Scientists have found promise in a new pest-busting approach aimed at wiping out possums and rats with a one-two punch of 1080 poison.

In vast and rugged parts of New Zealand, aerial 1080 drops remain the only weapon the Department of Conservation (DoC) has to rapidly beat back booms of pests predators.

But these operations are usually carried out only every five years or more, giving surviving possums and rats time to build their populations back up again.

Rat populations could recover remarkably quickly, bouncing back in just one or two years after a drop.

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Scientists have been trying to tackle this problem with an approach dubbed "rapid repeat" or "dual 1080", which aimed to completely wipe out pests in targeted areas.

If this could be achieved, pests could return only by reinvading purged areas.

Following workshops between DoC, Zero Invasive Pests and OSPRI, scientists proposed a specific method involving two different bait types of 1080 being applied two months apart, with each round preceded by two non-toxic pre-feeds to lure their targets.

The trials, carried out in Westland's remote New Creek, drew on a combination of radio-tracking, specially designed "chewcards", trail cameras, and separate bait acceptance experiments.

"Our hope was that if we repeated the 1080 baiting almost immediately, all of the survivors from the first baiting would be killed in the second baiting a few months later," said Manaaki Whenua-Landcare Research scientist Graham Nugent, who led the study.

"What we found was that many of the survivors of the first baiting had learned that the cereal bait we used was dangerous, so they avoided eating it."

To overcome that, the researchers sowed some pre-feed bait between the first and second 1080 baitings, and then sowed the second lot of 1080 bait.

After the second hit, rat numbers were immediately knocked back to nothing, with one EDR-coated bait proving particularly effective.

But the approach didn't work as well with their main targets, possums.

The second 1080 baiting only managed to kill half of the possums, most of them likely just juveniles.

"What we did see with survivor possums, though, was that they would still eat a very different kind of bait even if they didn't eat the usual cereal bait - so we are now exploring whether we can use that different bait for the second baiting."

In vast and rugged parts of New Zealand, aerial 1080 drops remain the only weapon the Department of Conservation has to rapidly beat back booms of pests predators. Photo / File
In vast and rugged parts of New Zealand, aerial 1080 drops remain the only weapon the Department of Conservation has to rapidly beat back booms of pests predators. Photo / File

Zero Invasive Pests was using a similar concept to the "rapid repeat" in several of its own trials, one of which was being run now.

The approach had the potential to make a big contribution to the Predator Free New Zealand (PFNZ) goal of wiping out stoats, possums and rats by 2050.

"It will obviously be of help if the cost of dual baiting is not too high, and it greatly increases the length of time before either another 1080 operation is needed - or it enables the managers to be able find and eliminate any survivors or re-invaders at an affordable cost."

HOW DO WE MAKE CITIES PEST-FREE?

Meanwhile, another study has explored how pest predators might be wiped out in New Zealand's urban areas.

"Rat control is just as important in downtown Auckland as it is in remote Fiordland," said Associate Professor James Russell, a conservation biologist at the University of Auckland.

Although less than one per cent of New Zealand's land cover was urban, it was also where 86 per cent of us live.

The most common predators in our towns and cities were rats and possums, but also hedgehogs.

"Our pet cats and dogs can also be wildlife predators if they aren't cared for responsibly, and this is definitely a topic where both natural and social sciences need to be combined to find," Russell said.

Stoats, however, tended to avoid urban areas, likely because they couldn't find a reliable food source.

The war on pests is just as important in downtown Auckland as it is in remote Fiordland, says, University of Auckland conservation biologist Associate Professor James Russell. Photo / File
The war on pests is just as important in downtown Auckland as it is in remote Fiordland, says, University of Auckland conservation biologist Associate Professor James Russell. Photo / File

In cities, a big challenge lay in the fact scientists simply didn't understand the basic biology of urban pests like they did of those animals in the wild.

"This is further complicated by the fact that the tools we successfully used for their pest control in forest can't always be used in inhabited areas," Russell said.

"A good example is when we look at somewhere like Herald Island in the Waitemata Harbour.

"It's only a small island on which rat eradication would usually be straightforward, but between the complex infrastructure acting as habitat and the presence of people and pets, we almost have to start from scratch."

The new study, which Russell led with fellow University of Auckland ecologist Professor Margaret Stanley, concluded that success in urban areas greatly depended on people living within them.

"We made some particular recommendations around starting long team research and monitoring projects in urban areas like we've had in nature reserves for decades, but we think this should be easy given all the citizen science happening for pest control at the moment."

Given how complex city environments were - and the fact aerial drops couldn't be used like they were in the wild - approaches would have to be ground-based.

On the plus side, it was easier to recruit an army of volunteers who could regularly check traps.

"But at the same time we'll have to work with people to find out what's important about conservation and PFNZ to them," Russell said.

"The urban communities of New Zealand are running far ahead of the scientists in this case, with exciting projects like Predator Free Miramar demanding scientists provide solutions to some of the unique problems of working in urban landscapes."