The mystery of butterflies that emerged from across the Atlantic | Climate and Environment

In October 2013, Gerard Talavera, a researcher at the Botanical Institute of Barcelona, ​​found three specimens of the carder butterfly (Vanessa Cardui) on a beach in French Guiana, South America. It was a surprising find, since this lepidopteran, so named because it feeds mainly on thistles, is present in Europe, Africa, Asia and North America, practically everywhere on the planet except Australia, Antarctica and the South American continent. How did these butterflies get to French Guiana? “Their wings were badly damaged and they were lying in the sand, unable to fly,” Talavera recalls of the special moment when he began an investigation that lasted more than 10 years around the world, until he managed to reconstruct the incredible journey of these insects across the Atlantic Ocean, a work published this summer in the journal. Nature.

Because of their proximity, the most logical alternative would be that they had arrived in French Guiana from North America, flying over Central America and its tropical jungles or over the Caribbean Sea. However, for the Catalan entomologist, this common butterfly with a wingspan of 5 to 9 centimeters, with wings in brown, orange, black and white, was already an old acquaintance and had a somewhat more radical explanation for its mysterious journey: leaving Africa. A year earlier, Talavera, together with another CSIC researcher, Roger Vila, had put forward the hypothesis that Vanessa Cardui migrate annually from our continent, Europe, to sub-Saharan Africa. This idea seemed incredible, since it would mean that this small winged insect would have to cross the Sahara Desert, but it was confirmed in 2016. “The Cardera performs a migratory circle consisting of several generations, that is, every month a new generation of butterflies replaces the previous one (adults live for three to five weeks) and makes stops. So every year they travel 15,000 kilometers between Scandinavia and the African equator,” Vila summarizes.

Although it didn’t make intuitive sense, when Talavera discovered the specimens from French Guiana, he immediately thought of Africa. “In 2013, knowing or sensing the existence of massive migratory movements across the Sahara, I thought it made the most sense that the three butterflies had come not from the United States but from Africa,” he says. At the time, it was just a hunch. And the question was what types of winds would have to be created to make the butterflies’ hypothetical flight across the Atlantic possible.

The first thing Talavera did after returning from his trip was to study the winds that blew in the area before the discovery of the Lepidoptera. With the help of researcher Eric Toro Delgado, in the following years they managed to develop several complex models that combined wind circulation with the physiological abilities of the butterflies. Thus, they were able to reconstruct the trajectory of the air currents thanks to the HYSPLIT computer program, owned by the US National Oceanic and Atmospheric Administration (NOAA). “The results showed that a few hours before the butterflies were spotted in French Guiana, air currents arose on the African coast, near Mauritania and Senegal. This was the same throughout the altitude layer from 500 to 2000 meters. And the average speed of these winds was 27 kilometers per hour,” Toro Delgado summarizes.

“We already knew that carders could fly at such altitudes during migration. They were using the trade winds, the same ones that Christopher Columbus used to reach America,” says Vila. But even if they had shown that the winds were in favor of their crazy theory of a transatlantic flight, that wouldn’t have been enough evidence to suggest that the three individuals from French Guiana had accomplished such a feat. Someone could have bought them online and left them there. How could they prove they came from Africa?

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To continue putting the pieces of the puzzle together, they turned to genetics. Their next step was to genetically compare these three individuals with others from other parts of the world. To do this, they spent three years collecting samples Vanessa Cardui in more than 30 countries (USA, Mexico, Canada, Hawaii, Senegal, Benin, Ivory Coast, Malaysia, Japan, etc.), until the DNA of 1,200 samples was sequenced. The work on sequencing the genome of these populations was carried out in a molecular laboratory led by the prestigious biologist and entomologist Naomi Pierce at Harvard University (USA), and subsequent analyses were carried out in Barcelona. “In 2018, we had the genetic results ready, and we found that the samples from French Guiana belonged to a population migrating between Europe and Africa,” says Talavera.

Africa was starting to seem like a serious option, but there were still some unanswered questions about demonstrating an oceanic journey. There was another possibility. Was it possible that they were descendants of other butterflies that had made the journey, but at an earlier point in the past? To prove that the three French Guiana specimens had crossed the Atlantic themselves, the researchers had to develop their own technology to track possible pollen fragments in the specimens.

Three butterflies, placed in separate envelopes, were sent to the W. Schafer Institute of Botany in Krakow, Poland, to the lab of Tomasz Suchan, a researcher who specializes in phylogenetics and phylogeography. Never before had DNA from migratory butterflies been sequenced using the metabarcoding method. But, as Gerard Talavera puts it, the three samples were in the hands of a “real lab wizard.” The first piece of good news was that the specimens were loaded with pollen, despite the “long journey they had hypothetically undertaken.” Secondly, the pollen also pointed to Africa. “We found several African plants. The most abundant species we found was Hyera senegalensisand then Ziziphus spina-christi“two shrubs that are only found in the sub-Saharan region and are not found anywhere else in the world,” explains Suchan. “These are species that bloom from September to November in Senegal, Benin, Ivory Coast, etc. Right in the area and at the time they had to leave for South America,” adds Roger Vila.

The pieces of the puzzle were falling into place, but there were still some mysteries to be solved. Were the butterflies on the beach in French Guiana born in Africa or Europe? The research continued in Canada between 2018 and 2023, where isotope geolocation specialists Clement Bataille and Megan Reich from the University of Ottawa made a truly astonishing discovery by analyzing something very small: the hydrogen and strontium isotopes present in the wings of the butterflies. “When a caterpillar feeds on a host plant, the isotopic composition from which it is formed is incorporated into the tissues of the caterpillar, and when the resulting butterfly emerges from the chrysalis, this isotopic composition is retained in the wings,” he explains. “Butterfly wings retain the chemical signature of where they were born. Our results show that the origin of the three butterflies studied corresponds to the territories of France, Spain and Portugal,” Bataille concludes.

Now Gerard and his team of collaborators could claim to have demonstrated the first transatlantic journey Vanessa CarduiAt least three of these specimens traveled 4,200 kilometers, from the African coast between Western Sahara and Senegal to French Guiana. Verifying such an amazing journey is not only inconclusive, but also opens up countless new questions. What does such a transoceanic flight entail for a butterfly? Researchers knew that Lepidoptera have orientation systems that allow them to determine both magnetic north and the position of the sun depending on the time of day. “They must have combined moments of active flight with other wing beats sufficient to stay aloft,” says Talavera.

Although they did not see the butterflies flying in the middle of the ocean, the researchers even calculated energy models that show the cost of a butterfly’s energy and the distance it can travel. “Without the help of the wind, the butterflies could only travel 780 kilometers. We estimate that the journey could have taken between five and eight days. Although the most remarkable thing is that the wind was particularly favorable,” says Toro Delgado. After 10 years, scientists have managed to solve the great mystery of butterflies Vanessa Cardui appeared on the other side of the ocean, although many questions remain. To what extent was this a coincidence? Are such trips more common than we think? How might the arrival of migratory insects carrying viruses and bacteria affect the ecosystem?

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