Millions of bogong moths normally line the walls of caves in the Australian Alps over summer, but for the past two years there have been zero moths in some caves.

Key points: Every year moths travel thousands of kilometres to reach the Snowy Mountains, but they haven't turned up in some places

Every year moths travel thousands of kilometres to reach the Snowy Mountains, but they haven't turned up in some places Scientists say the decline may be caused by drought conditions in their breeding areas and climate change

Scientists say the decline may be caused by drought conditions in their breeding areas and climate change The disappearance of moths could cause a number of alpine animals such as the endangered mountain pygmy possum to starve

It's a trend that concerns scientists like Eric Warrant.

Every year Professor Warrant returns from Lund University in Sweden to his house — and field laboratory — in Adaminaby in New South Wales to study the moths and their incredible migratory skills.

Last year he was shocked to find two caves he regularly visited had no moths at all. A third, larger cave in the Snowy Mountains had fewer than previous years, but still millions of moths, he said.

When he came back again this year, what he found was "catastrophic".

"I went again to those two caves and found the same thing — no moths. But what was really scary was the bigger cave was also completely empty," Professor Warrant said.

"We're talking about caves that normally would have tens of millions of moths in each, easily.

Each spring, Bogong moths migrate from their breeding grounds in southern Queensland, north and western New South Wales and Victoria to the Australian Alps.

Bogong moths fly more than 1,000km to reach the cosy caves of Mount Kosciuszko and nearby alpine areas. ( Supplied: Eric Warrant )

The decline in moth numbers is likely to be caused by a lack of rainfall due to winter drought in these breeding areas and climate change, according to Professor Warrant.

"Normally there is at least enough rain for the vegetation to be able to grow sufficiently to feed the caterpillars, but that hasn't been the case this year or last year," he said.

"It's a reflection of climate change and the way things are in the world at the moment. It's confronting to see this to be honest."

However, not all the caves in the area have been depleted.

Professor Warrant and his team found another cave covered in moths, but he said, they can't tell whether the numbers in this cave are different from previous years for lack of data.

He said the differences in the cave numbers suggested moths came from different breeding grounds, where one location wasn't as badly affected by the drought as the other.

"I don't think they will disappear completely, but there's certainly a possibility that they could become locally extinct at their breeding grounds."

Billions of bogong moths — about 17,000 per square metre — normally line the walls of alpine caves during summer. ( Supplied: Eric Warrant )

Food source for threatened possums

Bogong moths are a very important source of protein in the Alps for wildlife including the threatened mountain pygmy possum and other insectivorous mammals and birds.

"Bogong moths bring a huge influx of nutrients and productivity into the Alps," said alpine biologist Kate Umbers from the University of Western Sydney.

"Losing them would affect everything," she said.

The endangered mountain pygmy possum is one species that relies on the bogong moth in summer as a source of protein. ( ABC News: Lucy Barbour )

Reports of starving mountain pygmy possums with dead young in their pouches suggest it already has.

Ecologist John Morgan from La Trobe University has just returned from teaching fieldwork in the Mount Hotham region, where he said he hasn't seen bogong moths for two years.

"The mountain pygmy possums we trapped last week are pretty skinny for this time of year," Professor Morgan said.

"Many of them were about half their normal size, and there's been babies dead in the mother's pouch."

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As males only live for around four years, another summer of failed reproduction would mean a very limited number of mates, he added.

Euan Ritchie, a wildlife ecologists at Deakin University, said a big collapse in insect populations could have a profound effect on a whole range of species.

There's also an eerie parallel to the plight of the eastern quoll, which nearly disappeared in Tasmania when their preferred food source was diminished.

"Originally it was thought the [quoll] decline was due to climate change directly," Professor Ritchie said.

"But it turned out that the quolls were really dependent on a type of beetle larvae which are affected by changes in weather."

The grubs were depleted and so were the quolls, with those left shifting their diet onto other insects and small mammals.

"So there are more knock-on effects for other species too," Professor Ritchie said.

Alpine insects face perfect storm of threats

Long-term trends in the total numbers of bogong moths are unknown.

According to Dr Umbers there isn't much counting of anything going on in the Alps due to a lack of resources.

"The biggest tragedy is we don't have the data or support for longitudinal research to know if there's been changes in alpine insects," she said.

"Even the grasshoppers that I study, I can't tell you whether there has been a change over the past 12 years, I have no data."

While insect populations around the world are suffering from human disturbance and climate change, Dr Umbers said alpine insects, as well as other alpine wildlife, were facing their own perfect storm of threats.

"Feral animals like foxes and cats are known to eat insects, and feral horses damage the alpine habitat," she said.

"The vulnerability of the Australian Alps to climate change is the worst in the world because we've got these short little mountains so when it gets warmer, there is nowhere for these cold-adapted species to go."

Professor Ritchie said insects are right on the frontline of climate change.

"People think insects are resilient to climate change, but species in rainforests or mountains for example have a limited thermal tolerance, so even small increases in temperature can knock insects for six," he said.

"I think people are realising more that insects are actually being directly affected by changes in weather and climate, not just things like pesticides.

"If it continues down this path we're likely to see these quite diabolical extinction cascades where you lose one species after another because everything's interconnected," Professor Ritchie said.