How a flock of butterflies flew 4,200 kilometers across the Atlantic Ocean without stopping

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Painted ladybugs venture far and wide with their impressive migratory patterns that stretch for thousands of miles – but they often travel overland so they can stop to rest.

Scientists have now found evidence that a group of winged travelers flew more than 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 discovery puts an end to a decade-long mystery that began when entomologist and lead author of the study Dr. Gerard Talavera came across about ten thistle butterflies, known by the scientific name Vanessa cardui, on a beach in French Guiana in October 2013. The insects, which are not normally found in South America, were worn and had holes and tears in their wings.

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

But crossing an entire ocean was unheard of for butterflies, even ones as worldly as Painted Ladies. Talavera and his colleagues had to rule out a few factors before concluding that these butterflies had accomplished what had previously been thought impossible.

How far a butterfly can fly

An October 2016 study that Talavera co-authored found that painted ladies from Europe migrate long distances of about 2,500 miles (about 4,000 kilometers) to sub-Saharan Africa, encountering obstacles such as the Mediterranean Sea and the Sahara . Still, the butterflies usually stay above land, where they “can stop and refuel, feed on flowers and then gain energy to keep going,” Talavera said.

It could take a Painted Lady butterfly five to eight days to cross the Atlantic Ocean, depending on several variables, according to new research.

Based on analyses of energy constraints, researchers concluded that the butterflies could fly up to 780 kilometers (480 miles) nonstop, but that favorable wind conditions allowed them to make the long journey, Talavera said.

“This is actually a record for an insect, especially a butterfly, to make such a long flight without the ability to stop,” said Talavera, who also directs the Worldwide Painted Lady Migration Project, a global citizen science project that maps the insects’ migration routes.

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

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

“Being suspended at just the right height in the air column to take advantage of the trade winds is nothing short of remarkable,” says Dr. Floyd Shockley, collections manager for the entomology department at the Smithsonian National Museum of Natural History in Washington. DC, which 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 about 10 butterflies that were out of place, versus the occasional lone discovered that were likely caught in storms, could be enough evidence that this was a coordinated migration event for the group of insects, Shockley said.

Following a butterfly

Researchers have taken some crucial steps to confirm that these out-of-place butterflies really did travel across the ocean.

First, to rule out that the insects didn’t travel overland from North America, the researchers analyzed their DNA and found that it matched that of European-African populations. The team then used a technique known as isotope tracing, which looks at the composition of the butterflies’ wings to find evidence of the types of plants they ate as caterpillars, said study co-author Dr. Megan Reich, a postdoctoral researcher at the University of Ottawa in Ontario. Using this method, the scientists concluded that the butterflies’ birthplace was in Western Europe, North Africa or West Africa, she added.

Scientists concluded that the butterflies' birthplace is in Western Europe, North Africa or West Africa using isotope research, which looks at the composition of the butterflies' wings to find evidence of the types of plants they eat. ate like caterpillars.  -Gerard Talavera

Scientists concluded that the butterflies’ birthplace is in Western Europe, North Africa or West Africa using isotope research, which looks at the composition of the butterflies’ wings to find evidence of the types of plants they eat. ate like caterpillars. -Gerard Talavera

But the real key to finding the route the butterflies took was a method first described in a September 2018 study led by Talavera, which found that pollen sticking to butterflies can reveal something about their migration journey by the plants they fed on. The butterflies spotted in October 2013 had pollen from two West African plants, Guiera senegalensis and Ziziphus spina-christi. The tropical shrubs bloom in August and November, according to the study, and this blooming season matches the timeline of the butterflies Talavera discovered in South America.

In addition, an analysis of weather data from 48 hours before the discovery of the stranded butterflies showed that they were “exceptionally beneficial for the butterflies’ dispersal across the Atlantic Ocean from West Africa,” the authors noted in the study.

If the insects had traveled from their likely birthplace of Europe to Africa and South America, the butterflies’ journey could have taken 7,000 kilometers or more.

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

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

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

If future research shows that the butterflies’ journey is likely a regular migration pattern, it will be one of the longest insect migrations in the world, he added.

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