Now they're so scarce that endangered mountain pygmy-possums reliant on them for food have been found starving, their babies dead.
Until recently, the moths were so plentiful that their epic spring migration from the plains of southern Queensland, NSW and Victoria to cool caves in the Australian Alps was an annual annoyance along the eastern seaboard.
In Canberra, dense swarms would routinely descend on Parliament House at night when its bright lights were shining.
But in 2017 and 2018 the moths largely failed to arrive.
In December the species was among 124 new Australian plants and animals added to a global red list of threatened species compiled by the International Union for Conservation of Nature.
The mountain pygmy-possum — Australia's only hibernating marsupial — has been on the same list since 2008.
It's in the critically endangered category, one notch below extinct in the wild.
There are thought to be fewer than 2000 left in three populations in the alpine and sub-alpine rocks and boulders of the Bogong High Plains, Victoria's Mt Buller and Mt Kosciuszko in NSW.
Threats to the possum include climate change, loss of habitat and predators like feral cats and foxes.
Some conservationists fear the loss of springtime food, crucial for them to refuel and rear healthy young, could be enough to seal the possum's fate.
After the moths stopped coming in 2017 and 2018, many female mountain pygmy-possums were found underweight and unable to produce enough milk.
In some cases that resulted in the loss of entire litters.
The Australian Conservation Foundation said it was sobering that the bogong moth and the possum now sit side by side on the IUCN red list — the leading scientific assessment of extinction risks, globally.
ACF nature campaigner Jess Abrahams said it was a sad example of the world's extinction crisis and how the loss of one species frequently causes or contributes to the loss of another.
The moth's population collapse a few years ago was staggering, with numbers shrinking by an estimated 98 per cent. Scientists believe a mix of extreme droughts, pesticides and changes in agricultural practices was to blame.
Caves that once heaved with tens of millions of moths were empty when researchers went looking for them.
There was some evidence of a modest population recovery last year, but experts say numbers remain very low.
As awful as the impacts have been for the mountain pygmy-possum, Mr Abrahams said they were at least on the radar. But he worries about the other effects that are yet to reveal themselves.
"There's birds, and reptiles and other marsupials in the mountains that also rely on the bogong moth," he said.
"In food chains you have these flows of energy and the bogong migration was a massive flow of high nutrients up into the mountains that helps these animals that have survived a harsh winter be fully fed for spring and summer.
"We can see the the first few cascade effects, like starving pygmy-possums, but who knows where these flow-on effects end.
"Of the new Australian species added to the red list, 54 are classed as vulnerable, endangered or critically endangered, with another 70 of concern, but at a lower level.
“Australia's largest bat — the grey-headed flying fox, with a wingspan of over one metre — has been listed as vulnerable.”
The megabat, which is also among the world's largest, has been in trouble for a while. It's found in Queensland, NSW and Victoria but is suffering from the effects of habitat clearing, and culling by farmers before the species was protected.
Mr Abrahams said Australia could not afford to lose the vast number of plants and animals that were already in deep strife, with the full effects of locked-in climate change yet to play out.
A UN-backed report, released two years ago, warned the window to save the planet's biodiversity was closing, with one million species at risk of extinction.
"Australia has a terrible record on species extinctions," Mr Abrahams said.
"We are blessed with some of the most incredible places, beaches, animals and plants on the planet. But we are losing them before our very eyes, and this red list is evidence of that."
He said urgent action was needed to turn around the extinction crisis and that demanded strong federal leadership on climate change, better national environment laws, and crucially the money that's required to nurse species back from the brink.
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