Solving the bogong moth mystery

By Tim the Yowie Man 2 July 2020
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Painstaking research is shedding light on one of nature’s epic insect migrations.

IT’S A PUZZLE that’s mystified scientists for decades. Just how does the bogong moth navigate each spring from the hot and arid plains of southern Queensland, north-western New South Wales, western Victoria and South Australia to the cool environment of the Snowy Mountains, and then complete the return journey 3–4 months later?

During some years the much-maligned moths are blown off course or attracted by city lights, as famously occurred in September 2000 when the opening ceremony of the Sydney Olympic Games was besieged by the flying insects.

When, three years later, they descended in their millions on Parliament House in Canberra during a visit by US president George W. Bush, some politicians, annoyed by bogongs swimming in their coffee, called for the use of insecticide to discourage the multitude of interlopers.

Moths that successfully make it all the way to the Australian Alps, a remarkable journey that’s longer in some cases than 1000km, shelter and feed at lower altitudes until the snow melts and they can access summer hide-outs on the roof of Australia. Here, in myriad caves and crevices, the bogongs aestivate in a summertime state of dormancy similar to winter hibernation before flying back in autumn to their birthplace to breed.

“It’s one of the world’s extraordinary animal migrations,” says Dr Eric Warrant, an Australian biology professor at Sweden’s University of Lund. After 10 years of research, he is close to solving the mystery of how the bogong, with a brain smaller than a grain of rice, navigates this long-distance return flight.

In 2015 Eric and his colleagues from Lund, and the Australian National University (ANU) where he is a Visiting Fellow, partly unlocked the secret when they discovered that, as for sea turtles and birds, the bogong can use Earth’s magnetic field as a compass.

However, this groundbreaking research revealed that the moth was also using visual cues to aid navigation.

“It’s akin to a bushwalker using a compass to find a direction, then looking for something obvious, such as a hill in the desired direction, to walk towards, thereby allowing them to put the compass back in their pocket and continue walking towards this visual beacon instead,” Eric explains. “But whether these visual cues for moths were terrestrial, celestial or a combination was a mystery.”

Because the bogong moth migrates at night, Eric and his team decided to look first at obvious celestial cues, such as the Moon and stars. And it wasn’t long before they hit the jackpot. “We now have some very promising evidence that supports the possibility that the moths are able to use the starry night sky as a compass,” he reveals.

“We’d known from our previous research that African ball-rolling dung beetles use the night sky for keeping a random straight line while rolling dung over short distances at night, so we thought we’d test it on the bogong moths.”

To test this hypothesis, Eric and his Lund colleague Dr David Dreyer captured live moths during their migration south from Queensland to the Snowy Mountains, tethered them to tungsten pins, and then, in a specially designed non-magnetic flight simulator, subjected them to a series of tests that involved projected Australian starry skies.

Eric and David conducted these experiments in a magnetic vacuum at a purpose-built laboratory near the NSW town of Adaminaby so the moths couldn’t use Earth’s magnetic field to navigate. “While this research is still some way from being finalised, initial results indicate that the Australian night sky might be a crucial part of the moth’s navigational compass,” Eric says.

To further understand the bogong moth’s navigation abilities, and just how they use both the magnetic field and the night sky, Eric and colleagues Andrea Adden and Dr Anna Honkanen are now undertaking a series of physiology experiments.

“We have a non-magnetic electrophysiology rig where you can insert electrodes into nerve cells inside the brain and record the responses of these cells to various kinds of stimuli,” Eric explains. “We are looking for cells that respond to rotations of the night sky and might be involved in a possible star compass system, which is really quite exciting.”

While he and his team may be close to solving one of the most enduring mysteries of the bogong moth, Eric holds serious concerns for the species’ long-term future.

“In a good year, more than 4 billion of the moths arrive in the Australian Alps, but over the last 40 years there has been a decreasing trend in the number of aestivating moths, with numbers especially collapsing since 2017,” Eric says.

Dr Ken Green, an alpine ecologist in the College of Asia and the Pacific at ANU, offers a reason why. “In the Murray- Darling Basin, the main winter breeding ground of bogong moths, changes in farming practices, such as increasing land clearing for crops, which currently removes around a quarter of a billion moths annually from the mountains compared with pre-European levels, and an elevated use of insecticides, may have facilitated the decline in bogong moths observed from 1980.

“The crash in moth numbers in the last three years is likely due to severe drought in the moths’ breeding grounds,” adds Ken, who is investigating the effect that increasing alpine cave temperatures are having on the bogong moth population. “Even though we have found little evidence that increasing global temperatures per se are responsible for the decline in bogong moths, the Australian climate has nonetheless become continuously drier and warmer over past decades, possibly hampering the survival of immature stages in the breeding areas and confining adult aestivation to gradually higher elevations.”

These increasing temperatures could push the bogong moth to the brink of extinction. “If warming alpine conditions continue, this would lead to fewer suitable aestivating locations and a further collapse in bogong moth numbers,” Eric explains. “Because the moths are a crucial food source for alpine marsupials and birds, as well as a major long-constituent of soil, this could have serious negative knock-on effects for the entire alpine ecosystem.”

Perhaps this scenario is one those policymakers who themselves migrate to Capital Hill every parliamentary session might contemplate next time a bogong moth is found swimming in their cappuccino.

This article is featured in issue 157 of Australian Geographic.