Nature Today | Wow, do our thistle butterflies fly to South America?

Every year there are thistle butterflies in our country, but the numbers vary greatly. In good years, in August, many thousands of thistle butterflies can be seen when the hatchlings are in flight. These will then sit in our garden and collect nectar for the return trip. Over the years it has numbered a few or a dozen. It has to do with how many people come here in spring, April and May. They leave Africa for the north at the end of winter. If there is good breeding in Africa, so many leave, they are also dependent on (weather) conditions along the way. Winds, depressions, heavy rains and more can be worse. If conditions are poor, many thistle butterflies will not come here to breed, so there won’t be many offspring to be found here in late summer and start the journey south. Good thistle butterfly years are 2016, 2003, 2009 and 2019.

An international team of researchers analyzed the peak year of 2019 and looked at how the migration of the thistle butterfly continued in different ways. They used surveys (citizen science), airway modeling, genetics, pollen metabarcoding, and ecological niche modeling. Using isotopes, where the butterflies emerged from the chrysalis was also studied. The results are fascinating, because it turns out that thistle butterflies can cross the Atlantic Ocean in five to eight days. Butterflies from Africa fly to French Guiana! For this they need the help of ‘good’ air currents. Like us, this crossing is definitely not successfully completed every year. The thistle butterfly is also found in North America, but by looking at the pollen carried by thistle butterflies, it can be determined that it is from African plants.

Even more amazing is that the origin of those South American butterflies can be determined and they may have come from Western Europe. So a thistle butterfly emerges from its chrysalis here in Ostwartersplassen, flies to Africa, crosses the Sahara, and eventually returns to breed in French Guiana. That’s over 7,000 kilometers, a truly incredible feat!

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Text and photos: Cars Welling, Butterfly Foundation
Figure: Suchan et al., 2024
Maps: Blanca Marti