Early detection of mouse plagues

Because mice have a high potential rate of population increase, early detection of mouse plagues is essential for effective control to be implemented.

Early detection of a plague can be difficult due to:

  • normal seasonal peaks in mouse numbers which cause minimal impact on agricultural production;
  • failure to monitoring population changes, using tools such as census cards;
  • failure to recognise and report unusual population changes.

It is critical that landholders acknowledge these difficulties so that they do not result in the failure to monitor for mouse plagues each year. Persistence in maintaining an accurate monitoring process will have its rewards.

Quite often the rapid-increase phase of mouse populations is not fully recognised until numbers have started to approach plague proportions. At this point damage will already have become serious and averting further damage to crops is made extremely difficult because of the size and distribution of the mouse population.

Theoretically, plagues can be predicted using rainfall patterns and vegetation response. However, predictions are not always correct, particularly on a local scale.

In recent decades, mouse plagues have occurred more frequently as isolated outbreaks rather than as the widespread problems experienced in the past which affected nearly all grain growing areas in south-eastern Australia.

For this reason it is increasingly important for landholders to be vigilant in monitoring their own crops.

Normal fluctuations in mouse numbers

Mice are present throughout the year in nearly all grain growing areas of Australia. Mouse numbers depend on many factors but the most important is the difference between non-breeding winter populations and breeding populations of spring and summer. Mice are not long lived and the size of the over-wintering population is usually small and difficult to detect.

The trigger for commencement of breeding, which is not yet fully understood, and the prevailing factors of weather and food supply determine whether a population develops into a plague.

Breeding usually starts around October each year and continues through to the following May, however, occasionally some females (10–20%) breed through winter. Knowledge of these changes in breeding performance and the size of the over-wintering population are important factors to assess when predicting the likelihood of a plague. Peak numbers can be expected by early autumn, with numbers then declining, usually due to lack of food and to stress-related diseases.

Winter conditions can also cause mice to invade homes, thereby becoming more noticeable to the public. This occurrence should not be confused with plague conditions.

Mice can breed at an alarming rate, with litters of 2–11 every three weeks. If conditions are right, plague proportions can be reached after only 4–5 generations.

Mice will generally not move far when the food supply is adequate. However, they will invade new crops or habitats in search of food or as a result of increasing population pressure. In these situations mice can move long distances.

Impacts of mice plagues

Mice cause damage to most crops, including wheat, oats, barley, soybean, maize, sunflower, sorghum, rice, lucerne, seed and other leguminous crops, as well as horticultural crops such as melons, pumpkins and tomatoes. Mice will also be active in most farm produce storage areas, while pig and poultry sheds and grain storage facilities are particularly favoured. Mice plagues can cause damage to machinery, vehicles and electric motors, including their plastic and rubber components.  Mice can transmit a number of diseases to humans and livestock. In particular, mice can transmit:

  • salmonella to humans and domestic animals
  • encephalomyocarditis (EMC) virus to pigs
  • leptospirosis to humans and animals, including pigs and cattle
  • tapeworms, roundworms and fungal skin diseases (eg ringworm) to humans and cats.

Mouse droppings can contaminate human and livestock foods.

Identifying the problem

Personal observations, record keeping and timely reporting are still the most reliable methods of identifying and addressing increasing mouse numbers. Farmers are encouraged to conduct regular (weekly) walk-throughs of their crops and record their observations. Once mice are in plague proportions there is little that can be done to control mouse numbers, so early identification of population increase and appropriate responses are vital. Warning signs to watch for inlcude:

Signs of mouse activity around the perimeter of crops

Burrows, soil removed from within burrows and worn tracks are often visible. The intensity of burrowing indicates the number of mice present. In irrigation areas mouse activity is most obvious around channel banks early in the maturation of the crop. After the crop has matured and/or when water is taken off, mouse tracks are usually visible in the wet soil.

Signs of crop damage

Mice have been known to dig freshly sown seeds out of the ground before they germinate. The digging is funnel shaped and the husk of the seed is usually nearby. Mice may damage immature crops by gnawing stems just above the nodes causing the heads of the plants to die off and become visible as brown patches in a green crop. In more mature crops, damaged heads tend to stand higher due to the removal of the grain. Other signs include mounds of gnawed grain at points within the crop, particularly around burrow entrances.It is necessary to walk through the crop to identify these signs. Grain damage to some crops, such as sunflower, can be differentiated from the damage caused by birds as birds tend to remove the entire seed from the head whereas mice gnaw at the seed while it is still attached.

