Traps sell out as mouse population increases

With local mouse populations increasing and traps and poisons flying off shelves, local people are being urged to carefully consider control measures, as some can harm native wildlife which can help control rodents, including owls, raptors and reptiles.

Liz Bell

TRYING to buy a mouse trap in Gippsland at the moment is like trying to buy toilet paper during lockdown.

With a mice plaque rapidly marching into Victoria from New South Wales, hardware stores and supermarkets are running out as soon as stock arrives, leaving frustrated customers staring at empty shelves.

Experts say the mouse population explosion – which is much worse in New South Wales – has been brought on by the ideal conditions of warm and wet weather and plenty of food.

Agriculture Victoria is aware of increased mice populations in some parts of the state, but believes it is not widespread.

A spokesperson said while the mouse numbers were not as high in Victoria as other parts of the country, mice were still a concern and could damage newly-sown crops by eating the sown grain, and cause some damage around harvest time.

The authority says potential shortages in rodent management products could be tied to a range of reasons, including annual seasonal demands and increased mice numbers interstate.

And it advises landholders to use mouse traps or baits with extreme care to control mice in areas where young children or pets frequent, or where non-target animals may be affected.

However wildlife groups are urging residents to consider rejecting baiting methods, with small mammals (possums and bandicoots) often killed by ingestion of baits, and owls, raptors and reptiles that eat rodents often killed from secondary anticoagulant rodenticides (rat poisons).

Wildlife Victoria advises the used of safer, first generation rodenticides to avoid unnecessary animal deaths.

The volunteer rescue service receives more than 400 reports each year about sick or injured owls and raptors.

In 2018 the number of reports was more than 800, with an unusually high number between May and September, sparking an investigation by Wildlife Health Australia.

At the time, BirdLife Australia and the Department of Environment, Land, Water and Planning named rat bait as being the most likely cause of death in barn owls.

According to a report, other birds such as Nankeen kestrels and Boobook and several other species were also affected.

Forty-eight dead birds were submitted for examination and although most had died from starvation and trauma, 38 per cent of the Barn owns and Nankeen Kestrels tested had second generation anticoagulant rodenticides detected in their livers at levels which were considered possible or likely to cause toxicity.

Holly Parsons from BirdLife Australia said second generation rodenticides were powerful enough that a single feed could be lethal.

But because of the time lag between taking a bait and feeling the effects, rodents could consume a more-than-lethal dose and still be wandering around – “like walking time bombs”.

“Predators that naturally eat rodents, like owls and birds of prey, can then easily consume multiple poisoned rodents, in turn becoming poisoned themselves,” she said.

Second generation anticoagulant rodenticides don’t break down quickly – some can stay in tissues and organs for months, even years.

“Unfortunately this just makes it easier for these bigger animals to get a lethal dose of toxins,” she said.

“We know that many of our nocturnal birds and day time are in decline and removing these poisons from their food chain is one way we can help them.”

A Gippsland wildlife carer said the sad irony was that anticoagulant rodenticides killed many animals that actually provided rodent-control services, and there were easy and safer alternatives to using baits.

Safer first generation rodenticides contain the active ingredients Warfarin (Ratsak Double Strength) and Coumatetralyl (e.g. in Racumin), which work more slowly and break down more quickly.

Owls and other wildlife are unlikely to die from exposure to Ratsak Double Strength or Racumin. Second generation rodenticides present a greater risk to non-target animals, and contain the active ingredients Brodifacoum (most Ratsak brands), Bromadialone (some Ratsak products) and Difenacoum (Talon, Mortein, Ratsak Fast Action, Pestoff Rodent Bait 20R, Klerat).

BirdLife Australia recommends non-poison pest control first, like snap traps which do not pose a poisoning risk to children, pets and wildlife.

It says there were also steps to take to make the home less friendly for rodents including securing compost heaps, removing access to pet foods and rodent proof aviaries and chicken pens, and sealing up openings in roofs.

Farmers can also subscribe to Mouse Alert, which shares mouse activity observed at mousealert.org.au and Twitter @MouseAlert.