Insect Migration Mysteries: Butterflies Crossing Oceans and Moths Navigating by Stars
Trillions of insects embark on epic, largely unnoticed journeys every year across mountain ranges, deserts, and seas. It is only now, as their numbers suffer huge declines, that scientists are actively tracking these movements to unravel the mysteries of insect migrations.
Historic Observations in the Pyrenees
On a cloudless sunny day in October 1950, ornithologists Elizabeth and David Lack stood on a mountain pass in the Pyrenees and witnessed a once-in-a-lifetime spectacle: clouds of migrating insects. At the Puerto de Bujaruelo pass, up to 500 butterflies fluttered past them every hour through the 2,200m-high passage on the French-Spanish border. By mid-afternoon, dragonflies skimmed through, outnumbering butterflies by ten to one, with thousands of tiny flies filling the spaces between.
This day marked the first record of fly migration in Europe, revealing skies packed with tiny travellers on remarkable long-distance journeys unknown to science. Decades would pass before the concept of insect migrations was widely followed up by researchers.
Butterflies Crossing the Atlantic Ocean
Today, we know that insects, many with wings smaller than a human fingernail, are among the planet's most prolific migrants. In 2013, Spanish entomologist Gerard Talavera made a groundbreaking observation on a beach in French Guiana: tired-looking painted lady butterflies, not thought to live in South America. This launched a decade-long investigation to answer whether butterflies can truly cross the Atlantic Ocean, defying existing understandings about insects' capabilities.
Talavera assembled a team of biologists, geneticists, and atmospheric scientists to reconstruct this seemingly impossible journey. The findings, published in Nature Communications in 2024, represent the first direct proof that any insect had crossed the Atlantic. Although exceptional, Talavera notes, "it's likely it happens quite often." Painted lady butterflies migrate thousands of miles, using environmental cues like day length and temperature, and strategically harnessing winds to reach breeding grounds where females can lay over 1,000 eggs.
Moths Navigating by the Stars
Not all insect migrations are invisible. For thousands of years, people have known that bogong moths migrate in Australia, with First Nations communities smoking them out of caves for festivals. By the 1970s, scientists suspected these moths used stars to guide them, similar to night-migrating birds.
Research confirmed that bogong moths, with brains one-tenth the size of a grain of rice, use the starry sky as a compass over hundreds of kilometres nightly. "That was a truly new discovery," says Dr David Dreyer from Lund University. On cloudy nights, moths fall back on Earth's magnetic field, but light pollution from towns may disorient them, causing swarms in cities like Canberra.
Bogong moth populations have seen catastrophic declines, driven by severe drought, habitat loss from agriculture, and pesticide use. A 99.5% drop during the 2017 drought impacted species like the mountain pygmy possum, which relies on moths for food. Although partial recovery has occurred, future droughts threaten their survival.
Innovative Tracking Methods
Studying insect migrations is uniquely challenging due to their small size and abundance. Scientists have employed creative methods, such as tracking hawk moths in aircraft or using "glitter bombing" with fluorescent powder. Dr Jason Chapman from the University of Exeter revolutionized the field by using radar and a blimp to quantify insect movements high above.
His team set up radars and a net-suspended blimp at Cardington airfield in Bedfordshire, fishing insects from the sky over seven months. They found more than 3 trillion bugs travel over southern England yearly, with about 5% of UK insect species being migratory, including ladybirds and aphids. These migrations often unfold over generations, with no individual completing the entire journey.
Ecological Impacts and Declines
Seventy years after the Lacks' observations, scientists returned to the Pyrenean pass, finding 17 million insects fly through annually. An estimated 90% are pollinators, moving pollen hundreds of kilometres to enhance plant genetic diversity. Migratory insects like hoverflies decompose waste, recycle nutrients, and control agricultural pests, while providing food for birds.
However, climate breakdown, habitat loss from industrial farming, and pesticide use are causing sharp declines. A German study noted a 97% drop in aphid-eating migratory hoverflies over 50 years. As scientists finally unravel these migration mysteries, insects are vanishing, highlighting urgent conservation needs.
