Growers should monitor paddocks to get an accurate idea of their mouse problem as numbers can vary significantly within a district.

Monitor by walking a transect of 100 metres across the crop and counting active holes in a 1-metre-wide strip (gives an area assessed of 100 square metres). Pull back stubble to check closely for mouse activity. Repeat several times across the paddock.

Active holes can be identified by sprinkling talcum powder or corn flour around holes and inspecting the level of disturbance the following morning. One burrow per 100 square metres equals 100 burrows per hectare, or 200 mice per hectare. Burrows usually contain 1 to 4 mice but can have up to 40 mice.

Checks should be made across a paddock as populations can be patchy. Hole counts vary by soil type. In cracking soils, holes may be difficult to identify. In sandy soils, mice may dig many holes in search of seed, which can look similar to nesting burrows. In hard-setting soils, there may be few holes, but each can contain many mice – up to 40 per hole during plagues.

Chew cards

Mouse chew cards - template and instructions are most reliable for gauging numbers in late autumn/winter when food is scarce. They are less reliable as crops mature because the crop provides a more attractive food source than the oil-soaked card.

Using the chew cards:

• Each sampling area requires 10 pieces of strong paper or light card (10cm by 10cm), marked with a 1cm grid and soaked in canola or linseed oil.
• Place the cards randomly across a paddock and peg them to the ground.
• If more than 10 squares per card are eaten overnight, significant mouse populations are emerging.
• If more than 20 squares per card are consumed in immature crops, there is a significant mouse problem.

Economic damage is likely when there are 200 to 300 mice per hectare at sowing (1 to 2 active burrows per 100 m transect) and baiting is critical. Two hundred mice per hectare can eat 1 per cent of the crop sown each night, or 14 per cent in two weeks.

MouseAlert

Growers are encouraged to report and map mouse activity – presence and absence – using the MouseAlert website and via Twitter using @MouseAlert so other growers can see what activity is being observed in their neighbourhood.