Mouse Plague in Australia Prompts Emergency Approval of Stronger Bait

Australian Mouse Plague Intensifies: Emergency Rodenticide Approved for Farmers

"That's real cause for concern. It's a plague under any circumstances."
— Steve Henry, CSIRO expert

A severe mouse plague is devastating agricultural regions in Western Australia and South Australia, causing significant crop damage and prompting the Australian government to approve an emergency permit for a stronger rodenticide.

Extent of the Plague

Reports indicate alarmingly high mouse densities across multiple farming areas.

In Western Australia, the national science agency CSIRO has received reports of 3,000 to 4,000 burrows per hectare in the northern grain belt, including the Morawa region. The CSIRO defines a plague as more than 800 mice per hectare. Some estimates in farming communities in Western Australia reach up to 8,000 mice per hectare.

In South Australia, mouse numbers on the Adelaide Plains and Yorke Peninsula are at their highest in at least four years, with monitoring showing up to 400 mice per hectare. The infestation has spread from farmland into towns, affecting homes and businesses.

Affected Areas

Timeline and Background

Mouse plagues have occurred on Australian grain farms since the 1880s, typically every four to five years following periods of increased rainfall that boost food availability.

• 1872: The first documented plague in Saddleworth, South Australia.
• 1993: A major plague across South Australia and Victoria caused an estimated $96 million in damage.
• 2020-2021: A plague in eastern Australia caused an estimated $1 billion in damage across South Australia, western Victoria, New South Wales, and southern Queensland.

CSIRO expert Steve Henry noted that historically Western Australia had fewer mouse problems, but recent favorable cropping years have increased food availability, creating the conditions for this current crisis.

Regulatory Response

The Problem: Previously, the standard bait available to farmers contained 25 grams of zinc phosphide per kilogram (ZP25). Farmers and the group Grain Producers Australia applied for emergency permits to manufacture and use a double-strength bait (50 grams per kilogram, ZP50) , stating that the standard dosage was not effectively controlling the mice.

The Delay: The Australian Pesticides and Veterinary Medicines Authority (APVMA) initially rejected the application, citing that submitted data was "not of sufficient regulatory quality."

The Science: CSIRO research officer Steve Henry stated that published studies found the 50-gram rate more effective than the 25-gram rate. He also noted that research so far indicates no increased risk of secondary poisoning to birds.

The Approval: Following further lobbying from Grain Producers Australia, supported by the Grains Research and Development Corporation and the CSIRO, the APVMA conditionally approved an emergency permit for the ZP50 bait.

• The permit includes strict controls to reduce risks to people, wildlife, and the environment.
• A key condition is that farmers must be trained and accredited to purchase and use the ZP50 bait.
• Some farmers had already seeded crops with ZP25 bait and may now need to reseed.

Impact on Communities and Supply

Western Australia:
Residents in Morawa report mice in homes, cupboards, and food supplies. Morawa Shire President Karen Chappel stated that trapping alone is insufficient and that paddock control is needed to reduce mice in towns. WA Agriculture Minister Jackie Jarvis expressed disappointment at the initial APVMA delay.

South Australia:
Wilhelm Rural, the state's sole importer of mouse bait ingredients, confirmed that 100 tonnes of bait was sent to Western Australia, raising concerns about potential supply shortages for local farmers. A new shipment of ingredients is not expected in South Australia until the end of May.

Expert Statements

"Farmers need access to higher dose zinc phosphide baits, as the ZP25 strength does not convert to adequate control in the paddock."
— Andrew Weidemann, Grain Producers Australia

"Emergency permits are issued to support farmers while maintaining examination of product safety and impacts."
— Scott Hansen, APVMA CEO

Steve Henry urged farmers to monitor local mouse populations and bait before numbers escalate, preferably at sowing time, to prevent the situation from worsening.