SOME growers are unaware of the level of risk or damage from mouse activity in their crops.
Increased mouse activity was anticipated this year due to the volume of grain left behind in paddocks after a record-breaking harvest, however some growers have not actively monitored their crops before or during seeding.
Growers have been encouraged to actively and frequently monitor for mouse activity, especially given the recent detection of significant mouse damage to crops in Merredin and surrounds.
With the Bureau of Meteorology predicting warmer, drier conditions for WA in July, it could be a perfect storm for further extensive mouse damage in the region, as milder weather contributes to more active mouse breeding.
CSIRO research officer Steve Henry said mouse numbers could build rapidly under the right conditions, leading to crop damage throughout the growing season.
"Mice can do damage at all phases of the crop, so if they're taking whole plants out when the crop is being sown, there is a related loss of yield when the crop is harvested," Mr Henry said.
"The key message now is to be vigilant in the late winter/early spring and be prepared to bait if you see signs of mouse activity."
Once mouse numbers are very high, it is very difficult to reduce damage and control strategies can be costly.
Surveillance and extension
The Grains Research and Development Corporation (GRDC) has invested in additional surveillance and extension activities to alert WA growers and advisers of the presence of mouse outbreaks across the State throughout the 2022 season.
A GRDC investment, led by Farmanco, was developed in response to feedback from GRDC's 2021 National Grower Network forums, which showed better mouse management was a priority for WA growers, who were heavily impacted by mouse damage last year.
Data from May showed mouse activity hotspots across all five port zones, with particularly high levels of activity around Coorow, Northam, Jerramungup and Beaumont.
GRDC crop protection manager - west, Georgia Megirian said growers in these regions needed to be especially vigilant over the coming months to avoid mouse infestations and widespread damage to grain crops.
"Adding more surveillance sites across the State, alongside the surveillance conducted by the Great Northern Rural Services in the Geraldton port zone and Ravensthorpe Agricultural Initiative Network in the Ravensthorpe area, has given us an indication of mouse populations," Ms Megirian said.
"This should provide growers and industry information on potential risk of crop damage in their areas from mice at sowing and throughout the growing season.
"However, it is still vital that growers monitor their paddocks to determine if they have a mouse issue and determine whether baiting may be required."
Accurate monitoring and early spring management are key to reducing the risk of mouse damage in developing winter crops.
Growers are encouraged to complete regular checks for signs of active burrows or crop damage.
"In cereals this may be chewing at the node or stem, which causes the head to fall over or from a distance can look like frost damage, whereas in canola and legume crops, growers should be inspecting flowers and pods for damage," Mr Henry said.
"At the first sign of crop damage, growers need to be prepared to bait, preferably with 50 grams per kilogram of zinc phosphide (ZnP) spread at 1kg per hectare.
"The best chance of success is to bait before the milky stage or before pod development as mice may not eat baits if there are lots of high-quality seeds or pods available."
Mr Henry said controlling mouse populations during late winter and early spring would effectively reduce the number of mice when breeding started in September.
Mice build up locally within paddocks and can travel around 100 metres to forage for food.
There are simple in-paddock monitoring techniques, such as mouse chew cards, which can be used when conditions indicate there is a risk of increasing mouse numbers.
"Mouse chew cards are set out overnight and the proportion of the card that has been chewed by mice is recorded when it is collected the following day," Mr Henry said.
"This method is most reliable when alternative food is scarce and particularly handy in bulky crops where active mouse holes may be difficult to find under the canopy."
Chew card measuring 10 centimetres by 10cm can be made using standard printer/ photocopy paper, with a 1cm printed grid.
The cards need to be pre-soaked in canola and/or linseed oil for about 10 to 15 minutes.
Growers should select a few paddocks which are representative of the farm and place the chew cards approximately 30m in from the edge of the paddock by pegging each chew card to the ground in a line of 10 cards spaced at one card every 10m.
Following the furrows makes it easier to find the cards again the next morning and after retrieving the chew cards the following morning, growers should assess for evidence of mouse damage by averaging the damage across the 10 cards.
Currently ZnP is the only rodenticide registered for in-crop use.
"ZnP baits can be laid through aerial or ground application," Mr Henry said.
"Baiting at the time of sowing is most effective for protecting recently sown crops but baiting is also effective for controlling mouse damage during vegetative growth, flowering and seedset."
It is critical to always adhere to label instructions when baiting and be aware of withholding periods.
Bait on the ground is more likely to be taken before mice climb plants to eat developing seed heads and ideally should be applied where there is a forecast for at least three to four dry days.
Mouse bait should be put out before other pest/nutrient treatments as it gives mice the chance to encounter ZnP bait before they discover any other new substances in a paddock and reduces the likelihood of a sub-lethal dose being consumed and therefore bait aversion.
"Do not mix mouse bait with snail/slug bait and do not apply mouse bait with a surface application of urea," Mr Henry said.
"Zinc phosphide can be scraped off the surface of the treated grain when agitated with other substances."
Growers can also co-ordinate baiting strategies with their neighbours for area-wide management and highest impact.
"After baiting, continue to monitor mouse activity," Mr Henry said.
"Re-baiting should occur more than four weeks after the first bait application to overcome bait aversion."
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