The bogong moth migrates up to 1000 km each year to a few select caves in the Australian Alps – and now we know how it does it. Photo: Ajay Narendra, Macquarie University.
Australia’s bogong moth is officially the first known invertebrate to use the stars as its guide on its long, annual migrations.
The “world-first discovery” made by a team of international researchers from the Australian National University (ANU) – as well as the Lund University in Sweden, University of South Australia (UniSA) and others – was published in the Nature science journal earlier this month.
“Until now, we knew that some birds and even humans could use the stars to navigate long distances, but this is the first time that it’s been proven in an insect,” said Lund University zoology professor Eric Warrant, also a visiting fellow at the ANU and an adjunct professor at UniSA.
“Bogong moths are incredibly precise. They use the stars as a compass to guide them over vast distances, adjusting their bearing based on the season and time of night.”
Each spring, billions of bogong moths (agrotis infusa) emerge from breeding grounds across southeast Australia and fly up to 1000 km to a small number of caves and rocky outcrops in the Snowy Mountains – a journey they’ve never made before.
The moths lie dormant in the cool, dark shelters throughout summer and in autumn make the return journey to breed and die.
The researchers used sophisticated flight simulators and brain recordings in controlled, magnetically neutral environments to see how the moths do it.
A wall of bogong moths “aestivating” – the opposite of hibernating – in a cave in the Australian Alps. Photo: Eric Warrant.
Under natural starry skies and no magnetic field, they found the moths consistently flew in the correct migratory direction for the season – southward in spring, northward in autumn.
When the starry skies were rotated 180 degrees, the moths reversed direction accordingly, but when the stars were scrambled, their orientation vanished.
“This proves they are not just flying towards the brightest light or following a simple visual cue,” Prof Warrant said.
“They’re reading specific patterns in the night sky to determine a geographic direction, just like migratory birds do.”
Recent bogong moth population numbers. Image: Peter Casey.
There’s more – even when the stars were obscured by clouds, the moths still kept their direction.
The team discovered the moths were relying on the Earth’s magnetic field at this point, due to “specialised neurons in the moth’s brain … that fire most strongly when the moth is facing southwards”.
“It’s a remarkable example of complex navigational ability packed into a tiny insect brain,” Prof Warrant said.
It’s hoped the discovery could inform technologies in robotics, drone navigation and even conservation strategies for species threatened by habitat loss or climate change.
Bogong moth populations have declined sharply in recent years to the point they’re now classified as vulnerable.
Drought, as well as disruptions to their migration route such as light pollution and habitat destruction, are thought to be largely to blame.
However, recent surveys suggest numbers are slowly picking up again, even if it’s yet to reach the level of two decades ago.
In 1990, Parliament House was forced to turn off many of its exterior lights during the migration season and in 2000, the Bureau of Meteorology mistakenly issued a forecast for rain after its weather radar picked up a swarm of moths.
Parliament House now has UV filters fitted to all its external lights. Photo: DPS/AUSPIC.
Prof Warrant said we should still expect the moths to “accidentally” end up over Canberra every few years, mostly due to strong winds.
“Before ascending to their final cave destinations – which at this time of year are completely covered and inaccessible by snow – moths generally head to lower elevation areas in the mountains to temporarily ‘camp’,” he explained.
“When moths invade Canberra, what they are really doing is simply camping as they would’ve done elsewhere lower in the mountains.
“After a week or two, the moths leave Canberra and continue their journey upwards into the Alps.”
In recent years, a solution has also been found for both Parliament House and Telstra Tower which involves covering the buildings’ lamps with with ultraviolet (UV) light filters.
“The UV part of the light spectrum … is the most attractive wavelength range for moths and other nocturnal insects,” Prof Warrant said.
“This has helped a lot to reduce the number of moths attracted to these places.”
The full research paper, entitled “Bogong moths use a stellar compass for long-distance navigation at night” is available on the Nature website.
