The painted lady was known to be a tireless migratory butterfly, completing a 15,000-kilometer journey from Europe to Africa and vice versa over several generations. Now we discover that this 150-milligram insect with a wingspan of less than 7 centimeters is also capable of crossing the Atlantic from east to west. A "one-way journey," and an accidental one at that, said Vanessa cardui specialist Gerard Talavera (Barcelona Institute of Botany, Harvard University).

The Spanish entomologist had discovered a few dying specimens on a beach in French Guiana, which is located on the northeastern coast of South America, at 6am, on October 28, 2013. After a meticulous multidisciplinary investigation, he described in Nature Communications on June 25, how he and his colleagues determined their origin.

A review of the meteorological data revealed that easterly winds from the west coast of Africa had been blowing steadily toward French Guiana in the week before the butterflies were captured, at an average speed of 27 kilometers an hour. These are the same air currents that regularly transport millions of tons of Saharan dust to the Caribbean and the American continent.

The researchers then compared the DNA of the Guyanese painted ladies with that of 126 individuals from North America, Europe and Africa. Their genomes matched those of populations migrating between Europe and Africa.

The third lead consisted in analyzing the pollen grains found on the bodies of the dying butterflies. They belonged to between eight and 15 plant species, depending on the classification adopted. One was endemic to the Sahel region (in Africa) and another to sub-Saharan zones, both flowering between August and November, at the end of the rainy seasons in West Africa.

Finally, the researchers used isotope analysis to determine the ecological and geological substrate on which the individuals had grown. "We have isotopic maps that enable us to locate the type of signature transmitted to the tissues of herbivores," which include butterflies, said Clément Bataille (University of Ottawa), who led this part of the investigations. His conclusion? "These painted ladies were probably born in southwest France."

So, one could imagine that they followed the normal southward migration, which takes them in autumn, in a single generation, to the warmth of West Africa. This is where things took an unexpected turn when a gust of wind blew them out to the open sea with no way of returning.

Talavera and his colleagues calculated that without the assistance of air currents – and without the possibility of recharging their batteries – these butterflies could have flown a maximum of 780 kilometers. If these painted ladies flew 4,200 kilometers above the waves, it was probably not by adopting an active flight, which even with tailwind would have enabled them to cover "only" 1,900 kilometers, but by flapping their wings only to stay aloft and hover, 85% of the time. Under these conditions, it would have taken them five to eight days to cross the Atlantic, provided their energy reserves at departure were at least 14% of their total mass.

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