Zinc phosphate mice baits should be twice as strong: CSIRO study

At the height of the 2021 mouse plague, ag vet regulatory APVMA passed an emergency permit, allowing farmers to access baits with a double dose of zinc phosphate.

CSIRO researcher Steve Henry said although there were a number of pre-plague reports from people not satisfied with the standard bait, despite multiple applications, it was a 150km detour to talk to a couple of farmers that really piqued his interest.

"One farmer couldn't stand still, he was so frustrated - at one point I thought he was going to grab me and shake me," Mr Henry said, chuckling.

"He said 'I keep putting the bait out, they keep eating it and then keep eating my crop'."

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With funding from the Grains Research and Development Corporation (GRDC), CSIRO researchers began investigating. At first, they believed the mice had become resistant to zinc phosphate after decades of baiting.

However, they found no difference in mice from an area that had been baited for 20 years and those that had never been baited. The team went back to first principles and re-tested the sensitivity of mice to zinc phosphate.

"We found mice were half as sensitive to zinc phosphate as was first reported by the 1980 USA study," Mr Harvey.

But the mis-reported sensitivity wasn't an issue when the bait was first introduced to Australia, Mr Harvey said, because at the time conventional farming systems left little else to eat in paddocks except crops and bait.

"Mice would come across, one, two, three, four grains of bait very quickly and get a lethal dose," Mr Harvey said.

But with the changes in farming practices, paddocks are a "much more complex habitat", with more food and shelter.

"Mice can get one grain of bait, and go a long time without finding the next one, because they're spread three grains per square metre," Mr Henry said.

"If they don't find the next grain before they get sick, they're not gonna take the next one. They become adverse, they get a bellyache and know it was something new they ate, so they avoid eating it again."

Field tests of the double dose baits - which contained 50 grams of zinc phosphate per kilogram - confirmed the theory, killing 80 per cent of the mouse population more than 90pc of the time.

"These results highlighted the importance of every bait grain needing to be a lethal dose as there is no guarantee that mice will find and consume more than one baited grain, and consumption of a sub-lethal dose leads to aversion," Mr Henry said.

The research has been passed on to bait producers and the APVMA for consideration.