What can growers do?

MONITORING – Reacting to mouse plagues was standard practice in the past, however with increases in our understanding of mouse ecology, integrated and preventative management is now possible. Once populations reach plague levels, control efforts and damage minimization becomes more difficult. Plagues are generally considered to be anything above 500 mice per hectare and serious plagues occur when there are >1000 mice per hectare. Be aware however that even numbers as low as 100-200 mice per hectare can still cause significant damage. Signs of an overall increase in mouse activity include: numerous burrows, mouse droppings on soil and plants, large numbers of mice seen at night in paddocks and on roads, occasional day time sightings, signs of digging around seed beds and direct chewing to plants.

Once mouse activity is detected, there are several methods that growers can employ to assess population trends over time in their paddocks:

• Hole counts are an established method of estimating mice densities within paddocks. To do this, mark out a 100 m straight line and walk along this transect counting the number of active holes within a 1 m wide strip (i.e. 100 m² in total). Holding a ‘T’ shaped stick out in front of you is an easy way to measure a continuous 1 m width. Multiplying the number of holes by 200 provides an estimate of mice per hectare, based on 2 mice per hole (which is a conservative estimate). The actual number of mice per hole will differ depending on the availability of refuges or other cover within the paddock, as well as soil type. If you are uncertain about the activity of a hole, sprinkle talcum powder around it and inspect disturbance of the powder the next morning.  

• Census cards typically consist of 10 cm² pieces of light card soaked in canola oil and pegged to the ground randomly across a paddock. Upon checking the cards the next morning, an average of 10% of the card area missing indicates an emerging mouse problem, and 20% or more missing suggests a significant problem may be present. Be aware however that the presence of other food sources may decrease the likelihood of mice chewing on census cards.

• Conventional snap-back traps can also be used in the paddock to gauge mice activity. Traps can be baited with a small piece of cardboard or leather soaked in canola oil, or smeared with peanut butter and set up in transects. Place at least 20 traps at 10 m intervals in a straight line, then check early the next morning. If 10% of traps capture mice, there is potential for significant crop damage.

INTEGRATED MANAGEMENT - Farm hygiene, in particular minimizing the availability of feed through good harvesting techniques and cleaning up around sheds and silos, can go a long way towards preventing a mouse problem. Consideration should also be given to controlling weeds and volunteer plants in autumn before seed-set, and spraying, slashing or burning to remove potential refuges. Natural predators of mice include birds, foxes, feral cats and snakes. Although predators are unlikely to control mice in a plague situation, an increase in predator activity may signal a rise in mice numbers in a paddock.

When mice do pose a threat, growers should aim for early and rapid establishment of strong plants to give crops the best chance of ‘outgrowing’ the mouse threat at establishment. Increasing seeding rates, sowing as deeply as appropriate and cross-harrowing to remove sowing lines can be useful techniques. Dry sowing should be avoided (if possible) as seed could be in the ground for some time before germination, leaving it vulnerable to mice searching for food. Heavily grazing paddocks after harvest will help clean up split grain.

The most immediate method of combating a mouse problem is baiting. There are two rodenticides currently registered for field use. Zinc phosphide applied to sterilised wheat (MouseOff^® ZP) can be applied to broad-acre crops, at a rate of 1kg/ha, which results in an even coverage of 2-3 grains per m², or about 20,000 lethal doses per hectare. This rate is thought to be sufficient in the majority of instances and should provide greater than 90% control. A secondary baiting is only likely to be needed in localised situations. Lower rates should be avoided as trials have shown significantly less control is achieved and re-infiltration is possible. MouseOff®BD (containing 0.05 g/kg bromadiolone) is currently registered for use in domestic and commercial premises around grain storages, farm buildings & animal housing only. However, bromadiolone-based baits (containing 0.05 g/kg bromadiolone) are available only in New South Wales for use around crop perimeters under APVMA permit number PER11331. This permit is in force from 31 March 2009 until 30 June 2011, and specifically requires that baits are prepared by a Livestock Health and Pest Authority (LHPA). If the current bait production rate is sustained and there are no supply-chain limitations, it is expected that ACTA will be able to keep up with the demand for MouseOff^® baits across all affected regions this season.

Growers should consult product labels for instructions before applying baits and recognise that baiting in a single paddock alone may not be effective if mice are problematic in surrounding paddocks. This is because mice are highly mobile, particularly when in search of food. They have been recorded travelling several hundred metres in one night, and much larger distances over several consecutive nights. It is important to understand the potential for reinvasion of mice intro baited areas before assuming a baiting failure. Growers are reminded that the preparation of ‘home-made’ baits is not only illegal, but also potentially dangerous to personnel as well as being harmful to non-target animals.