How a group of butterflies flew 2,600 miles across the Atlantic Ocean without stopping

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Painted lady butterflies venture far and wide with their impressive migratory patterns that stretch for thousands of miles — but they often travel across land, so they can stop to rest.

Scientists have now found evidence that a group of the winged travelers flew over 2,600 miles (about 4,200 kilometers) across the Atlantic Ocean without stopping, according to a new study published June 25 in the journal Nature Communications.

The finding ends a decade-long mystery that began when entomologist and lead study author Dr. Gerard Talavera came across around 10 painted lady butterflies, known by the scientific name Vanessa cardui, on a beach in French Guiana in October 2013. The insects, which are not usually found in South America, were worn out with holes and tears in their wings.

“They looked exhausted. They even couldn’t fly very much — they kind of jumped instead of flying,” said Talavera, a Spanish National Research Council senior researcher at the Botanical Institute of Barcelona. “The only explanation that came to my mind was that these were long-distance migrants.”

But crossing an entire ocean was unheard of for butterflies, even ones as worldly as the painted ladies. Talavera, along with his colleagues, had to rule out a few factors before concluding that these butterflies accomplished what was previously thought impossible.

An October 2016 study that Talavera coauthored found that painted ladies from Europe migrate in great distances of around 2,500 miles (about 4,000 kilometers) to sub-Saharan Africa, facing obstacles such as the Mediterranean Sea and Sahara Desert. But even so, the butterflies remain mostly over land where they can “stop and refuel, feed on flowers and then get energy to keep going,” Talavera said.

Crossing the Atlantic would take a painted lady butterfly five to eight days, depending on different variables, according to the new study.

Based on analyses of the energy constraints, researchers concluded that the butterflies could fly a maximum of 485 miles (780 kilometers) or so without stopping, but favorable wind conditions are what allowed them to complete the lengthy journey, Talavera said.

“This is actually kind of a record for an insect, especially for a butterfly, to perform such a long flight without possibilities to stop,” said Talavera, who also leads the Worldwide Painted Lady Migration Project, a global citizen science project that tracks the migratory routes of the insects.

There have been other instances in which experts suspect butterflies and other migratory insects of traveling longer distances than usual, turning up on boats, remote islands or countries where they are not usually found, Talavera said.

The researchers believe that these butterflies took part in their annual migration south from Europe but got lost when the wind blew them into the ocean, he added. The butterflies then likely rode out the trade winds, which blow east to west near the equator, until they reached land in South America.

“Getting suspended in the air column at just the right height to take advantage of the trade winds is nothing short of remarkable,” said Dr. Floyd Shockley, collections manager for the department of entomology at the Smithsonian National Museum of Natural History in Washington, DC, who was not part of the new study. “It kind of begs the question, have they been doing this for a long time, and we just never documented it because we weren’t looking for it in South America?”

The discovery of around 10 out-of-place butterflies, versus the occasional discovered singleton that was likely caught up in storms, could be sufficient evidence that this was a coordinated migratory event for the group of insects, Shockley said.

Researchers took a few crucial steps to confirm these out-of-place butterflies really did travel across the ocean.

First, to rule out that the insects did not travel over land from North America, the researchers analyzed their DNA, finding it to match with that of European-African populations. Next, the team used a technique known as isotope tracing that looks at the composition of the butterflies’ wings for evidence of the types of plants they ate as caterpillars, said study coauthor Dr. Megan Reich, a postdoctoral fellow at the University of Ottawa in Ontario. By this method, the scientists concluded the butterflies’ birthplace to be in either Western Europe, North Africa or West Africa, she added.

But the real key to finding the route the butterflies took was a method first described in a September 2018 study led by Talavera that found that pollen clinging to butterflies can be telling of their migratory journey through the plants on which they fed. The butterflies spotted in October 2013 had the pollen of two West African plants, Guiera senegalensis and Ziziphus spina-christi. The tropical shrubs flower through August and November, according to the study, and this blooming season lines up with the timeline of the butterflies that Talavera discovered in South America.

Additionally, an analysis of weather data from 48 hours before the discovery of the beached butterflies had shown to be “exceptionally favorable for the butterflies to disperse across the Atlantic from West Africa,” the authors noted in the study.

If the insects traveled from their likely birthplace of Europe, then to Africa and South America, the butterflies’ journey could have consisted of 4,350 miles (7,000 kilometers) or more.

“A lot of people think of butterflies as really fragile creatures. I think this really shows how strong and resilient they are and these amazing journeys that they take — they really shouldn’t be underestimated,” Reich said.

The researchers hope to use the same techniques to investigate the migration patterns of other species of butterflies, she added.

“This is just the first step in sort of a long process of trying to understand why this happened and how this happened,” Shockley said.

If future research finds that the butterflies’ journey is likely a regular migration pattern, it’s among the longest insect migrations in the world, he added.