One morning in late October 2013, Gerard Talavera, an entomologist, saw something highly unusual: a flock of painted butterflies stranded on a beach in French Guiana.
The painted lady, or the species Vanessa cardui, is one of the most widespread butterflies in the world, but it is not found in South America. Yet there they were, lying in the sand on the continent’s eastern coast, their wings tattered and riddled with holes. Judging by their condition, the bleary-eyed Dr. Talavera, who works at the Barcelona Botanical Institute in Spain, guessed they were recovering from a long flight.
The insect is a champion of long-distance travel, routinely crossing the Sahara on a journey from Europe to Southern Africa, covering up to 14,500 kilometers. Could they also have made the 4,200-kilometer journey across the Atlantic Ocean without anywhere to stop and refuel? Dr. Talavera wanted to find out.
Tracking insects’ long-range movements is challenging. Tools like radio tracking devices are too large for insects’ small, delicate structures, and radar only allows monitoring of specific locations. Scientists have had to rely on educated guesses and observations from citizen scientists to piece together travel patterns.
“We see butterflies that appear and disappear, but we’re not directly proving the links, we’re just making guesses,” Talavera said.
In 2018, he developed a way to use a common genetic sequencing tool to analyze pollen DNA. Pollen grains attach to pollinating insects, such as butterflies, when they feed on flower nectar. 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 geographic flora to trace the insect’s path.
In a paper published Tuesday in the journal Nature Communications, Dr. Talavera and his team describe a crucial clue to unlocking the mystery of the stranded butterflies: pollen clinging to butterflies in French Guiana matches flowering bushes in West African countries. These shrubs bloom from August to November, which corresponds with the arrival schedule of butterflies. This suggested that 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 trace their lineage and discovered that they had European-African roots. This ruled out the possibility that they had flown over land in North America. They then used an insect tracking tool called isotope tracking to confirm that the butterflies’ natal origins were in Western Europe, North Africa and West Africa. By adding meteorological data that showed favorable winds blowing from Africa to America, they were building towards a monumental discovery.
“This is brilliant 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 the painted butterflies made the first transoceanic journey ever recorded by an insect.
It is likely that they were on their typical route across Africa when they were blown off course by a strong wind. Once over the ocean, the butterflies continued flying until they reached the shore.
Insect migrations are the largest movement of biomass worldwide. In southern England alone, 3.5 billion insects migrate annually. Their ability to transport pollen, fungi and even plant diseases over great distances highlights the global impact of these small creatures. 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 withstand a difficult and dangerous journey, which probably lasted between five and eight days. It also demonstrates how much scientists still need 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.”
