The painted lady butterfly is a migratory species in Europe, and common visitor to the Channel Islands, previously known to migrate from Europe to the Afrotropics during the autumn. Butterflies are obviously much harder to track on migration than birds and the fate of this butterfly species and its offspring remained unknown. Butterfly migration can be very different to that in birds where a bird like the swallow moves between, often very distant but well demarcated, summer and winter areas. The best known butterfly migration is perhaps that of the monarch in North America where despite there being well known and well demarcated summer and winter areas, generations of monarchs will never see the winter areas but are essential in the species life cycle (see the fascinating story of the monarch here).
Researchers were now able to demonstrate that painted lady butterflies return from the Afrotropical region to recolonise the Mediterranean in early spring, travelling an annual distance of 12,000 km across the Sahara Desert.
While the Palearctic-African migratory circuit is typically associated with birds, scientists from the Institute of Evolutionary Biology (IBE), a joint research centre of the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF), in Barcelona, Spain, found that the painted lady Vanessa cardui endures annual trans-Saharan circuits like some birds do.
This butterfly species travels 12,000 km and crosses the Sahara Desert twice to seasonally exploit resources and favourable climates on both sides of the desert. Few species are known to perform annual long-range trans-Saharan circuits, and that of the painted lady is the longest migratory flight known in butterflies to date.
In a previously published study, the researchers demonstrated that painted lady butterflies migrate from Europe to tropical Africa by the end of summer, crossing the Mediterranean Sea and Sahara Desert.
The fate of these migrants and that of their offspring remained unknown. “Our hypothesis was that the species initiates a reverse northward migration towards Europe in spring, thus completing a regular migratory cycle,” states Roger Vila, one of the researchers.
The answer is in the wings
With the aim of confirming this hypothesis, they studied the natal origin of the butterflies that reached the Mediterranean region in early spring. To do so, they analysed the stable hydrogen isotopes of the butterflies sampled in Morocco, Andalusia and Catalonia in Spain, Crete, Egypt and Israel.
An isotope is a form of a chemical element whose atomic nucleus contains a different number of neutrons compared to protons in the nucleus. In water, the proportion of hydrogen and its stable isotope depends on the geographical location. When absorbing water, this proportion is maintained in plants; it later remains in the caterpillars that feed on these plants, and, eventually, in adult butterflies.
By analysing the hydrogen stable isotopes found in the wings of adult butterflies, the researchers could determine where they had developed as caterpillars.
“It is difficult to study the movement of insects by means of observations, marking or radio tracking, since there are millions of individuals and they are very small. This is why finding out where a butterfly grew up before undergoing the metamorphosis by means of stable isotope analysis turns out to be extremely useful. It feels like magic,” says Gerard Talavera, who led the research.
The results show a major proportion of specimens stay in the Afrotropics during winter and that those recolonising the Mediterranean are most probably their offspring. This scenario closes the loop for the Palearctic-African migratory system of Vanessa cardui and shows that the annual distance travelled by the successive generations may reach about 12,000 km, including crossing of the Sahara Desert twice.
Whether the painted lady does regular migratory circuits similar to those of the monarch butterfly in North America was a matter of scientific debate. This research reveals the parallelisms in such a unique evolutionary adaptation.
Access the paper Round-trip across the Sahara: Afrotropical Painted Lady butterflies recolonize the Mediterranean in early spring here