Sightings

Increasing mouse activity is quite often revealed along the roadside, particularly near crops, at night by car headlights. If mouse damage is suspected, a quick inspection of a crop at night with a torch can also reveal mouse activity.

Presence of nests

In irrigated crops, particularly rice, nests are often visible, appearing as leaf and stem platforms at water level. This sign usually occurs once a plague has developed.

Droppings

Droppings are useful in identifying the presence of mice in larger crops such as sunflower and maize. Droppings are found at the junction of leaves and stem, or on the seed head.

Presence of mice in storage facilities

Mice are usually present all year in facilities such as grain storage silos. Any unusual increase in numbers or damage can indicate an increase in field populations.

An increased number of predators working the cropping area

The most obvious predators are the raptors (hawks, kestrels and kites). If large numbers are hovering in the crop area it may be worth investigating. However, it has been noted in previous plagues that raptors are slow to respond to increasing mouse populations and are generally not apparent until damage has become evident.

Monitoring mouse numbers

If mouse numbers are increasing farmers should be proactive. All observed changes in mouse populations should be reported to Local Land Services which can provide advice and assistance in controlling mice. If the risk of a build-up of mouse numbers is high or if early signs are evident, farmers are encouraged to attempt some form of direct assessment of mouse numbers in the field. There are two simple ways of doing this: the use of census cards and trapping.

Census cards

Census cards are squares of paper soaked in canola oil that are pegged to the ground at 10-15m intervals throughout the survey area, starting at the perimeter of the paddock and moving into the cropped and pasture area. If mice are present they will chew away the paper to varying degrees depending on the numbers of mice present and the availability of alternative food. Census cards are occasionally known as bait cards but they do not kill mice.

This method only suggests whether mice are present at the time: it does not indicate the likely progression of the population towards a plague. They should be put out in a variety of habitats early in the season. If cards are eaten in or around early-maturing crops this indicates potential problems. Field experience indicates that card consumption of around 20% or more in immature crops is a reasonable guide that the mouse problem is real. Cards can become less reliable as crops mature and provide greater amounts of food for the mice.

Trapping

Simple snap-back traps can be a useful means of checking a mouse population. For ease of baiting the traps, permanently attach a small piece of leather to the trigger and occasionally add a few drops of linseed (or similar) oil. Alternatively, smear some peanut butter on the trigger each time they are set. Step out straight lines of 20–25 traps, with traps 10 m apart, in a variety of habitats, for three consecutive nights.

Trap success is the number of mice caught divided by the number of traps per line times the number of nights, e.g. 30 mice from 25 traps over 4 nights = 30/(25 x 4) = 30%.

Trap success can be used as the index of mouse abundance. It is difficult to determine what rate constitutes a risk but anything around 20% in an early-maturing crop would indicate problems.

Breeding status

A trapped sample also allows the population to be examined for its breeding status. The earlier a population starts to breed, the more likely a plague will occur. Signs to look for are obviously pregnant females and females that have already started to breed (indicated by prominent mammary glands). Another indication of early breeding can be obtained from the size of the mice caught. Adult mice usually measure over 72 mm from the tip of the nose to the base of the tail. Mice with body lengths less than 72 mm, particularly if they are much smaller, indicate that juveniles have already been born into the population that season.

Baiting

In order to prevent build-up of mice into plague numbers control measures should be implemented as soon as mice are detected. There are a number of control options available for mice control. Mouse poisons are available from a variety of outlets including rural merchants and Local Land Services. Local Land Services can supply Bromadiolone (mixed with grain supplied by the landholder) for perimeter baiting of crops or for use around sheds and farm structures. Perimeter baiting is a control measure designed to prevent mice moving into a crop, so the timing of this control method is important. The use of perimeter baiting won't protect a crop if the mice are already in the paddock. The use of restricted chemicals must be used in accordance with the relevant pesticide permit and the potential impacts on wildlife and non-target species must be adequetly managed.

Alternate mouse control strategies

Mouse control should be part of an organised and ongoing program that will lessen the damage caused during a plague. Actions to consider include mouse-proofing facilities, mowing channel banks, minimising rubbish around farm buildings and adopting good farm hygiene practices.

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Further information

For further infrmation, or if you need assistance in interpreting your assessment of mouse numbers and the likelihood of an approaching plague, consult Local Land Services.