Early one morning in late October 2013, entomologist Gerard Talavera spotted something very unusual – a flock of painted butterflies stranded on a beach in French Guiana.
Painted Lady, or species Vanessa cardui, is one of the most widespread butterflies in the world, but does not occur in South America. Yet there they were, lying in the sand of the continent’s eastern shores, their wings torn and riddled with holes. Judging by their condition, a bleary-eyed Dr. Talavera, who works at the Botanical Institute in Barcelona, ​​Spain, guessed they were recovering from a long flight.
Insects are champions of long-distance travel, routinely crossing the Sahara on a journey from Europe to southern Africa that covers up to 9,000 miles. Could they also make 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-distance movement 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 had to rely on educated guesses and citizen-scientist observations to piece together travel patterns.
“We see butterflies that appear and disappear, but we don’t directly prove the connection, we only make assumptions,” said Dr. Talavera.
In 2018, he developed a way to use a common genetic sequencing tool to analyze pollen DNA. Pollen grains cling to pollinating insects like butterflies do 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 plants it came from. Later, it was possible to trace the DNA back to the geographical flora and map the path of the insects.
In an article published Tuesday in the journal Nature Communications, Dr. Talavera and his team describe a major key to solving the mystery of the stranded butterflies: Pollen clinging to butterflies in French Guiana matched flowering shrubs in West African countries. These shrubs bloom from August to November, matching the timeline of the arrival of butterflies. This indicated that the butterflies had crossed the Atlantic. The thought was exciting. But Dr. Talavera and his team were careful not to jump to conclusions.
In addition to studying the pollen, the scientists sequenced the genomes of the butterflies to trace their lineage and found that they have European-African roots. This ruled out the possibility of them flying over the mainland from North America. They then used an insect-tracking tool called isotope tracking 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 built a monumental find.
“It’s a great 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-like tracking supported the conclusion that painted butterflies made the first transoceanic journey ever recorded by an insect.
It is likely that they were on their typical journey across Africa when strong winds blew them off course. Once the butterflies crossed the ocean, they flew on until they reached the shore.
Insect migrations are the largest movement of biomass in the world. A staggering 3.5 trillion insects migrate through southern England alone each year. Their ability to carry pollen, fungi and even plant diseases over vast distances highlights the global impact of these tiny creatures. Experts say the oceanic migration of painted ladies may give scientists a better way to track those journeys.
The find showed that the delicate creatures could withstand the difficult and dangerous journey, which most likely lasted five to 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 migration.”