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Painted lady butterflies venture far and wide with their impressive migratory patterns that span thousands of kilometres, but often travel overland to stop for a 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 ends a decade-long mystery that began when entomologist and lead study author Dr. Gerard Talavera came across about 10 painted female butterflies, known scientifically as Vanessa cardui, on a beach in French Guiana in October 2013. The insects , not normally found in South America, were worn with holes and tears in their wings.
“They looked exhausted. They couldn’t even fly much — they kind of hopped instead of flying,” said Talavera, a senior researcher at Spain’s National Research Council at the Botanical Institute in Barcelona. “The only explanation I could think of was that these were long-distance migrants.”
But crossing an entire ocean was unheard of for butterflies, even such mundane ones as painted ladies. Talavera, along with his colleagues, had to rule out several factors before concluding that these butterflies had achieved what was previously thought impossible.
An October 2016 study 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, facing obstacles such as the Mediterranean Sea and the Sahara desert. But even so, the butterflies mostly stay over land, where they can “stop and refuel, feed on flowers and then get energy to continue,” Talavera said.
Crossing the Atlantic Ocean would take a painted lady butterfly anywhere from five to eight days, depending on various variables, according to the new study.
Based on analyzes of energy constraints, the researchers concluded that the butterflies can fly a maximum of 485 miles (780 kilometers) or so without stopping, but favorable wind conditions are what allowed them to complete the long journey, Talavera said.
“This is actually something of a record for an insect, especially a butterfly, to make such a long flight without stopping,” said Talavera, who also directs the Worldwide Painted Lady Migration Project, a global citizen science project that tracks insect migration routes .
There are other cases where experts suspect butterflies and other migrating insects are traveling longer distances than usual, appearing on boats, remote islands or countries where they are not normally found, Talavera said.
The researchers believe these butterflies were involved in their annual migration south from Europe but were lost when the wind blew them out into the ocean, he added. The butterflies then probably emerged from the trade winds that blow from 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 nothing short of remarkable,” said Dr. Floyd Shockley, collections manager in the Department of Entomology at the Smithsonian National Museum of Natural History in Washington, DC, who did not was 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 didn’t look for it in South America?”
Finding about 10 butterflies out of the site, compared to randomly found single butterflies that were likely caught in storms, could be enough evidence that this was a coordinated migration event for the insect group, Shockley said.
The researchers took several important steps to confirm that these unusual butterflies do indeed travel across the ocean.
First, to rule out that the insects did not travel overland from North America, the researchers analyzed their DNA, finding that it matched that of European-African populations. The team then used a technique known as isotopic tracing, which looks at the composition of the butterflies’ wings for evidence of the types of plants they ate as caterpillars, said study co-author Dr. Megan Reich, a postdoctoral fellow at the University of Ottawa in Ontario. Through this method, the scientists concluded that the butterflies’ birthplace was either Western Europe, North Africa or West Africa, she added.
But the real key to finding the route the butterflies took is a method first described in a September 2018 study led by Talavera, which found that the pollen stuck to the butterflies can tell the story of their migratory journey through 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 is consistent with the butterfly schedule that Talavera found in South America.
Additionally, analysis of weather data from the 48 hours before the beached butterflies were found showed that it was “extremely favorable for the butterflies to disperse across the Atlantic from West Africa,” the study authors note.
If the insects traveled from their likely birthplace of Europe, then to Africa and South America, the butterflies’ journey could have been 4,350 miles (7,000 kilometers) or more.
“A lot of people think of butterflies as really fragile creatures. I think it really shows how strong and resilient they are and these incredible journeys that 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 something of a long process of trying to figure out why this happened and how this happened,” Shockley said.
If future research finds that the butterfly journey is likely a regular migration pattern, it would be among the longest insect migrations in the world, he added.
