Early one morning in late October 2013, entomologist Gerard Talavera noticed something most unusual: a swarm of Painted Lady butterflies stranded on a beach in French Guiana.
The Painted Lady butterfly or the species Vanessa cardui, is one of the most widespread butterflies in the world, but is not found in South America. Yet there they lay in the sand of the continent’s east coast, their wings tattered and full of holes. Judging by their condition, the sleepy Dr. Talavera, who works at the Institut Botànic de Barcelona in Spain, suspected that they were recovering from a long flight.
The insect is a master of long-distance travel and regularly crosses up to 14,500 kilometers of the Sahara on its journey from Europe to sub-Saharan Africa. Could it have managed the 4,200-kilometer journey across the Atlantic without stopping to refuel? Dr. Talavera wanted to find out.
Tracking the long-distance movements of insects is challenging. Devices such as radio-tracking devices are too large for insects’ small and delicate bodies, and radar devices only allow monitoring of specific locations. Scientists have had to rely on educated guesses and observations from citizen scientists to reconstruct travel patterns.
“We see butterflies appearing and disappearing, but we cannot directly prove the connections, we are only guessing,” said Dr. Talavera.
In 2018, he developed a method that allows one to use a common genetic sequencing tool to analyze pollen DNA. Pollen grains stick to pollinating insects, such as butterflies, as they feed on nectar from flowers. Dr. Talavera used a method called DNA metabarcoding to sequence the pollen’s DNA and determine which plant it came from. Later, the DNA could be traced back to the geographic flora to record the insect’s path.
In a paper published Tuesday in the journal Nature Communications, Dr. Talavera and his team describe a key clue that may solve the mystery of the stranded butterflies: Pollen found on 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. 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 determine their ancestry and found that they have European-African roots, ruling out the possibility that they had flown overland from North America. They then used an insect tracking tool called isotope tracking to confirm that the butterflies were born in Western Europe, North Africa and West Africa. By adding weather data showing favorable winds blowing from Africa to the Americas, they moved closer to a groundbreaking discovery.
“This is a brilliant piece of biological detective work,” said David Lohman, an evolutionary ecologist at the City College of New York who was not involved in the work. Dr. Talavera’s forensic detective tracking supported the conclusion that the painted ladies made the first transoceanic journey by an insect ever recorded.
They were probably on their usual route through Africa when a strong wind blew them off course. The butterflies continued flying over the ocean until they reached the coast.
Insect migrations are the largest biomass movement in the world. An incredible 3.5 trillion insects migrate across southern England alone every year. Their ability to transport pollen, fungi and even plant diseases over long distances underlines the global importance of these tiny creatures. The ocean migration of painted ladies could help scientists better track these journeys, experts say.
The finding showed that the delicate animals were able to survive a difficult and dangerous journey that most likely lasted 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,” adding that they “will help us understand migrations.”