Early one morning in late October 2013, Gerard Talavera, an entomologist, saw something very unusual – a flock of painted female butterflies stranded on a beach in French Guiana.
The Painted Lady, or species Vanessa cardui, is one of the most common 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 shores, their wings tattered and pitted with holes. Judging by their condition, a bleary-eyed Dr. Talavera, who works at the Institut Botànic de Barcelona in Spain, assumed they were recovering from a long flight.
The insect is a champion of long-distance travel, regularly crossing the Sahara on its trek from Europe to South Africa, covering up to 9,000 miles. Could they have also made the 2,600-mile journey across the Atlantic without a place to stop and refuel? Dr. Talavera wanted to know.
Tracking the movements of insects over long distances is challenging. Tools such as radio tracking devices are too large for the insects’ small and delicate frames, and radar only allows observation of certain locations. Scientists had to rely on educated guesses and citizen-scientist observations to piece together the travel patterns.
“We see butterflies appearing and disappearing, but we’re not proving the connections directly, we’re just making assumptions,” Dr Talavera said.
In 2018, he developed a way to use a general genetic sequencing tool to analyze pollen DNA. Pollen grains adhere 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 pollen and determine which plant it came from. Later, the DNA can be traced back to the geographic flora to chart the insect’s path.
In a paper published Tuesday in the journal Nature Communications, Dr. Talavera and his team describe a crucial clue to solving the mystery of the stranded butterflies: the pollen stuck to the butterflies in French Guiana matches the flowering shrubs in the West African countries. These shrubs bloom from August to November, which matches the arrival schedule of butterflies. This suggested that the butterflies had crossed the Atlantic. The idea was tantalizing. 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 lineage and found that they have European-African roots. This ruled out the possibility that they had flown overland from North America. They then used an insect-tracking tool called isotopic tracing to confirm that the birth of the butterflies was in Western Europe, North Africa and West Africa. By adding weather data showing favorable winds blowing from Africa to the Americas, they made a monumental discovery.
“This is a brilliant piece of biological detective work,” said David Lohmann, an evolutionary ecologist at the City College of New York, who was not involved in the work. Dr. Talavera’s forensic tracing supported the conclusion that painted female butterflies made the first transoceanic journey ever recorded by an insect.
They were probably on their typical route through Africa when they were blown off course by strong winds. Once over the ocean, the butterflies continued to fly until they reached the shore.
Insect migrations are the largest movement of biomass in the world. A staggering 3.5 trillion insects migrate annually over southern England alone. Their ability to transport pollen, fungi and even plant diseases over vast distances highlights the global impact of these tiny creatures. Experts say that with the painted ladies’ oceanic migration, scientists may have a better way to track those journeys.
The find showed that the delicate creatures could withstand a difficult and dangerous journey, which most likely lasted between five and eight days. It also demonstrates 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 research, called the study’s methods “innovative,” adding that they “will help us understand migrations.”
