Early one morning in late October 2013, Gerard Talavera, an entomologist, saw something very unusual – a flock of painted lady butterflies stranded on a beach in French Guiana.
Painted Lady, or Vanessa cardui species, It is one of the most widespread butterflies in the world, but it is not found in South America. Yet there they were, lying on the sands of the continent’s eastern shores, their arms torn apart and riddled with holes. Judging by their condition, the bleary-eyed Dr. Talavera, who works at the Institut Botànic de Barcelona in Spain, thought they were recovering from a long flight.
The insect is a champion of long-distance travel, routinely crossing the Sahara on a trip from Europe to sub-Saharan Africa, covering up to 9,000 miles. Could they also have made the 2,600-mile journey across the Atlantic Ocean without a place to stop and refuel? Dr. Talavera wanted to find out.
Tracking the long-range movements of insects is challenging. 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 have had to rely on educated guesses and citizen scientist observations to piece together travel patterns.
“We see butterflies appearing and disappearing, but we’re not proving the connections directly, we’re 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 are attached to pollinating insects such as butterflies when they feed on flower nectar. Dr. Talavera used a method called DNA metabarcoding to sequence the pollen’s DNA and determine which plant it came from. Later, DNA could be traced to geographic flora to determine the insect’s path.
In a paper published Tuesday in the journal Nature Communications, Dr. Talavera and his team describe a crucial clue to unraveling the mystery of the trapped butterflies: Pollen clinging to butterflies in French Guiana matches flowering shrubs in West African countries. These shrubs bloom from August to November, which coincides with the arrival time of the butterflies. This suggested that the butterflies had crossed the Atlantic. The idea was misleading. But Dr. Talavera and his team were careful not to jump to conclusions.
In addition to studying the pollen, the researchers sequenced the genomes of the butterflies to trace their ancestry and found that they had European-African roots. This ruled out the possibility that they had flown overland from North America. Next, they used an insect-tracking tool called isotope tracing to confirm that the butterflies’ native origins were in Western Europe, North Africa, and West Africa. Adding weather data showing favorable winds blowing from Africa to the Americas, they were building to a monumental find.
“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 trail as a forensic detective supported the conclusion that painted lady butterflies made the first transoceanic journey ever recorded by an insect.
It is likely that they were on their typical route through Africa when they were blown off course by a strong wind. Once over the ocean, the butterflies continued to fly until they reached the shore.
Insect migrations are the largest movement of biomass worldwide. In Southern England alone, 3.5 trillion insects migrate each year. Their ability to transport pollen, fungi and even plant diseases over great distances highlights the global impact of these tiny animals. With the oceanic migration of painted ladies, experts say, scientists may have a better way to track these journeys.
The discovery showed that the delicate creatures could endure a difficult and dangerous journey, which 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 “groundbreaking,” adding that it “will help us understand migrations.”