Migration: Butterflies cross the Sahara

When she emigrates, Belle-Dame (Vanessa Cardui), a species of butterfly, half is not: it can cross the Sahara Desert to reach Europe, shows a study published June 29, 2021 in the American journal PNAS.

data collected over a long period of time

These butterflies are exclusively present in sub-Saharan Africa. In Europe, their numbers vary greatly from year to year. But until now, there was no explanation for this phenomenon. Where do butterflies on the Old Continent come from? Could it be that they are originally from Africa and sometimes struggle to migrate? Entomologists know that belle-dames migrate in the spring after the breeding season which is in winter. To track the movement of this species, an international research team used long-term monitoring data obtained by thousands of volunteers. Biologists have also taken into account atmospheric and climate data from Africa and Europe.

Butterflies that travel between 12,000 and 14,000 km

So by combining several data the researchers were able to get a more accurate view of the Beles-Dames migration. They concluded that these butterflies are capable of covering thousands of kilometers, crossing the Sahara Desert and the Mediterranean Sea to reach Europe. This visit does not happen every year, but when the weather allows it. The Bels-Dames then travel, between 12,000 and 14,000 km, flying non-stop during the day and resting at night to successfully cross the Sahara. This is the longest known insect migration ever. But be careful: this is done in several stages from a demographic point of view. “The number of generations for the entire annual circuit is about 6 to 8 generations, explain to science and the future Chinese researcher Gao Hu, lead author of the study. No one person can cover the entire migration journey, so crossings take place over several generations.“.

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This map shows the areas crossed by the Belles-Dames during their stay. Credits: sing hoo et al

And this journey is only possible if there is sufficient vegetation growth along the route, whether in savannas in winter or in North Africa in spring, which leads to an additional breeding season just before crossing the Mediterranean. Indeed, plants whose growth is adapted to the humid climate allow the caterpillars Vanessa Cardui to feed In addition, winds between Africa and Europe also allow for intercontinental migration of Bels-Dames. The latter flies one to three kilometers above sea level to take advantage of this.

According to the researchers, this study could help them better assess the migratory movements of other insects and especially invasive species that are capable of ruining crops or being carriers of pathogens.

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