Double strength zinc phosphide mouse bait on the cusp of another APVMA emergency permit for 2023

Steve Henry, mouse researcher with CSIRO, said trial work done in the past year had shown marked improvement in efficacy using the double strength bait.

He said the trials were made possible while an emergency permit was in place for the Australian Pesticides and Veterinary Medicines Authority, allowing the use of 50 grams a kilo zinc phosphide baits, compared to the official label rate of 25g/kg was in place.

The emergency permit was initially registered with the APVMA by grower representative group Grain Producers Australia in the wake of the devastating mouse plague of 2020-21.

GPA research and development spokesperson Andrew Weidemann said the group was confident the APVMA would renew the emergency permit for 2023 and added GPA would continue to work with CSIRO and the Grains Research and Development Corporation to generate the necessary trial data to push forward to get a permanent registration.

"This will be really good for growers, there is no doubt we have seen the increased efficacy of the double strength baits, but equally it is going to require a willingness to adhere to the compliance rules," Mr Weidemann said.

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"All growers wanting to use the double strength bait will have to complete a short course via the GPA Training hub to be eligible to use the bait," he said.

"It is not time-consuming and will allow growers the chance to really get on top of their mouse numbers."

Mr Henry said the trials conducted this year helped formalise growers' own anecdotal experiences.

"We can now say official trial data shows the double dose works better," he said.

Mr Henry said a number of steps had been put in place to identify whether mice were becoming desensitised to sublethal doses of bait.

"Pleasingly, when doing trials with a farmer on the Yorke Peninsula who had been baiting for 16 years when we worked with nave mice, never exposed to zinc phosphide next to those trapped on the farm there was no difference in efficacy."

"We did the dosing directly into the stomach so there could be no variation and we found no difference between the different groups of mice."

"Confirmation there is no existing sensitivity to the zinc phosphide in the field population is a really good thing."

However, he said trials across the country had found the issue of sublethal doses of bait were an issue.

"The trial work has found that if mice do get a sublethal dose, as can happen with the 25g/kg bait then the aversion rate is really quite high."

"Those taking sub lethal doses, stopped taking the bait, we called it the 'dodgy curry effect', if you eat something that doesn't agree with you, you're put off it and essentially that is what has happened with the mice."

Mr Henry said he wanted to see more work done into baiting efficacy when there were alternative food sources available as can often be the case when farmers look to bait in the autumn.

"We did some work at Parkes this year but it almost replicated an old conventional system as we planted very late and there was very little in the way of alternative food sources.

"What happens there is that mice often get four grains, enough for a lethal dose,

"Now most people are no-tilling and there can be other sources of food which we have to weigh up when making baiting decisions."

In terms of the trial data, Mr Henry said there was a stark difference.

"With the 25g/kg bait against the 50g/kg bait, both applied at the same rates, at 25g/kg about 20 per cent of the time you can kill 80pc of the mice, but you ramp that up to 50g/kg and 80pc of the time you kill 90pc of the mice, which makes a huge difference in trying to run numbers down."

Mr Weidemann said it was important to keep an industry focus on preventing mice breeding up to plague proportions.

"There will be plenty of food for them with grain on the ground after the wet year and the lodging so we need to be vigilant and keep an eye on numbers."