Where are high-risk areas?
Any cereal growing area may be at risk from mice.
- Weed seeds and unharvested or spilt grain provide mice with an abundant food source.
- If the crop from the previous season was exceptionally good, or harvest was inefficient due to shatter, lodging or machinery problems, the possibility of mouse problems within the next crop is increased.
- Heavy early autumn rainfall also raises the probability of future mouse problems by allowing early plant growth (including weeds), which provides mice with additional food and harbourage.
How to assess mouse activity?
On farm monitoring of mice will indicate the extent and severity of the problem.
MOUSE NUMBERS CAN VARY WIDELY BETWEEN AND WITHIN PADDOCKS, SO ALL LANDHOLDERS SHOULD MONITOR THEIR OWN CROPS FOR EMERGING PROBLEMS.
Damage to crops is a frequent, but not continual threat, so at least an annual assessment of mouse activity and risks is needed. A detailed description of the process for identifying mouse activity is available as a pdf file. See http://www.animalcontrol.com.au/pdf/Mouse ID 2005.pdf (196 KB).
Counting mouse holes
In sandy or clay soils it is possible to walk and count mouse holes on a path 1 metre wide by 100 metres long (i.e. 100 m2). Allow for two mice per active hole (the range can be from 0 to 40 mice per hole). If an average of 5 active holes are counted per 100 m2, with 2 mice per hole, then there are about 1,000 mice per hectare (1 ha =10,000 m2). Note: This technique does not work well for cracking clay areas where mouse holes are difficult to define.
Trap lines
Snap-back traps baited with cardboard soaked in canola or linseed oil can be used to estimate mouse presence. Place at least 20 traps at 10 m spacings along lines ('transects'). Check traps early next morning. Repeat trapping every few months, but increase frequency if mouse problems emerge and crop is at risk. If, at any time, more than 10% of traps hold mice there is potential for significant crop damage. Breeding status can also be assessed. If females show evidence of pregnancy or lactation (i.e. extended nipples), the problem may worsen quickly.
Canola squares
New Canola Card
Canola Card showing mouse presence
Peg out at least 20 canola oil soaked paper squares (10 x 10 cm), at 10 m intervals along transects in crops, stubbles and other areas where mice may be living. The amount of paper eaten overnight provides a rough index of the abundance of mice in an area. Download a canola card template in PDF format (64kBytes).
If an average of 5-10% or more of paper is eaten then mice pose a threat. However, the amount eaten may also be influenced by the availability of other food. As mouse density can be patchy between and within paddocks, the more lines of squares put out, the better the indication of potential mouse problems.
Beware - canola squares underestimate mouse numbers if density is high or if alternative food is about. Check for other signs as well (e.g. active burrows and crop damage).
Grain stations
Place a small spoonful of grain under bird-proof covers (e.g. upturned flowerpot with mouse access holes or roofing iron with a stone on top) in a grid pattern or transect throughout the paddock. Measure the loss of grain (eaten by mice) after 1-5 nights. This will give a rough idea of how many mice come to the feeding station and the potential for seed loss, allow ~3 g per mouse per night.
Other indicators and warning signs of mice
Close and regular inspection of crops is needed, including:
- Check for circular patches of dead crop.
- Check for mice after dusk using a spotlight.
- Look for mice on roads at night - beware this can be a very late sign of a serious problem.
- Increased predator birds may mean that mice are about.
- Place sheets of galvanised iron or bags at strategic locations and note mice when lifted.
- Look for mouse tracks in soil e.g. well-worn paths between cracks on cracking soils or between holes in sandy or loam soils.
Distinguishing active from inactive mouse holes
Lightly cover over holes encountered in a transect, marking each to facilitate relocation. Then count the proportion re-opened next day. Talcum powder can also be used to identify active burrows. Sprinkle talc around holes then check next morning for disturbance and tracks. Note: rain and strong wind can mask the mouse tracks and this method does not tell how many mice are present in each active hole.
The zinc phosphide rodent bait MOUSEOFF® ZP should be used in conjunction with other measures to prevent mouse damage to crops.
