EXPERT REACTION: Tackling the NSW mouse plague

New South Wales is currently being hit hard by a mouse plague with crops and homes overrun with mice. The personal and financial impact on farming communities is substantial and a $50 million support package has been put together to help support regional communities.

Part of that support package involves mouse baiting, but there has been confusion in the media over the nature of the baits. There are two parts to the mouse control part of the package, free treatment of grain to distribute as bait and urgent authorisation of the rodenticide bromadiolone to use as a broadcast bait.

The free treatment of grain to use as bait is zinc phosphide, but a lot of media focus has been on bromadiolone.  

Bromadiolone has been described in the media as “outlawed”. Yet bromadiolone is approved to use in bait blocks and bait stations, it is currently not approved as a treatment for grain to use as a broadcast bait, although it has been in the past.

Bromadiolone is an anticoagulant rodenticide, these work by interfering with production of vitamin K, essential for blood clotting, so the animals die from uncontrolled bleeding. Bromadiolone is one of the second generation anticoagulants, which only require one exposure to a bait to kill rodents, while the first generation ones needed animals to consume bait over more than one day. While bromadiolone is highly effective, it is not like “napalming mice” as has been reported in the media.

Second generation anticoagulants are lethal to all mammals, native marsupials and birds. Bromadiolone is currently only authorised in baits and bait stations because of the risk to native and domestic animals. The enormous scale of the mouse plague means that baits and bait stations cannot hope to control the mice in the fields, so a broadcast bait is needed.

If the authorisation goes ahead then bromadiolone will probably replace the current zinc phosphide as a grain treatment for broadcast bait. The sheer scale of the mouse plague means that traps and non-lethal approaches have no hope of controlling the mouse numbers.

Both zinc phosphide and bromadiolone are toxic to humans, but with the recommended safe handling procedures farmers should have negligible risk. Broadcast grain baiting procedures prevent baited grain from entering grain intended for consumption.

How toxic/common is Bromadiolone and is its use in this situation likely to have impacts on the environment, native animals or human health?

"Bromadiolone is a “second generation” anticoagulant. It is highly potent. It is a single feed rodenticide. Because there is a delay in the onset of symptoms, rodents cannot associate eating the rodenticide with becoming sick. They normally die three to seven days after eating the baits.

“First generation” anticoagulants (eg Warfarin) are older but rodents need multiple feeds to receive a lethal dose.

Second generation rodenticides are readily available for use in bait stations for household use, in warehouses etc. The regulations around the use of the product are tightly restricted because of potential problems (thus the requirement to use bait stations).

A major concern with using second generation anticoagulants is that the bait can accumulate up through the food chain, leading to death of predators or scavengers of poisoned mice. There is an antidote to anticoagulant poisoning (Vitamin K1).

Anticoagulants are not currently registered for use “in crop”, so cannot be broadcast over crops.

On the other hand, zinc phosphide (an “acute rodenticide”) is currently registered for use “in-crop”. There are few secondary poisoning risks with zinc phosphide because when a mouse eats the poison, the phosphide converts to phosphine gas in the acid environment of the stomach and is released, and does not accumulate up through the food chain.

The APVMA (pesticide regulator) has recently approved the use of a double-strength zinc phosphide bait. Our work (funded by GRDC) confirmed that this double-strength bait now provides a lethal dose from one treated grain. Farmers and the grains industry have many years experience in using zinc phosphide treated baits."

Is this the best way to deal with the mouse plague? Are there other strategies we could also be using?

"This mouse plague is an extreme situation, and we want to ensure farmers have the best tools available to be able to manage the problem. That is why we explored improving the use of zinc phosphide baits to double the strength.

There are also complex issues about the role of alternative food on how effective baiting is. The more alternative food available to mice, the less they are inclined to take poison baits. We are doing more research into this question.

We recommend reducing alternative food as much as possible. We also recommend baiting as crops are sown. Protecting grain at sowing is the most critical stage. Farmers do not want to sow their crops more than once (it is expensive and takes a lot of time)."

Why is the mouse plague so bad this year?

"We are coming off several very dry years, then we have excellent rainfall and growing conditions over a very large area. Mice are simply responding to excellent conditions. They started breeding earlier than normal, and their survival was also probably higher. Mouse populations can increase very rapidly.

Combined with abundant high quality food, numbers have increased to what we see now. They also move into farm houses, grain stores and other infrastructure cause all sorts of damage.

Why is the mouse plague so bad this year?

"Mouse plagues are not new to Australia – a combination of huge food resources from grain and the mice’s ability to breed fast under a wide range of conditions means that occasionally, as in this case, a good growing season and the right rains has resulted in a ‘boom’ of plague proportions.  Many native species also show ‘boom and bust’ patterns like this but nowhere near as noticeable: the introduced house mouse in Australia is an invasive species par excellence."

How toxic/common is Bromadiolone and is its use in this situation likely to have impacts on the environment, native animals or human health?

"The use of rodenticides to fight this is an understandable response but we have to be aware that this will come with costs.  Bromadiolone is what is known as a second generation rodenticide that aims to kill a mouse that eats it after a single meal.  The result will be large numbers of dead and dying mice.  

The trouble is, these mice are likely to be eaten by native predators and scavengers – owls, kites, kestrels, kookaburras, ravens – will all then accumulate the toxin and die as well.  Our own research has shown that many reptiles – natural mice predators – will also bioaccumulate rodenticides and as reptiles seem to be able to survive a bit longer after rodenticide uptake they themselves then become ‘toxic time bombs’, waiting to poison any predator that might eat them.

Of course, baiting with these toxins can also impact our farm animals, pets and working animals – both during baiting and long after.  A recent review paper showed that, as large reptiles can be part of human diet both in Australia and other regions, ultimately humans can also be at risk of rodenticide poisoning."

Is this the best way to deal with the mouse plague? Are there other strategies we could also be using?

"Rodenticides probably still have role. What are called first generation rodenticides work more slowly but also break down more quickly and so have less of an impact on native animals that might eat poisoned mice.  The other thing we can do, and should be doing anyway, is encouraging the maintenance of habitat in farmland that supports natural predators – birds of prey, native carnivores, snakes and large lizards – they are our front line defence against mouse plagues."