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Painted lady butterflies venture far and wide with their impressive migratory patterns that span thousands of miles – but they often travel across land, allowing them to stop and rest.
Scientists have now found evidence that a group of winged travelers flew more than 4,200 kilometers over the Atlantic Ocean without stopping, according to a new study published June 25 in the journal Nature Communications.
The discovery ends a decade-long mystery that began when entomologist and lead author of the study, Dr. Gerard Talavera, encountered about a dozen painted butterflies, known scientifically as Vanessa cardui, on a beach in French Guiana in October 2013. The insects, which are not typically found in South America, were worn with holes and tears in their wings.
“They looked exhausted. They couldn’t even fly much – they were kind of jumping instead of flying,” said Talavera, a senior researcher at the Spanish National Research Council’s Botanical Institute in Barcelona. “The only explanation I could think of was that they were long-distance migrants.”
But crossing an entire ocean was a novel experience for butterflies, even those as worldly as painted ladies. Talavera and his colleagues had to eliminate a few factors before concluding that these butterflies had accomplished what was previously thought impossible.
An October 2016 study co-authored by Talavera found that painted ladies from Europe migrate great distances of about 2,500 miles (about 4,000 kilometers) to sub-Saharan Africa, facing obstacles such as Mediterranean Sea and the Sahara Desert. But despite this, the butterflies mostly stay above land where they can “stop and refuel, feed on flowers and then get energy to continue,” Talavera said.
According to the new study, the Atlantic crossing would take a painted lady butterfly five to eight days, depending on several variables.
Based on analyzes of energetic constraints, the researchers concluded that the butterflies could travel a maximum of 485 miles (780 kilometers) without stopping, but that favorable wind conditions allowed them to accomplish this long journey, a Talavera said.
“It’s actually kind of a record for an insect, especially a butterfly, to fly that long without being able to stop,” said Talavera, who also directs the Worldwide Painted Lady Migration Project, a global citizen science project that tracks the insects’ migration routes.
There have been other cases in which experts suspect butterflies and other migratory insects are traveling longer distances than usual, appearing on boats, isolated islands or countries where they are not usually found, a Talavera said.
Researchers believe the butterflies were part of their annual migration to southern Europe but got lost when the wind blew them into the ocean, he added. The butterflies then likely rode the trade winds, which blow east to west near the equator, until they reached land in South America.
“To be suspended in the air column at the right height to take advantage of the trade winds is just remarkable,” said Dr. Floyd Shockley, collections manager in the entomology department at the Smithsonian National Museum of Natural History in Washington, D.C., who was not part of the new study. “It kind of raises 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 displaced butterflies, compared to the occasional discovery of singletons likely caught in storms, could be sufficient evidence that this was a coordinated migratory event for the group of insects, Shockley said .
The researchers took some crucial steps to confirm that these displaced butterflies actually crossed the ocean.
To rule out that the insects did not travel overland from North America, the researchers analyzed their DNA and found that it matched that of European and African populations. Then the team used a technique known as isotope tracing, which examines 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 either Western Europe, North Africa or West Africa, she added.
Gerard Talavera
Scientists concluded that the butterflies’ birthplace was either Western Europe, North Africa or West Africa, using isotope tracing, which looks at the composition of the butterflies’ wings to gain evidence of the types of plants they ate as caterpillars.
But the real key to discovering the path butterflies take is a method first described in a September 2018 study led by Talavera, which found that pollen clinging to butterflies may be indicative of their migratory journey through plants which they feed on. The butterflies spotted in October 2013 had pollen from two West African plants, Guiera senegalensis and Ziziphus spina-christi. According to the study, these tropical shrubs bloom from August to November, and this flowering season matches the timeline of the butterflies Talavera discovered in South America.
Additionally, an analysis of weather data from 48 hours before the discovery of the stranded butterflies was found to be “exceptionally favorable for the dispersal of butterflies across the Atlantic from West Africa,” the authors note in the ‘study.
If the insects traveled from their likely birthplace, Europe, to Africa and South America, the butterflies’ journey could have taken 7,000 kilometers or more.
“Many people consider butterflies to be very fragile creatures. I think it really shows how strong and resilient they are and the incredible journeys they take – they really shouldn’t be underestimated,” Reich said.
The researchers hope to use the same techniques to study 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 it happened,” Shockley said.
If future research reveals that the butterflies’ journey likely fits a regular migration pattern, it will be one of the longest insect migrations in the world, he added.