The common house mouse (Mus musculus) is an introduced species to Australia. Their ability to live on a wide range of foodstuffs, including most of those eaten by humans, and their ability to adapt to a wide variety of environmental conditions has earned mice the reputation of being a pest.
Mice are always present under field conditions, usually in low numbers, and have usually been confined to refuge areas where they are not easily detected. New farming systems, which sow crops every year and retain crop residues (stubbles), are also likely to be providing ongoing habitat for mice. At low densities, mice live largely on the seeds of native grasses or weeds and make virtually no impact on crops or stored produce. However, a favourable combination of environmental and population factors can lead to a sudden growth in the mouse population.
In Australia, mice living under field conditions have a seasonal pattern of breeding. Breeding generally begins in early spring and continues until cold or wet weather develops in late autumn. The breeding period may be shortened for mice living under unfavourable seasonal conditions or extended for mice with nests in the warmth of buildings or haystacks. A combination of high numbers of offspring per litter and high survival rates of young mice can build up the population rapidly. Thus, if food supply, soil type and soil moisture are favourable, the population can increase to plague proportions within a short time.
Mice are mainly active at night and usually confine their movements to areas where adequate cover is available. They tend to use the same path when moving from refuge to feed source. Within buildings, these movements are often confined to walls or natural barriers, beams or pipes, and result in the formation of smear marks. In the field, distinct paths are formed through vegetation.
It is difficult to keep mice out of buildings and storage facilities because of their ability to swim, dig, jump up to at least 300 mm, jump down at least 2.5 metres without injury and squeeze through openings as small as 8 mm wide. In addition, they can run up most rough surfaces, run down ropes and electric wires, and climb upside down along wire mesh.
Adult mice eat between 3 and 5 grams daily of a wide range of foods. In food storage areas their diet can include cereals, vegetables, meat, fish, bread, biscuits, peanuts, cheese, rolled oats, rice, wheat germ, apple and melon. Mice living in field conditions can survive on the seeds of the native grasses and grains.
Mice will sample all foodstuffs available within their range and may not return to a particular feed type for several days. This information is important when determining the type of material to be used as bait, where to place the bait and how often to change the type of bait. Mice can live and breed without drinking water if their diet has a moisture content of at least 15 per cent. This moisture can be contained in the leaves, stems or roots of plants and may be available as dew. If mice live in dry conditions near buildings they need water to survive, so their activity can be limited by cutting off their access to water.
Mice are well recognised for invading households, poultry runs and buildings where they consume and foul food sources and chew insulation, electrical wiring, vehicle parts and all manner of infrastructure.
Mice can also cause damage in crop paddocks immediately after sowing by digging into loose soil to find larger seeds such as maize, sunflower, wheat, oats, barley, pulses, pumpkin and marrow. They also eat the newly sprouted seedlings before and after they emerge from the soil.
The impact of mice is not as great on plants beyond the seedling stage, at least not until seeds or grains begin to mature. Plants such as wheat are then damaged by mice gnawing at the nodes on the stems, causing developing seed heads to fall.
In maturing crops of wheat, oats, barley, pulses, sorghum and maize, losses of up to 30 per cent have been reported. Heavy losses can also occur in vineyards and vegetable crops from eating and fouling of produce.
Before committing resources to mouse control, it is important to confirm mouse numbers present warrant the control cost. It is generally recognised that 200 mice per hectare can cause significant economic damage. Around 200 mice/ha eat the equivalent of one sheep. A mouse population density of 1000/ha has the potential to eat 5 per cent of freshly sown crop seed per night.
Observation of mice activity includes mice on roads at night, spotlight monitoring or activity around buildings and silos will give an indication of populations that are higher than normal.
Cards or paper squares 10 cm × 10 cm soaked in canola oil can be pegged out overnight at 10-metre intervals across a transect and observed for the level of feeding damage. Five per cent or more eaten indicates significant mouse activity and baiting may be required.
