Scientists find first evidence that butterflies crossed an ocean

Early one morning in late October 2013, Gerard Talavera, an entomologist, saw something very unusual: a flock of thistle butterflies stranded on a beach in French Guiana.

The thistle butterfly, or the species Vanessa cardui, is one of the world’s most common butterflies, but it is not native to South America. Yet there they lay in the sand on the continent’s eastern coast, their wings torn and riddled with holes. Judging from their condition, a bleary-eyed Dr. Talavera, who works at the Institut Botànic de Barcelona in Spain, suspected they were recovering from a long flight.

A champion of long-distance travel, the insect routinely crosses the Sahara on a journey from Europe to sub-Saharan Africa, taking up to 9,000 miles. Could they have made the 4,200-kilometer journey across the Atlantic Ocean without any stops to refuel? 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 have had to rely on educated guesses and observations from citizen scientists to map travel patterns.

“We see butterflies appear and disappear, but we don’t prove the connections directly, we just make assumptions,” Dr. Talavera said.

In 2018 he has developed a way to use a common tool for genetic sequencing to analyze pollen DNA. Pollen grains stick to pollinating insects like butterflies when they feed on nectar from flowers. Dr. Talavera used a method called DNA metabarcoding to sequence the DNA of the pollen and determine which plant it came from. Later, the DNA could be traced back to geographic flora to map the insect’s path.

In published an article On Tuesday, Dr. Talavera and his team in the journal Nature Communications a crucial clue to unravel the mystery of the stranded butterflies: pollen that clung to the butterflies in French Guiana corresponded to 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 were native to Western Europe, North Africa and West Africa. By adding weather data showing favorable winds blowing from Africa to the Americas, they worked toward a monumental finding.

“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 bolstered the conclusion that the painted ladies were the first transoceanic voyage ever made 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. Over southern England alone, that is astonishing 3.5 trillion insects migrate annually. Their ability to carry pollen, fungi and even plant diseases over great distances highlights the global impact of these tiny creatures. With the oceanic migration of the thistle butterflies, experts say, scientists may have 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.”