Clement Bataille and his team in the field. (Courtesy of Clement Bataille)
On an early October morning in 2013, Gerard Talavera was walking along a beach in French Guiana when he was surprised to spot a flock of painted lady butterflies in the sand.
An entomologist from the Botanical Institute of Barcelona in Spain, Talavera knew these orange, gold and brown insects travelled long distances, ranging far and wide. But they weren’t supposed to range as far as this South American beach.
It was a mystery. The butterflies’ tattered wings offered a clue though: these painted ladies had just completed a marathon journey.
It would take a decade and an international team of researchers, including University of Ottawa isotope geochemist Clément Bataille, to figure out how these insects found their way to this unlikely spot. In a paper published in June in the journal Nature Communications, the researchers showed the butterflies flew – and glided with the help of favourable winds – at least 4,200 km over the ocean from West Africa to French Guiana.
The team figured this out using various techniques, including sequencing the DNA of pollen grains the butterflies were carrying and pinpointing two species of plants that only grow in tropical Africa. This showed the butterflies had dined on nectar from African flowers before launching themselves across the Atlantic Ocean.
The researchers also sequenced the butterflies’ genomes and compared them to populations in other parts of the world, which demonstrated the painted ladies on the French Guianese beach were better matched to those in Africa and Europe, and not North America. This in turn also suggested an oceanic journey, which the researchers believe took five to eight days.
What’s more, Bataille’s analysis of hydrogen and strontium isotopes in their wings showed the butterflies were probably born in Europe, extending their epic migratory flight to more than 7,000 km.
As Bataille explains, in their larval stage butterflies feed off certain plants with unique ratios of isotopes. The ratios reflect things such as local climate patterns, elevation, temperature and soil composition, and these ratios are then “locked into” the adult butterflies’ wings.
The team combined Bataille’s isotope maps with maps of suitable weather and habitat for larval growth. This showed the butterflies were probably born in western Europe, possibly France, Ireland, the United Kingdom, or Portugal.
“We have no idea if it’s a regular occurrence, but since we published our study, we’ve had people calling to say they’ve seen them in Barbados and the Antilles,” says Bataille. “So I think it’s much more common than we believe – maybe not crossing the ocean, but long distances.”
This is important, he says, because it highlights how migratory insects can impact ecosystems and agriculture far from home as they transport pollen, fungi and even plant diseases.
“We have a full table of elements – a full keyboard to play with,” says Bataille. “Anything that exists on the surface of the Earth has an isotopic signature that we can relate to spatial patterns to geolocate where they’re coming from.”
Leonard Wassenaar, an isotope specialist with the University for Continuing Education Krems in Austria, says it can be incredibly difficult to reconstruct a flying insect’s origins when they cross large bodies of water via high altitude winds.
“Even more challenging are rare instances when large groups of painted ladies or other species suddenly appear where they are not normally found,” says Wassenaar, who works with environmental stable and radioisotopes to study freshwater and terrestrial ecosystems.
“In this paper, Sr and H isotopes of wings were used in combination with genetics, pollen, and wind trajectories to rule out where painted ladies did not come from. And like a good Sherlock Holmes mystery, eliminating the improbable origins leaves only the plausible origins,” he adds.
But the technique isn’t just for understanding migratory patterns. It can help pinpoint the provenance of agricultural products and show where bombs were assembled. It can also help police solve cold cases by pinpointing the origins of people dead and alive.
“For example, I was born in Guadeloupe which has special geology – it’s volcanic, tropical and is affected by aerosols from the ocean,” says Bataille. “So my teeth look very different than someone who has lived in Canada all their life.”
That said, isotope geolocation isn’t a perfect tool. In the case of the painted lady butterflies, Bataille worked with the ratios of just two elements – more would have been helpful. It would, he notes however, also have been more expensive.