Proof that butterflies crossed an ocean

Excerpts from a story in The New York Times:

Early one morning in late October 2013, Gerard Talavera, an entomologist, saw something highly unusual — a flock of painted lady butterflies stranded on a beach in French Guiana.

The painted lady, or the species Vanessa cardui, is one of the world’s most widespread butterflies, but it isn’t found in South America. Yet there they were, lying in the sand of the continent’s eastern shores, their wings tattered and riddled with holes.

The insect is a champion of long-distance travel, routinely crisscrossing the Sahara on a trek from Europe to sub-Saharan Africa, covering up to 9,000 miles. Could they also have made the 2,600-mile journey across the Atlantic Ocean without any place to stop and refuel?..

Dr. Talavera and his team describe a crucial clue to cracking the mystery of the stranded butterflies: Pollen clinging to the butterflies in French Guiana matched flowering shrubs in West African countries. These shrubs bloom from August to November, which matches the timeline of the butterflies’ arrival...

In addition to studying the pollen, the researchers sequenced the butterflies’ genomes to trace their lineage and found they had European-African roots. This ruled out the possibility that they had flown over land from North America. Then, they used an insect-tracking tool called isotope tracing to confirm that the butterflies’ natal origins were in Western Europe, North Africa and West Africa.

It's amazing what science is capable of nowadays.  The full scientific report is available online at Nature Communications.  The painted lady resting on my finger is one I raised some years ago from a caterpillar found on local nettles.