Migration: butterflies cross the Sahara

When migrating, Belle-Dame (Vanessa cardui), a species of butterfly, is not halved: it can cross the Sahara Desert to reach Europe, reveals a study published on June 29, 2021 in the American Journal PNAS.

Data collected over the long term

These butterflies are especially present in sub-Saharan Africa. In Europe, their numbers vary greatly from year to year. But so far this phenomenon has had no explanation. Where do butterflies come from on the Old Continent? Could it be that they are from Africa and sometimes struggle for migration? Entomologists know that Belle-Dame migrates in the spring after the breeding season that takes place in the winter. To track the movement of this species, an international research team used long-term monitoring data collected from thousands of volunteers. Biologists also took into account atmospheric and climatic data for Africa and Europe.

Butterflies that cross between 12,000 and 14,000 kilometers

Therefore, by combining several data, the researchers were able to obtain a more accurate vision of Belles-Dames migration. They concluded that these butterflies were capable of migrating thousands of miles, crossing the Sahara Desert and the Mediterranean to reach Europe. This trip does not take place every year, but only when weather conditions allow. Then the Belles-Dames travel, a round trip, between 12,000 and 14,000 km, flying non-stop during the day and still resting at night to successfully cross the Sahara. This is the longest known migration of insects so far. But beware: it is performed in several stages from a demographic point of view. “The number of generations for the whole annual cycle is probably about 6 to 8 generations, explain Science and the future Chinese researcher Gao Hu, lead author of the study. No individual can cover the entire migration journey, so the transition takes place over several generations.“.

This map shows areas that Belles-Dame crossed during their migration. Credit: Gao Hu et al

And this journey is only possible when there is sufficient vegetation growth on the route, either in the savannah in winter or in North Africa in the spring, which then leads to an additional breeding season just before crossing the Mediterranean. In fact, plants, whose development is favored by a humid climate, then allow caterpillars Vanessa cardui feed. In addition, winds between Africa and Europe also allow Belles-Dames transcontinental migration. The latter fly between one and three kilometers above sea level in order to benefit from it.

According to the researchers, this study could help them better assess the migratory movements of other insects and especially invasive species capable of releasing crops or as pathogen vectors.

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