia, western Victoria, New South Wales, and southern Queensland. Previous plagues have also exposed people and pets to pesticides and disease, increased stress, affected livestock health through feed and water contamination, and caused economic, social, and environmental damage.
Control Measures and Regulatory Status
Farmers are primarily using zinc phosphide bait for mouse control. Seeding is currently underway in Western Australia, which is considered the most effective time for baiting.
Bait Strength Dispute
There is a current dispute regarding bait strength standards. Farmers and the group Grain Producers Australia (GPA) have applied for emergency permits to use a double-strength bait (50 grams per kilogram of zinc phosphide), as they report that the standard 25g/kg bait is not providing adequate control.
Farmer John Warr stated that a mouse dies after eating one bait with the higher poison rate but requires at least two baits with the standard rate.
CSIRO researcher Steve Henry stated that CSIRO research has found the 50-gram rate to be more effective than the 25-gram rate, and that the 25-gram bait "doesn't work consistently."
Grain Producers Australia states it received emergency permits for the higher-strength bait in 2021, but subsequent applications have been rejected. The Australian Pesticides and Veterinary Medicines Authority (APVMA) has not approved the current permit application, citing that submitted data "is not of sufficient regulatory quality." GPA is now gathering additional information to submit to the APVMA.
Henry added that studies so far indicate the stronger rate does not increase the risk of secondary poisoning to birds, though research is ongoing. GPA hopes to secure an emergency permit, with a longer-term goal of full registration for the bait.
Bait Supply
Wilhelm Rural, South Australia's sole importer of mouse bait ingredients, confirmed that 100 tonnes of bait has been sent to the Esperance region in Western Australia over the past fortnight. Managing director Brenton Wilhelm expressed concern that this diversion may strain supplies for South Australian farmers.
Wilhelm stated he currently has enough product to treat 500,000 hectares of farmland but expects it could be exhausted by the end of next week. A new shipment of ingredients is not expected in South Australia until the end of May.
Other Control Methods
Other strategies include mouse-proof grain storage, sealing gaps in homes, and homemade mouse-proof fences. Second-generation anticoagulant rodenticides (brodifacoum and bromadiolone) have been found at lethal levels in native owls, reptiles, and quolls. The federal regulator recently banned the retail sale of these products.
Statements from Officials and Experts
Steve Henry (CSIRO Researcher): Urged farmers to monitor local mouse populations and bait before numbers escalate, preferably at sowing time. He noted that mouse plagues have "very profound" psychological impacts because farmers cannot escape them.
Andrew Weidemann (Grain Producers Australia): Stated farmers need access to higher dose zinc phosphide mouse baits, as current ZP25 strength "does not convert to adequate control in the paddock."
Mark Fowler (WA Farmers grains section president): Said, "We have an urgent problem that exists right now."
Agriculture Minister Julie Collins: Stated that farmers are dealing with effects of the Iran war on fuel and fertilizer, and mice will be an additional challenge.