Sometimes millions of painted ladies swarm to Western Europe and Germany in the spring. An international team of researchers reports that the cause of these spectacular mass infiltrations is the exceptionally damp weather conditions in sub-Saharan Africa in the previous winter in the science journal PNAS. As a result, abundant plants grow on which the larvae of the black-white-red-orange butterflies can grow. Carried by suitable winds, the moths would then cross the Sahara.
The annual migration of the painted lady – a round trip of around 12,000 to 14,000 kilometers between tropical West Africa and Scandinavia – is the longest regular insect migration currently known, the researchers write. In spring, a generation of the painted lady (Vanessa cardui) hatches in the Maghreb region, i.e. in Morocco, Algeria and Tunisia. It flies to the Mediterranean area, where a new generation will be produced immediately, which will then move over the Alps to Germany and other countries in Central and Northern Europe. The butterflies reach as far as Great Britain and Ireland, some even as far as northern Sweden and Finland. In the fall they make their way back to northwest and sub-Saharan Africa. Six or more generations of painted ladies grow up throughout the journey.
In some years there is a massive emergence of moths, with millions of animals moving across the sky. Jason Chapman’s team from the University of Exeter evaluated monitoring data on population sizes of the moths from 1994 to 2015. They also analyzed environmental data from the respective habitats along the butterflies’ route, including information on temperature, precipitation, vegetation thickness and wind conditions.
The vegetation in the winter habitat determines the population
The analysis showed that the mass occurrence did not occur regularly during the investigation period of 22 years. Rather, the strength of the spring population in the Maghreb was mainly influenced by the density of vegetation in the winter habitat of the animals south of the Sahara. If the amount of precipitation there was unusually high in one year, an above-average number of animals reached the Maghreb region, the Mediterranean region and Northern Europe.
Indeed, the years of mass butterflies in Europe were preceded by relatively humid winters in the sub-Saharan region. Plants would then grow rapidly and not only serve as food for the larvae, but also presumably provide adult animals with nectar to strengthen them, the researchers write. Further analyzes showed that wind conditions repeatedly appeared, thanks to which the butterflies could cross the Sahara in about four days.