Mouse plague: bromadiolone will obliterate mice, but it’ll poison eagles, snakes and owls, too

Masked owl (Tyto novaehollandiae), one of many birds of prey at great risk of secondary poisoning
Belinda Davis, Author provided

Robert Davis, Edith Cowan University; Bill Bateman, Curtin University; Damian Lettoof, Curtin University; Maggie J. Watson, Charles Sturt University, and Michael Lohr, Edith Cowan UniversityIt’s the smell that hits you first. The scent of urine and decomposing bodies. Then you notice other signs: scuttles and squeaks, small dead bodies leaking blood, tails sticking out of hubcaps.

If you’ve lived through a mouse plague, you’ve seen this, and smelled the stench of mice dying of poison baits.

As a desperate measure to help combat the mouse plague devastating rural communities across New South Wales, the state government yesterday secured 5,000 litres of bromadiolone. This is a bait that’s usually illegal to roll out at the proposed scale.

This is a bad idea. While bromadiolone effectively kills mice, it also travels up the food chain to poison predators who eat the mice, and other species. And these predators, from wedge-tailed eagles to goannas, are coming out in droves to feast on their abundant prey.

When your prey is everywhere

Animal plagues in Australia are fuelled by the “boom and bust” of rainfall.

We have natural, flood-driven population explosions of the native long-haired rat, with accompanying booms of letter-winged kites, their predator. We also have locust plagues when the conditions are right, leading to antechinus or mice plagues which eat the locusts.

Since at least the late 1800s, we’ve had terrible plagues of the introduced house mouse (Mus musculus). But rarely has it been this bad, with conditions currently seeming worse than the last plague in 2011, which caused over A$200 million in crop damage alone.

High numbers of birds of prey — nankeen kestrels, black-shouldered kites and barn owls — are often reported feasting on plague mice.

Snakes, goannas, native carnivores such as quolls, and feral cats and foxes, also take advantage of the abundant food. Pets, especially cats and some dogs, are highly likely to consume mice under these conditions, too.

Poisoning the food web

Laying out poison baits is one way people try to end mouse infestations and plagues. So-called “anticoagulant rodenticides” are divided into first and second generations, based on when they were first synthesised and the differences in potency.

Wedge-tailed eagles are among the predators that take advantage of the house mouse plague.
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Second generation anticoagulant rodenticides have higher toxicities than first generation, and are lethal after a single feed. First generation rodenticides, on the other hand, require rodents to feed on them for consecutive days to be lethal.

But mouse-eating predators are highly exposed to second generation rodenticides. For most animal species, the lethal doses of rodenticide aren’t yet known.

A scientific review from 2018 documented the poisoning of 31 bird, five mammal and one reptile species. Second generation aniticoaugulant rodenticides were implicated in the death of these animals.

Our research from 2020 found urban reptiles are highly exposed to second generation rodenticides, too. This includes mouse-eating snakes, called dugites, which had up to five different rodent poisons in them.

We also found poisons in frog-eating tiger snakes, and in omnivorous bobtail skinks which eat fruit, vegetation and snails. This is even more concerning because it shows how second generation rodenticides can saturate the entire foodweb, affecting everything from slugs to fish.

Bobtail skinks don’t eat poisoned mice, but they’ve still been found with poison in their systems.
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Bromadiolone is particularly dangerous, even to humans

The NSW government secured bromadiolone baits as part of its $50 million mouse plague support package for regional communities.

Five thousand litres of the poison can treat around 95 tonnes of grain, and the government will provide it for free to primary producers once federal authorities approve its use.

Bromadiolone is usually restricted to use in and around buildings. But given the widespread impacts on wildlife, using bromadiolone at the proposed scale will do more harm than good.

Past research on bromadiolone has shown residues persist for up to 135 days in the carcasses of voles (another rodent species). In international studies, bromadiolone has been found in the livers of a host of birds of prey, including a range of owl species, red kites, sparrowhawks and golden eagles.

Humans can be exposed, too, by eating the eggs of chickens that ate the mice.
Shutterstock

And it’s not just a problem for wildlife, humans are also at risk of exposure. For example, we can get exposed from eating eggs from chickens that feed on poisoned mice, or more directly from eating other animals that may have ingested poisoned mice.

A 2013 study looked at chicken eggs for human consumption, and detected bromadiolone in eggs between five and 14 days after the chicken ingested the poison. It’s not yet clear how many of these eggs we’d have to eat for us to get sick.

So what are the alternatives?

There are highly effective first generation rodenticides that provide viable solutions for managing mouse plagues. They may take a little longer to kill mice, but the upshot is they don’t stick around in the environment. A 2020 study found house mice in Perth didn’t have genetic resistance to first generation rodenticides, which suggests they’re effectively lethal.

Another approach has been to use zinc phosphide, a poison which is unlikely to secondarily poison other animals that eat the poisoned mice. However, zinc phosphide is still extremely toxic and will kill sheep, cows, pets and even humans if directly eaten.

Rolling out double-strength zinc phosphide may be the lesser of the evils in causing secondary poisoning, but only if used very carefully.

And another way to help control the mouse plague is to limit food resources for mice on farms. Farmers can minimise grain on ground, and Australia should invest in research for grain storage facilities that are less permeable to mice.

Mouse plagues are a regular cycle in Australia. Natural predators not only help create healthy, natural ecosystems, but also they help with mouse control. Second generation rodenticides will only destroy and weaken the predator populations we need to help us combat the next plague.

