Stand outside one spring night in southeastern Australia and you may be able to witness one of the biggest insect migrations in the world, as billions of brown Bogong moths (Agrotis infusa) flit across the sky.
Each year in the spring, the moths migrate around 620 miles (1,000 kilometers) north to the Australian Alps, where they can avoid the heat by hiding in cool caves until the fall, when they return to their breeding grounds. While migration is not uncommon in insects, the Bogong moth’s migration has been of particular interest to experts — how does a moth travel to a place it’s never visited before?
Researchers believe they now have the answer: stellar navigation. This would make the Bogong moth the first insect to use the stars for long-distance navigation as it makes its extended migratory journey.
Stellar navigation has a long history for both humans and animals, from ancient Polynesians to migratory birds. Given the stars’ dominance in the night sky, it’s not surprising for experts to think that other animals, like insects, may also use these twinkling lights for navigating.
“We knew from a previous study that the moths can use the geomagnetic field to navigate, but they only seemed to be able to do so in combination with visual landmarks, so we were thinking about what kind of landmarks these could be,” explained Andrea Adden, a researcher at the Francis Crick Institute in the United Kingdom.
“If you go to the Australian bush, where these moths live, and look around you at night, one of the most obvious visual landmarks is the Milky Way, which is always visible to some extent, independent of time of night and season,” Adden said. “We know that daytime migratory insects use the sun, so testing the starry sky seemed an obvious thing to try.”
Clambering into caves
To test whether these moths are truly using the stars to navigate, the researchers captured several using a light-trap. This required the team to traverse into the dark, cold caves where the moths were resting during their migration, which, for some of the team, proved to be too challenging.
According to Eric Warrant, a researcher at the University of Lund in Sweden and the leader of the project, “One of the most embarr***ing [stories] was when Lena Nordlund from Swedish Radio (who was with us in Australia doing a doblockentary) asked why I always sent [the] youngsters in the cave and I always sat outside. I was forced to admit I was claustrophobic and was scared of going in — something that of course she included in the doblockentary.”
Though Warrant was not comfortable going into the caves, that didn’t stop his collaborator, David Dreyer, also a researcher at the University of Lund, from challenging Warrant to a little competition to see who could catch the first Bogong moth of the migratory season. This competition lasted over a decade, with 20 different seasonal opportunities to compete.
“[I] dominated this competition, [winning] 19 migratory seasons,” Dreyer explained jokingly. “[My] 19 wins would remain unreported, until now. Justice at last.”
Testing moth flight
After capturing the moths, the team then placed them in a planetarium-like flight simulator, which included multiple projectors that could be programmed to give specific scenery. The simulator also blocked Earth’s magnetic field, forcing the moths to try to navigate in the simulation by their eyesight alone.
The researchers also attached electronic sensors to the moths to measure their brain activity. As Bogong moth brains are around the size of a grain of rice, adding the sensors was incredibly time-consuming.
“Studying the neural basis of how these moths navigate reveals new processing mechanisms in the insect brain,” Adden noted. “Even though human brains and insect brains are obviously very different, it often turns out that the computational principles are remarkably similar, so perhaps we can even learn something from moths that, one day, helps reveal something about the human brain.”
Once the moths were prepared, the researchers waited for evening in the outback, and then began to test the moths by recording their virtual flight paths in the simulator.
“We continued this process until we had used all the prepared moths,” Dreyer said. “The following morning was dedicated to data blockysis. This routine continued until every moth from the previous catch had been tested — after which we would head out to catch a new batch.”
Stuck in the field
While studying the Bogong moths, the COVID-19 pandemic hit Australia, forcing a lockdown. For Adden, this meant being stuck out in the field.
“A colleague and I were just wrapping up the field season in early 2020 when Australia entered its first Covid-19 lockdown, and the two of us were locked down at the field station for about a month,” she said. “This wasn’t as bad as it might seem — with no other humans in sight, we spent our days blockyzing data, watching local wildlife and learning to sew.”
Adden even took the time to practice her astrophotography skills, taking photos of the very night sky her research subjects leveraged to navigate.
Seeing long-distance navigation
After years of blockysis, the researchers found that the Bogong moths fly in the seasonally appropriate direction (north or south) depending on the stars in the night sky, suggesting that they do in fact use the stars to guide them.
“The stars are a very consistent cue. Even though the starry sky rotates throughout the night, the brightest part of the Milky Way is always in the South of the Southern celestial hemisphere,” said Adden. “That makes it a very stable comp*** cue that is reliable not just across nights and seasons, but across centuries.”
From the moth’s brain activity, the team also saw responses specific to certain rotations of the night sky in the flight simulator, and determined that their brains were the most active when they were “flying” in the right direction of their migration.
While the Bogong moth is not the only insect to use the stars for guidance, it is the first to do so for long-distance journeys, scientists said.
“A previous study established that dung beetles use the stars to guide short-distance movements, but the beetles only travel a short distance (maybe 5-20 meters) as opposed to flying 1,000 km during a migration,” Ken Lohmann, a researcher at the University of North Carolina, Chapel Hill who was not involved in the study, told Space.com.
Preserving a species
Studying how animals like moths navigate is not only fascinating, but can also help ensure the moth populations remain at a healthy level.
“The Bogong moth population declined dramatically after the recent drought and 2020 bushfires,” explained Adden. “Understanding how their migration works, and which cues they use to navigate, may help us protect these insects, which in turn helps the entire alpine ecosystem of which the moths are an integral part — e.g., as food for pygmy possums and all sorts of birds during the summer months.”
Part of that conservation work is looking at the role urbanization and, more specifically, light pollution plays in affecting the moths’ migratory path.
“Light pollution may well be a problem for Bogong moths during their migration,” Adden said. “On their way from Southern Queensland to the Australian Alps, they p*** several major cities, such as Canberra, which can be disorienting and trap the moths. In fact, this happened several years ago, when a cloud of moths briefly took over the Australian Parliament.”
While the Bogong moth shows the ingenuity of animals, for researchers and conservationists alike, understanding the animal navigation process as a whole is key to understanding their lifestyle, and therefore being able to protect them further.
“A central lesson of animal navigation is that species almost always have multiple ways to guide themselves,” said Lohmann.
This study was published online today (June 18) in the journal Nature.
