May 2022

The Nation Reviewed

Where did all the bogongs go?

By Anthony Ham
Illustration by Jeff Fisher
The drastic decline of the bogong moth could have disastrous ecological consequences

The bogong moth always knew how to make an entrance. At Elton John’s concert in Melbourne in 1993, the performer swallowed a bogong moth during his encore, stormed off the stage and wouldn’t return. At the closing ceremony of the 2000 Sydney Olympics, moths drawn by the stadium lights descended in a cloud so large that weather forecasters mistook it for an approaching storm. And on more than one occasion over the years, bogong moths have swarmed into Parliament House in Canberra, setting off smoke alarms and forcing an undignified evacuation of parliamentarians. They were Australia’s version of the passenger pigeon with one notable variation: they swarmed across the skies in such numbers that they blocked out not the sun, but the moon.

That all changed six years ago. Until 2016 or 2017, there were an estimated 4.4 billion bogong moths. Since then, their numbers have crashed by an estimated 99.5 per cent. On the International Union for Conservation of Nature’s Red List, the species leapt from “least concern” to “endangered” in one go, an almost unheard of change.

Born underground as caterpillars in southern Queensland, the young moths fly to southern New South Wales and Victoria’s Alpine region, covering a distance of more than 1000 kilometres. Although the moths have never before flown the route they will follow, they are, according to Dr Ken Green, of the Australian National University and the country’s leading bogong moth expert, able to navigate by using the Earth’s magnetic fields, switching their sensors on when they need to find their way, then off to conserve energy. On clear nights, they can navigate by using the Milky Way. As Green told Guardian Australia in December last year, they can “pick where the moon is even when it’s seven degrees below the horizon”.

Once they arrive in the south they seek refuge in caves where, when their numbers are healthy, they gather at a density of 17,000 bogong moths per square metre; each one weighs barely one third of a gram, or less than a Tic Tac. Some fly to Mount Bogong, Victoria’s highest summit (“bogong” is derived from the north-east Victorian Dhudhuroa people’s name for the moth).

The bogong moth is not just remarkable for its own sake. We tend to think of apex predators as the ones that hold ecosystems together, but here it is a moth that does so. “The bogong moth is the keystone species in Australia’s Alpine zone,” says Dr Marissa Parrott of Zoos Victoria. “They appear in the scats of animals all across the food chain – everything from rodents, small carnivorous marsupials, frogs and several endangered lizards, right up to large mammals such as dingoes. The moths are also the second-biggest influx of nutrients into the Alpine zone, surpassed only by the sun.”

In 2021, archaeologists found traces of 2000-year-old bogong moths on grinding stones in Victoria’s Alpine region. The find is believed to be the oldest evidence of insects being used as a source of food for humans anywhere in the world.

The most obvious impact of the moth’s falling numbers is upon the critically endangered mountain pygmy possum, of which barely 2000 survive in the wild. In some areas, the bogong moth makes up 80 per cent of the possum’s diet. One recent study found that 95 per cent of these possums recently lost their young due to a lack of food.

But the bogong moth’s decline is about more than inter-species ecological interactions – it also ripples out across the landscapes they inhabit. This is a cautionary tale of what can happen when just one type of insect disappears from an ecosystem.

If bogong moths disappear, or their numbers remain at current levels, says Parrott, “it’s going to have massive ecosystem effects. If you don’t have the nutrients in the soil, you don’t have the plants, you don’t have the invertebrates, you don’t have the animals. By taking this one piece out of the puzzle in the Alpine zone, you could actually unravel the whole ecosystem.”

Parrott says the bogong moth’s demise is evidence of “the insect apocalypse”, which is happening around the world. “We know about it, but the invertebrates just don’t get the attention. So many of our crops need insects to pollinate them. Our forests need insects to pollinate the flowers. Our entire world really runs on insects pollinating the flowers, feeding everything. It’s a massive global issue.”

Scientists are still piecing together the causes of the decline. At the heart of the issue are changes in agricultural practices, especially the use of pesticides, the increased farming of monocrops, and the propagation of cotton and rice, requiring flooding the soil; where such flooding occurs, the caterpillars are unable to emerge from below ground to become moths. Light pollution along moth migration routes is another problem: the bright lights of Canberra and Sydney are what Parrott calls “giant light traps” that draw millions of moths to their deaths. And, more than anything else, droughts across the Murray–Darling Basin impact upon bogong moth numbers as they are unable to survive for sustained periods of hot, dry weather.

Scientists aren’t giving up hope. Despite the bogong moth’s catastrophic decline, enough survive from which to mount a recovery. Each female bogong moth can lay around 2000 eggs, and under the right conditions enough of these should survive to allow the population to begin its recovery. Even when numbers are healthy and they descend upon a landscape, bogong moths are not considered to be a major agricultural pest. And in the insect world, there are precedents, including from a much lower base: through intensive conservation measures, the Lord Howe Island stick insect went from just two to 18,000.

Programs designed to help the bogong moth recover, and to help those species that rely on the moth, are already under way. Zoos Victoria has developed a substitute for bogong moths as food – the “bogong bikkie” – that can provide for mountain pygmy possums while moth numbers are low. Zoos Victoria has also pioneered its Moth Tracker app and “Lights Off for the Moths” programs, which help to spread awareness. “The bogong moth story is an example of how people can help in their own lives and actually really make a difference,” says Parrott. “We need more people engaged with this and realising that what you do in your own backyard in Queensland or northern New South Wales can actually affect a possum on a mountaintop a thousand kilometres away.”

At the same time, there is no blueprint for a decline as rapid and as far-reaching as that of the bogong moth. “This is an extreme case. To have that decline from billions to nearly undetectable … that is terrifying. I don’t know of any comparable example in invertebrates where there has been such a decline in such a short period of time.”

And it’s impossible to predict what would happen were the bogong moth to fall extinct. “The scary thing,” says Parrott, “is that no one really knows.”

Anthony Ham

Anthony Ham writes about wildlife, conservation and current affairs for magazines and newspapers around the world.

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