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This note describes the history, distribution ,and impact of house mice in Victoria. The common house mouse (Mus musculus) probably originated in Central Asia. It is a marvellously adaptive species that has attached itself to humanity and has accompanied humans almost everywhere. The result is an almost world-wide distribution. In Australia, mice can be found wherever people are, and in many areas where they are not. Mice had made themselves at home in desert, tropical and alpine habitats: in cities, towns and wilderness. They are by no means tied to humanity but seem to wander where they will. Mice have historically been a problem to primary producers when large populations have arisen from time to time (in plague proportions). In areas where there are extensive grain crops, mice can cause tremendous damage. Populations can quickly rise to unmanageable proportions. The main objective of some recent research work on mice in plague conditions has been to define:
1. Rainfall and Soil Conditions Above average rainfall the previous winter followed by midsummer rain has the effect of ensuring that soil is in a good state for burrowing; that the subsoil will be damp and that food resources will be adequate throughout the summer. Heavy rain or food shortage after harvesting will then have the effect of forcing populations to invade houses, silos and storage places in autumn. A hot summer will also help mouse establishment in that it will ensure that soil is in good condition for burrowing. The type of soil seems to play an important role in population growth. A 1970 plague in much of south-eastern Australia was most severe in the soils of the Victorian Mallee and in the grey-brown earths of adjacent South Australia and New South Wales. 2. Food Availability Where crop harvesting is delayed due to seasonal conditions until the drier autumn months, there is a possibility that, if soil and weather conditions are suitable, young mice will be present in large numbers. Optimum availability of food, coinciding with peak summer breeding will result in a rapid and uncontrollable `take-off' in mouse numbers. 3. Breeding and Behaviour Studies have shown that house mice will breed throughout the year, but there is a pronounced burst of breeding activity in late spring-early summer. Sexual maturity of the young is reached at about eight weeks of age. Litters vary between 4-8, although there have been reports of larger litters. Pregnancy lasts about 19 days and the breeding cycle may be repeated almost immediately. Populations that arise in plague proportions are placed under enormous social stress. There is some evidence to suggest that a large percentage of plague mice are transients (about 80%) continually moving from areas of high density/low food availability to areas of low density/high food availability. There is evidence that plagues of mice move, although in a non-directional, random sense. This phenomenon can take on the appearance of a collectively organised movement in the following way: a densely populated area will be completely stripped of food supplies, the mice in that area will then move on to another area. This will result in a vacuum into which other mice will pour. Consequently, what begins as a random, patternless movement, soon takes on the aspect of a river flowing from an unfavourable area to a more favourable one. 4. Availability of Cover The movements of mice can be restricted if there is not enough cover for them to move in safety. Mice will not cross large expanses of clear ground unless they are forced to do so. There is often a large residual population of mice remaining in the stubble of harvested crops. It is this population that provides the basis for an explosion of summer population figures. This group can easily find its way into young summer crops, causing severe destruction if there is not a sufficiently wide area of fallow ground between crops. Similarly, an abundance of cover in areas close to storage areas, on the edges of creek banks and irrigation channels provides just what mice need to be able to move from property to property or to establish themselves near stockpiles and transport facilities. Damage The damage that plagues of mice cause is enormous. They will attack virtually all cereal and grain crops, plus many vegetables and fruits. A 1970 report on the mouse plague in the Murrumbidgee and Coleambally Irrigation Areas of southern New South Wales, showed an average loss estimated of 1.25 tonnes per hectare. In some areas losses were estimated to be up to 30% of the crop. It should be stressed that a mouse plague, while it may arise from seasonally variable conditions, is not necessarily limited to a season's duration. Plagues have been known to last through a mild winter into the following growing year. The damage from plagues can then run into millions of dollars. The ramifications of grain losses are not restricted to the rural sector. Anything that affects the production of grain and cereal crops will adversely affect the whole economy to some extent: local markets become unstable and prices may rise; export shipments may not be met and overseas buyers will look elsewhere for reliable suppliers. Conclusions The factors that coincide to bring about the "end" of a mouse plague are not yet clearly understood. More work needs to be done into the dynamics of mouse populations and the biology of the species itself. Some comments can be made, however, on the basis of recent research. Environmental catastrophes (from the mouse's point of view) can quickly reduce populations to a reasonable level. The harvesting of a crop; flood, drought, or other significant variation in climatic conditions or severe reductions in the amount of favourable cover, are all limiting factors. Further to this, there is some evidence that the continued stress of over-crowded conditions can effectively reduce the resistance of mice to disease. Outbreaks of fungal and bacterial disease have been noted in over-crowded conditions. Control methods are specified in a separate publication. It may be remarked here, that probably the most effective method of controlling mouse plagues is to reduce the mobility of the pest. This will have the effect of increasing the internal social pressures which can restrain population build-ups while simultaneously reducing widespread crop losses. The length of time between plagues can vary considerably. One of the objectives of research by the Department of Natural Resources and Environment is an understanding of the phenomena of mouse plagues that will allow for an accurate prediction of the likely times and places in which a plague could occur in Victoria. As a result of the variation in time between consecutive mouse plagues, there is a danger of adopting a classic "she'll be right" pose, allowing themselves to forget the potential damage that mice can cause. Preventative control methods are relaxed, long grass and cover is permitted to grow back on roadsides and irrigation channels, regular checks of populations in stubble are no longer carried out. The effect of neglect may not be felt for some time but when it is, the results are extremely costly and serious. Over twenty percent of Australia's land surface has suffered mouse plagues in the past twenty years. Populations can multiply at amazing rates; it is not enough to be thinking about what could be done if mice numbers get out of hand. Effective monitoring and restriction of available cover are essential programs that are highly cost-efficient in the long run. References
This note replaces note number PA0032 The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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© The State of Victoria, 1996 - 2008.
This document was published on 26/10/2008.