A more accurate and quicker assessment can be achieved by walking a series of transects across a paddock and counting the number of active burrows. An average of several transects will increase the accuracy of monitoring.
Walk in a straight line for 100 metres and count the number of active burrows in a one-metre width (burrows /100 m²). A fair assumption is that each active burrow contains two mice. If one active burrow is observed per 100 m, this equates to two mice per 100 m² or 200 mice per hectare, enough to cause economic damage. Five active burrows per 100 m equates to 1000 mice/ha.
Care should be taken to only count a one-metre width, as counting too far each side of the transect will produce an overestimate of the population. Ensure you only count active burrows. If in doubt, spread talcum powder around a series of burrows and return the next day. Active burrows will be those where mouse activity has established tracks through the talcum powder overnight. With close inspection and experience, these tracks will be recognisable in the loose, scratched soil at the entrance to burrows.
Once a widespread mouse plague has developed, it is unlikely that any eradication program will have much effect on mouse numbers. If control measures are to be at all effective, they need to be applied as early as possible in the breeding season and should aim at reducing the ability of mice to live and breed in areas near crops, storage facilities and buildings.
In the field, mice harbour in remnant vegetation, along fence lines, dam banks and water easements. Clean up these areas, the earlier the better, by burning, spraying new growth, heavy grazing or ploughing. Slashing is less useful because it leaves a mat of material that mice can use for cover and nesting.
Recent observations in cropping paddocks clearly indicate that grain lost at or prior to harvest encourages mice and an efficient harvest technique is very important to limit mice breeding. Observations also suggest that mice are less prevalent where the stubble is either cut short or mulched soon after harvest and that grazing also limits mice breeding. Grazing clearly competes for food supply and the shorter stubble reduces habitat, most likely exposing mice to predators.
A commercial mouse bait is available for treating cropping paddocks. The active ingredient is zinc phosphide on sterilised wheat. An application rate of 1 kg/ha, or 20,000 lethal doses/ha, can give a control level of typically greater than 90 per cent per baited paddock. Zinc phosphide mouse bait is registered for both aerial application and ground application over cropping paddocks using a variety of properly calibrated small-seed applicators.
Zinc phosphide products are classified as Schedule 7 (S7) DANGEROUS POISON products, so anyone wishing to use them in Victoria must hold an Agricultural Chemical Users Permit (ACUP) for private use, or a Commercial Operator Licence (COL) with a vertebrate pest endorsement for contract use, or an Agricultural Aircraft Operator Licence (AAOL) for aerial application. To apply for or renew a chemical permit, go to the Agriculture Victoria Agricultural chemical control areas and permits web page.
There is a low off-target risk to other animals if the bait is applied at the specified rate and kept away from uncultivated bare ground and native vegetation.
If baiting is undertaken prior to sowing, monitoring should continue through the season to ensure follow-up control is undertaken if warranted.
Warning: Rodenticides are potentially dangerous to humans, domestic animals and wildlife if misused. Store and use them where children are least likely to find them. Read labels carefully and use only as directed. The use of chemical products not registered for the control of mice is illegal and may have serious consequences for quality assurance in the grains industry as well as causing off-target damage.
Predators have a role in maintaining a constant level of mice in most years but are unlikely to have a significant impact during a mouse plague. A marked increase in the number of predators in an area usually indicates an increase in the number of mice.
In south-eastern Australia the main predators of mice are falcons, owls, kites, kestrels, hawks, kookaburras, foxes, feral cats and snakes. During a plague, mice also become a source of food for birds such as butcher birds, magpies, crows and ibis.
Mice can transmit disease to human beings and domestic animals, and the mites they carry can cause a skin rash. Use gloves or an implement to handle sick or dead mice. Burn or bury the bodies.
Families should also keep in mind that a mouse plague can cause emotional stress in some individuals.
Further information on mice and control options is available from: