The house mouse (Mus domesticus) was introduced to Australia by the first European settlers during the late 18th century. Its distribution is now widespread throughout the whole of the country, but it is in the key grain growing areas of south eastern Australia where mouse numbers have had the greatest impact. With the start of another surge of rodent activity being reported, Gerwyn Jones, Asia Pacific Manager for PelGar International outlines some of the reasons for mouse plagues and potential outcomes.

Pile of mice from 1917

A pile of field mice that were poisoned by Australian farmers during the mouse plague of 1917. Although mice plagues are rare elsewhere in the world now, evidence suggests that have blighted humans throughout our history.

Mouse plagues have been regularly reported in Australia, the first being cited in a Victorian newspaper in 1917. On this occasion, in the town of Lascelles in the state of Victoria, around 200,000 mice (3 tons in weight) were caught in one night. By the end of June 1917, a total weight of 544 tons, equating to some 32 million mice, had been reported from this one plague.

Since then significant mouse plagues have appeared regularly in the country causing major losses in agricultural production. In 1993 one of Australia’s worst mouse plagues hit the grain producing areas of Victoria and South Australia, causing an estimated A$96 million worth of damage. The plague caused destruction of standing and stored crops, livestock was attacked, vehicles damaged and electrical and rubber insulation in buildings destroyed. In 2011 mouse numbers built to plague proportions in southern Queensland, New South Wales, western Victoria and South Australia. “Now again in 2019 we are seeing an alarming rise in rodent numbers,” explains Gerwyn Jones. “It is therefore crucial that we monitor and put in place strategies for control”.

Plague triggers
A mouse plague is defined as an excessive and increasing population of mice that are having an economic and/or environmental impact. The density of mice in crops, in non-plague years is normally <50 mice ha-1 but at peak densities can exceed 1,000 mice ha-1. Mouse numbers are dependent on many factors but what is key is the difference between non-breeding winter populations and the breeding populations in spring and summer. As mice are not long lived the size of the overwintering population is often hard to detect, but the trigger for the commencement of breeding appears to relate to the prevailing weather and food supply.

The mouse breeding season starts in October continuing through until the following May. As mice can breed at an alarming rate with litters of up to 11 offspring every three weeks it is not surprising that plague proportions can be achieved after only four to five generations.

What triggers an actual plague, compared to the normal annual fluctuation of the mouse population, seems to be dependent on a number of factors, in particular when:

  • there is a supply of quality food and water;
  • environmental temperatures are not extreme;
  • soil is moist and easy to dig;
  • nesting conditions are favourable;
  • diseases, parasites and predation levels are low.

It is also suggested that changes in agricultural practices may be increasing the frequency of mouse plagues. Stubble retention for example will provide additional habitat and extra food from split or unharvested grain, as was evident in the 2010 winter crop harvest. Plagues will often occur following a period of drought with a subsequent wet spring and summer which provides extra food and cover.

Mice Plague 2010

Mice at the Penong farm of Bill Oats, west of Ceduna in South Australia during the 2010 mouse plague. Source: News Corp

Impact of mice plagues
Wheat is the main winter cereal crop grown in southern and eastern Australia and is the crop which suffers most from mouse plagues. However, mice can also damage other cereal crops, soybean, maize, sunflower, rice, lucerne and a range of horticultural crops. They can impact farm storage areas and animal husbandry facilities, such as pig and poultry sheds and cause damage to farm machinery and vehicles.

Damage to the crops is caused by the mice consuming the grain and digging out newly sown seeds and germinating seeds which results in farmers frequently having to re-sow their crops.

In storage areas mice can enter via the smallest of holes so maintaining a mouse-free store is virtually impossible. Their droppings will contaminate stored grain and livestock feed.

They are also known to transmit a range of diseases and pathogens to both livestock and humans including:

  • salmonella to humans and domestic animals;
  • encephalomyocarditis virus to pigs;
  • leptospirosis to humans and animals;
  • tapeworms, roundworms and fungal diseases.

Control tactics
Guidelines issued by the New South Wales Government in Australia recommend monitoring and observation techniques before use of rodenticides for control. Monitoring helps to determine an increasing mouse population. Simple monitoring by observation, especially around buildings will provide a good indication of an increasing presence of mice through:

  • an increase in droppings;
  • gnawing;
  • burrows;
  • smudges on walls; dark runways;
  • tracks and worn pathways;
  • smell, sight and sounds
  • mounds of soil and seed
Mouse in grain

Mice seen in grain stores can be an indicator of a bigger problem to come. They will consume the grain and contaminate it with their droppings.

Visual signs are also valuable indicators. Damage to seed heads and chewed stems and seed husks at the base of plants are more indicative of mouse damage rather than bird and insect damage. The number of burrow entrances along a unit area or evidence of freshly dug soil will also suggest an active burrow. In irrigation areas syphons which have not been used for a while may have a very ‘mousy’ smell and mice may be seen running out of the ends. An increase in mouse numbers may also attract an increase in predators such as raptors.

Simple canola-soaked monitoring cards, which when chewed, can reveal the high probability that mice will take bait is the first recommended step.

Bait stations filled with a measured amount of grain and strategically placed in fields can also be used as a crude monitoring aid. If left overnight the loss of grain (eaten by the mice) is then determined by weighing the grain the following day. By assuming that a mouse will eat 4 grams each night (20% its body weight) then a rough estimate of the number of mice can be calculated by dividing the weight of grain lost overnight by four.

Once monitoring has taken place and a problem identified then use of rodenticides may be the only option. The most commonly used anticoagulant rodenticides used in Australia are: brodifacoum, coumatetralyl, flocoumafen and warfarin but these are not for field use and can only be used in and around farm buildings.

Two rodenticides currently permitted for field use are:
bromadiolone – a grain-based bait for crop-perimeter baiting only. Here the landowners supply the grain to the Livestock Health and Pest Authority (LHPA) who mix the grain with the bromadiolone.

zinc phosphide – this produces toxic phosphine gas on ingestion. The bait is registered for in-crop use and can only laid if a monitoring programme shows significant mouse activity.

It is important that an integrated pest management strategy should be employed to reduce damage caused during a plague. This includes mouse-proofing buildings, reducing the amount of farm rubbish around buildings and employing good farm hygiene practices. These strategies will help to reduce mouse populations exploding when conditions are optimal. It is also crucial that the threat to non-target species is minimal and as in-crop and perimeter baiting is an accepted method of minimizing the effect of mouse plagues on the Australian grain harvest assessing the impact of rodenticides on non-target species is recommended.

Problems for manufacturers
From a manufacturer’s point of view it is quite difficult dealing with a mouse plague,” explains Gerwyn Jones. “I have personally been in the industry during two mouse plagues in Australia, and there were some challenging problems that we faced.”

“As an overseas manufacturer, PelGar usually plans to have product stored in Australia based on the previous season and forecasts given to us by customers and end users. When a plague strikes you can throw all of that out of the window. Within a few weeks, stocks that would normally last months are gone. It is a real Catch 22 situation – you want to have stock to tackle a plague, but then you don’t want to be overstocked. I have seen manufacturers stock their warehouse with product, in preparation for a bad mouse season, which then did not materialise. You are then left with ageing stock and the cost of warehousing product until the next season.”

Research work, which focuses on understanding the mechanisms leading to increasing mouse populations, and developing predictive models, are only able to achieve 70% accuracy. It is therefore vital that farmers and growers on the ground recognise and monitor the early signs of increasing mouse numbers and act to put control measures and management practices in place early on. It is far easier to control a small mouse infestation than the economic and environmental impact of a plague.  With weather and the climate certainly having an influence on seasonal mouse populations, it is likely that climate change will have a significant impact on the number and magnitude of plagues in the future.

This article appeared in International Pest Control, May/June 2019 volume 61:3