FAQs on mice plagues

ouBelow are 13 frequently asked questions about mice and mice plagues. Browsers should also have a look at the Grain Research & Development Corporation’s Mouse Management Fact Sheet on their website www.grdc.com.au which is also a valuable resource.

What is a ‘mouse plague’?Mouse plague

The CSIRO Rodent Research Group generally considers anything over 500 mice per hectare as representing a plague. The entire cycle of a plague is one to two years, with competition between mice causing high populations to crash.

Are mouse plagues becoming more frequent in South Australia?

Yes. Although they occur irregularly, the frequency of recognised ‘mouse plagues’ somewhere in SA has increased from an average of once every six to seven years before 1970, to about once every four years since then. Also, substantial localised damage and control costs are now occurring in many other years.

When was the last mouse plague in South Australia and what did we learn from that event?

In 2010, severe mouse damage at seeding caused substantial yield losses in central and western Eyre Peninsula, while scattered severe problems occurred elsewhere in the state. Mouse numbers were generally higher in crops where:

  • The previous year’s grain yields were exceptionally high
  • Hail or wind damage dropped maturing heads before harvest
  • More grain was spilled at harvest
  • On lighter or stony soils versus heavy flats
  • On stubbles versus pastures
  • On ungrazed versus grazed stubbles or pastures
  • Where adjacent verge areas had heavy nut-grass or spear-grass cover
  • Where there were tall standing stubbles; and
  • In areas with persistent weed problems (including melons, onion weed, horehound).

When do mice breed, what spurs on a plague and what leads to their downfall?

Mice are primarily seed-eaters and their population levels in crops are mainly limited by the availability of high quality food and shelter. Mouse populations are at their lowest in late winter, when seed is scarce. Breeding begins as soon as fresh seed sets in spring, it peaks in late-spring and gradually declines until seed supply is removed by consumption, germination and spoilage.

Breeding and juvenile survival are usually compromised by moisture stress in the height of summer, but mice can modify their behaviour to minimise moisture stress when grain is super-abundant and summer rain, which produces a flush of summer weeds, can provide a ready supply of moisture. Breeding success is also heightened by dense cover. In these circumstances, continuous breeding can lead to steady, huge population increases from October to May.

Can they breed outside of the usual breeding periods?

Yes, mice can begin breeding early if early autumn rain germinates winter weeds and advances seed-set from August/September to June/July. At the other end of the season, breeding can be prolonged if summer rain produces a flush of fresh weed seed in late summer.

How do I know if I’ve got a mouse problem or going to have one?

Monitor your paddocks and farm. Low numbers around haystacks and buildings do not necessarily mean the paddocks are not badly infested, and vice-versa. If numbers have been high in autumn, continue your monitoring through winter and spring. Signs that mice numbers have increased include:

  • Numerous burrows
  • Mouse droppings on soil and plants and a typical mousey smell
  • Large numbers of mice seen at night, in paddocks or on roads
  • More birds of prey (e.g. kestrels or kites) around; and
  • Signs of seeds being dug up, plants being gnawed or pod and head damage.

What are the key management options to prevent a mice plague this year?Mouse Plague

The priority issue in the immediate future is to prevent seed set on summer weeds. Other stubble management options that were considered beneficial last year included rolling, slashing, prickle-chaining and burning, particularly if conducted in summer. These need to be carefully considered for impact on soil stability. If grazing is used as part of that process, it will be most effective if grazed hard and early – to remove grain in summer, before mice have an opportunity to use it.

In mice infested paddocks what’s the best methods of reducing mouse damage at sowing time?

Early sowing; increasing the sowing depth – say 4cm to 6 cm, instead of shallow sowing; and incorporating stubbles before or at sowing all reduce mouse damage, but in severely infested paddocks it may be insufficient without baiting.

How can I improve the success of baiting?

Last year on Eyre Peninsula, landholders reported that applying bait at sowing (or immediately after) was far more successful than leaving it for a few days after sowing. Use registered zinc phosphide products at recommended application rates and keep on top of summer weed control, especially melons. Persistent weed problems increase mouse numbers and provide an alternative food supply that make them harder to control with bait.

Which baits can I use and what’s the difference between the main ones?

Zinc phosphide is registered for in-crop use only and is the only bait registered for that purpose. Growers need to consult the label for specific use instructions. This bait can be laid if mouse activity is at a sufficient level to justify baiting.

Bromadiolone is an anticoagulant poison which is a sold as a grain-based bait and registered for use only on crop perimeters and around storage areas. It can be used on fence lines but not within the crop. Again, growers should consult the label for specific use instructions.
Perimeter and in-crop areas should be monitored after baiting.

What’s the risk to other animals from mouse bait or baited mice?

Mouse bait and mice that have died from bait represent a small but nevertheless potential risk to dogs and other animals. It’s recommended that you restrain your working dogs and pets during the baiting program.

Is farm hygiene important in limiting mouse numbers?

Yes, minimising spilled grain in paddocks is vital in limiting mouse populations and damage to next year’s crop. Set harvesters to minimise grain loss and monitor how much grain is left in the paddock. Heavy grazing can help clean up high harvest grain losses, but sufficient ground cover should be left to minimise risk of soil erosion.

Also clean up any concentrated spills of grain around field bins, augers, silo bags or other grains storage. It’s a good idea to remove or reduce cover, including plant material, rubbish and general clutter around farm buildings, silos and fodder storage – these all provide protection and hiding places for mice.

Will climate change benefit or impede mice plagues?

If climate change produces more extreme weather events mice will potentially benefit further. More cost-effective baiting options may be able to address part of this problem, but the root lies in the quantity of seed left in stubbles. New strategies are needed to manage that problem; innovations such as the Harrington Seed Destructor may provide multiple benefits for the control of herbicide resistant weeds, cereal disease and mice.

March 2011