Robert Davis, Senior Lecturer in Wildlife Ecology, Edith Cowan University; Bill Bateman, Associate professor, Curtin University; Damian Lettoof, PhD Candidate, Curtin University; Maggie J. Watson, Lecturer in Ornithology, Ecology, Conservation and Parasitology, Charles Sturt University, and Michael Lohr, Adjunct Lecturer, Edith Cowan University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Tiny desert mice could help save Australia’s grasslands from invasion

Christopher Edward Gordon, University of Wollongong and Mike Letnic, UNSW Australia

You should stop skylarking about with those bloody desert mice and try and stop those woody weeds. I could see clear through that paddock in the ‘60s. Now look at it. That scrub costs us tens of thousands of dollars in lost fodder and it’s almost impossible to muster the livestock.

That blunt assessment of our research, first offered by a local farmer in Australia’s arid rangelands almost seven years ago, raised an irresistible question for us as field ecologists. Why are Australia’s (and many others around the world) grasslands becoming woodier?

It certainly was a question worth asking. Shrub encroachment – an increase in the cover of woody shrubs in areas once dominated by grasses – is not just an issue in Australia.

In two recent papers published in the journals Ecography and the Journal of Animal Ecology, we looked at one key reason why trees are invading grasslands, and how we could stop them. And it all comes down to tiny desert mice.

Shrub invasion

“Invasive native vegetation”, as bureaucrats call it, is a major problem for livestock producers in drylands throughout the world. This is because the shrubs compete for space and light with the grasses needed to feed their cattle and sheep.

Shrub encroachment ‘inside’ the Dingo Fence.
Dr Ben Moore

It is a hard problem to tackle. Clearing and fire are the most common methods of controlling woody shrubs. But these methods are laborious and often hard to implement on large scales.

Removing shrubs is also contentious because these are typically native species that provide important habitat for wildlife. The New South Wales parliament’s controversial relaxation in November of regulations governing vegetation clearing were designed partly to allow farmers to remove invasive native vegetation.

What’s going on?

The causes for the spread are complex and poorly understood. Shrub encroachment is often attributed to overgrazing by livestock, which favours the growth of shrubs over grasses. It has also been linked to a reduction in bushfires that wipe out the shrubs and an increase in atmospheric carbon dioxide, which can promote their growth.

However, we suspected another important factor could be at play. And it was those little desert mice that provided us with a big clue – and a possible solution.

Since European settlement, livestock grazing and the introduction of foxes, feral cats and rabbits have decimated Australia’s native mammals, especially in arid and semi-arid areas.

The bilbies, bettongs, native rodents and other small mammals that became rare or extinct across much of the continent in the early 20th century once played essential roles in Australian ecosystems, by shifting vast amounts of soil and consuming vegetation and seeds.

Historical accounts suggest that shrub encroachment quickly followed European settlement and mammal extinctions in many areas. This coincidence led us to ask: could the loss of native mammals be making Australia’s drylands woodier?

Hopping to it

To answer this question, we went to the northwest corner of NSW. Here the Dingo Fence marks the border with Queensland and South Australia.

The Dingo Fence.
Ben Moore

We wanted to know whether the local extinction of a native mammal, the dusky hopping mouse, which eats shrub seeds and seedlings, would allow more shrubs to grow. The Dingo Fence was the perfect study site because dusky hopping mice are common on the northwest side, “outside” the fence, where dingoes are present.

Dingoes keep fox numbers down, which are the mouse’s major predator. However, dusky hopping mice are rare on the “inside” of the fence (the NSW side), where dingoes are less common and foxes roam.

We first used historical aerial photographs to show that shrub cover was consistently higher inside the dingo fence (rodents rare) than outside (rodents common). We then did field surveys, which showed that the numbers of shrubs, their seedlings and their seeds were greater where rodents were rare.

We also showed that dusky hopping mice were major consumers of shrub seeds and capable of keeping the numbers of shrub seeds in the soil down.

Fieldwork in the Strzelecki Desert.
Dr Ben Moore

Going wild again

These results are exciting because they suggest that the loss of native mammals such as the dusky hopping mouse may be an important and overlooked driver of shrub encroachment, not only in arid Australia but also globally.

Perhaps more exciting, however, is how we can apply our work. Our research suggests that “rewilding” drylands by re-establishing rodents and other native mammal species that eat shrub seeds and seedlings, such as bettongs and bilbies, could curb the shrub invasion.

Although an abstract and even controversial idea, rewilding of native mammals would provide a long-term solution to a problem that has affected pastoralists for more than a century.

Further, it would represent a natural and cost-effective strategy with enormous benefits for the conservation of imperilled native mammals.

Before we can do so, we have to control foxes and feral cats across vast areas, which is no small feat. However, the economic and conservation potential make it an approach that is well worth taking seriously.

Christopher Edward Gordon, Associate Research Fellow, University of Wollongong and Mike Letnic, Associate Professor, School of Biological, Earth and Environmental Sciences, UNSW Australia

This article was originally published on The Conversation. Read the original article.

2,000 mice parachute into Guam to kill snake invaders

Guam Mice Drop

The link below is to an article that looks at efforts to control the Brown Tree Snake on Guam.

For more visit:
http://usnews.nbcnews.com/_news/2013/12/02/21724382-two-thousand-mice-dropped-on-guam-by-parachute-to-kill-snakes