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Painted ladybirds venture far and wide with their impressive migratory patterns that stretch thousands of miles – but they often travel over land 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 ends a decade-long mystery that began when entomologist and lead author of the study, Dr. Gerard Talavera, came across about 10 painted lady butterflies, known by the scientific name Vanessa cardui, on a beach in French Guiana in October 2013. Not usually found in South America, their wings were worn down with holes and tears in them.
“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 for butterflies, crossing an entire ocean was unheard of, even for butterflies as worldly as the painted ladies. Talavera and his colleagues had to rule out a number of factors before concluding that these butterflies were accomplishing what had previously been thought impossible.
An October 2016 study that Talavera contributed to found that thistle butterflies 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. But even then, the butterflies mainly stay on land, where they “can stop and refuel, feed on flowers and then gain energy to keep going,” Talavera said.
According to the new study, crossing the Atlantic Ocean would take five to eight days, depending on different variables.
Based on analyzes of the energy constraints, researchers concluded that the butterflies could fly up to 780 kilometers without stopping, but favorable wind conditions prevented them from completing the long journey, Talavera said.
“This is actually a record for an insect, especially a butterfly, to do 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 tracks the insects’ migratory 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 were making their annual southward migration from Europe but got lost when winds blew them out to sea, 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.
“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 not in the right place, as opposed to the single butterfly discovered that was likely swept away by a storm, could be evidence enough that this was a coordinated migration event for the insect group, Shockley said.
Researchers have taken some important steps to confirm that these butterflies actually traveled across the ocean.
To rule out that the insects didn’t travel overland from North America, the researchers first 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 provide evidence of the types of plants they ate as caterpillars, said 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.
Gerard Talavera
Scientists came to the conclusion that the birthplace of the butterflies is Western Europe, North Africa or West Africa. They did this by conducting isotope research. In this, they looked at the composition of the wings of the butterflies to find out which plants they ate as caterpillars.
But the real key to finding the path the butterflies took was a method first described in a September 2018 study led by Talavera, who found that pollen stuck to butterflies can reveal information about their migratory paths through the plants they feed 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, the study says, and that blooming 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 showed “exceptionally favorable conditions for the butterflies to disperse across the Atlantic Ocean from West Africa,” the authors noted in the study.
If the insects had traveled from their likely birthplace in Europe and then to Africa and South America, the butterflies’ journey could have taken 7,000 kilometers or more.
“Many people consider butterflies to be very vulnerable creatures. I think this really shows how strong and resilient they are and the amazing journeys they go on – they really shouldn’t be underestimated,” said Reich.
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 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.