THERE have been reports of an increase in mice in properties across the State, especially due to the increase in onfarm residues.
A recent study of the 2021 harvest found about $300 million worth of grain was left on the ground, which is heaven for mice.
A difficult harvest has led to significant amounts of grain being left in paddocks, providing optimal conditions for mice across southern cropping systems in the lead-up to sowing the 2023 crop, said CSIRO research officer Steve Henry.
These conditions have highlighted the importance of having effective control strategies for mice to minimise the negative impact they have at different stages of the crop.
Recent research by Mr Henry found that each bait grain needs to carry a lethal dose of zinc phosphide (50 grams) in order to ensure effective control.
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Following numerous reports from growers that baits were not working as expected, CSIRO with investment from Grains Research and Development Corporation (GRDC), has embarked on a series of experiments to investigate potential reasons for the lack of effectiveness of the baits.
"It is really important to ask questions about why things aren't working the way they should," Mr Henry said.
"It is critical to have a control technique for mice that is effective."
The initial laboratory trial was carried out to identify potential new bait substrates that might be more attractive to mice.
"The thinking behind this experiment was if I am a mouse living in a barley stubble, why would I transition away from safely eating barley to eating wheat with zinc phosphide on it?" he said.
"When we identified a preferred food and offered it coated with zinc phosphide to mice in the lab we found that even though most of the mice in the study consumed the bait, we only killed half the number of mice that we expected to kill."
Another key finding of this work was that mice that consumed a sub-lethal dose (ate the toxin and lived) stopped eating the bait.
A subsequent laboratory study undertaken by CSIRO, which investigated the sensitivity of mice to zinc phosphide, showed that mice had not become less sensitive due to frequent exposure to the toxin over the past 20 years of use, but they were less sensitive than had been reported in trials undertaken in the United State in the late 1980s.
The key result of this work is that it takes two milligrams of zinc phosphide to kill a 15g mouse, not the 1g that was previously reported in the 1980s.
The results of this work have led to the provision of permits by the Australian Pesticides and Veterinary Medicines Authority to manufacture baits mixed at 50g of zinc phosphide per kilogram of wheat, instead of 25g of zinc phosphide per kilogram.
After two mammoth harvests, Australia has begun to struggle with the pests, even though Western Australia traditionally hasn't struggled with mice as much as their eastern counterparts.
Mr Henry said the increase in mice was a combination of increased food, moisture and shelter.
He said it was surprising that some towns, such as Dandaragan, were experiencing a mice problem now given the dry summer.
"Obviously there has been enough food and shelter, and probably enough stored moisture in the soil profile," Mr Henry said.
The Eastern States are experiencing patchy outbreaks across paddocks, and Mr Henry said it was important to bait with a paddock focus - rather than a blanket coverage.
"It's the paddocks that had significant loss where they are most likely to be," he said.
Now is the time to bait mice, either prior to sowing or as they sow, especially with cereals.
If farmers don't bait them during seeding, Mr Henry said mice would dig down each furrow and eat the freshly sown grain seeds.
"You'll just see these little lines of holes along the farrow line where they've dug down directly above and plucked them out," he said.
"That does happen with canola, it's really hard to imagine that they could find those tiny little canola seeds, but they do manage to find them."
Mr Henry has heard of farms where mice have gone through and devastated the canola crop just as it has germinated, at the two-leaf stage.
Especially when canola and cereal crops were worth a lot of money last year, every bit of damage had a large impact on farmers.
Mr Henry said not being able to escape the mice had a large psychological impact on farmers, which hadn't been well researched.
