Early one morning in late October 2013, entomologist Gerard Talavera saw something very unusual. It was a flock of painted lady butterflies stranded on a beach in French Guiana.
The Painted Woman, or Vanessa Cardui, Although it is one of the most widespread butterflies in the world, it is not found in South America. But they lay on the sands of the eastern shore of the continent, their wings tattered and gaping. A bloodshot-eyed Dr. Talavera, who works at the Botany Institute in Barcelona, Spain, saw their condition and assumed they were recovering from a long flight.
These insects are champions of long-distance travel of up to 9,000 miles, regularly crossing the Sahara Desert as they travel from Europe to sub-Saharan Africa. Could they have traveled 2,600 miles across the Atlantic without anywhere to stop and refuel? Dr. Talavera wanted to know.
Tracking insects’ long-distance movements is difficult. Tools such as radio tracking devices are too large for the insects’ small and delicate frames, and radar can only monitor specific locations. Scientists have had to rely on educated guesses and observations from citizen scientists to piece together travel patterns.
“We see butterflies appearing and disappearing, but we can’t directly prove the connection, we’re just making assumptions,” Dr. Talavera said.
In 2018, he developed a method that uses common genetic sequencing tools to analyze pollen DNA. When insects, such as butterflies, feed on nectar from flowers, pollen grains stick to the pollinating insects. Dr. Talavera used a method called DNA metabarcoding to analyze the DNA sequence of the pollen and determine which plant it came from. He was later able to chart the insects’ routes by tracing their DNA into geographical flora.
In a paper published Tuesday in the journal Nature Communications, Dr Talavera and his team describe a key clue that could solve the mystery of the stranded butterflies. That means the pollen on a butterfly in French Guiana matches a flowering shrub in the West African country. This shrub blooms from August to November, which coincides with the arrival of butterflies. This means that the butterfly crossed the Atlantic Ocean. The idea was interesting. But Dr. Talavera and his team were careful not to jump to conclusions.
In addition to studying pollen, the researchers sequenced the butterflies’ genomes to trace their lineage and discovered that they have European-African roots. This ruled out the possibility that they flew over land in North America. They then used an insect tracking tool called isotope tracking to determine that the butterflies’ native places were in Western Europe, North Africa, and West Africa. By adding meteorological data showing tailwinds blowing from Africa to the United States, they were building up a monumental discovery.
“This is 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 study. Dr. Talavera’s forensic detective-like pursuit supported the conclusion that painted ladybirds made the first recorded transoceanic journey by an insect.
They were likely on their usual route through Africa when they were blown off course by strong winds. Once across the sea, the butterflies continued to fly until they reached the shore.
Insect migration is the largest biomass migration globally. In southern England alone, 3.5 trillion insects migrate each year. Their ability to transport pollen, mold and even plant diseases over long distances highlights the global impact of these tiny animals. Experts say the maritime migrations of painted women may give scientists a better way to track these journeys.
The discovery showed that the fragile creature could endure the arduous and dangerous journey, which probably 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 methodology “innovative,” adding, “It will help us understand migration.”
