Early one morning in late October 2013, Gerard Talavera, an entomologist, spotted something highly unusual: a flock of painted lady butterflies stranded on a beach in French Guiana.
The painted lady, or the species Vanessa cardui, is one of the world’s most common butterflies, but is not found in South America. Yet there they lay in the sand of the continent’s east coast, their wings torn and full of holes. Judging from their condition, the bleary-eyed Dr. Talavera, who works at the Institut Botànic de Barcelona in Spain, said they were recovering from a long flight.
The insect is a champion of long-distance travel and routinely crosses the Sahara on a migration from Europe to sub-Saharan Africa, covering as much as 9,000 miles. Could they have made the 2,600-mile journey across the Atlantic without any refueling stops? Dr. Talavera wanted to find out.
Tracking the long-distance movements of insects is a challenge. Tools such as radio tracking devices are too large for the small and delicate frames of insects, and radar only allows monitoring of specific locations. Scientists had to rely on educated guesses and observations from citizen scientists to map travel patterns.
“We see butterflies appearing and disappearing, but we are not directly proving the connections, we are just making assumptions,” said Dr. Talavera.
In 2018, he developed a way to use a common genetic sequencing tool to analyze pollen DNA. Pollen grains stick to pollinating insects such as butterflies when they feed on nectar from flowers. Dr. Talavera used a method called DNA metabarcoding to sequence the DNA of the pollens and determine which plant they came from. Later, the DNA could be traced back to the geographic flora to map the insect’s path.
In an article published Tuesday in the journal Nature Communications, Dr. Talavera and his team provided a crucial clue to unravel the mystery of the stranded butterflies: pollen clinging to the butterflies in French Guiana matched flowering shrubs in West African countries. These shrubs bloom from August to November, which matches the timeline of the butterflies’ arrival. That suggested the butterflies had crossed the Atlantic Ocean. The idea was tempting. But Dr. Talavera and his team were careful not to jump to conclusions.
In addition to studying the pollen, the researchers sequenced the butterflies’ genomes to trace their ancestry and discovered that they had European-African roots. This ruled out the possibility that they had flown overland from North America. They then used an insect-tracking tool called isotope tracing to confirm that the butterflies’ natal origins were in Western Europe, North Africa and West Africa. By adding weather data showing that favorable winds were blowing from Africa to America, they built up to a monumental find.
“This is a brilliant piece of biological detective work,” says David Lohman, an evolutionary ecologist at the City College of New York, who was not involved in the work. Dr. Talavera’s forensic-detective-style tracing supported the conclusion that the painted lady butterflies made the first transoceanic journey ever recorded by an insect.
They were probably on their typical route through Africa when they were swept off course by a strong wind. Once above the ocean, the butterflies continued to fly until they reached the shore.
Insect migrations represent the largest movement of biomass worldwide. As many as 3.5 trillion insects migrate over southern England alone every year. Their ability to transport pollen, fungi and even plant diseases over great distances underlines the global impact of these little creatures. Experts say the painted ladies’ oceanic migration gives scientists a better way to track these journeys.
The find showed that the delicate creatures could endure a difficult and dangerous journey, which most likely took between five and eight days. It also shows how much scientists still have to learn. Jessica Ware, an evolutionary biologist at the American Museum of Natural History who was not involved in the study, called the study’s methods “innovative” and added that it will “help us understand migrations.”