Research published in the Proceedings of the National Academy of Sciences has revealed the wintering grounds of Painted Lady, a long-distance migrant butterfly familiar across Europe.
Until now, it has been a mystery where the butterflies spend the winter after crossing south over the Sahara and arriving in countries like Chad in the autumn. Researchers have long suspected there must be important areas for the species further south, but they have not had any knowledge on where the insects might be.
An international team of fieldworkers led by Gerard Talavera, from the Botanical Institute of Barcelona, visited a range of sites across sub-Saharan Africa in the months of December and January between late 2017 and early 2020 in order to ascertain whether the butterflies spent the winter in subtropical regions near the equator. A further 15 sites were surveyed year-round for two years, adding to the extensive search effort covering nine countries, including Cameroon and Benin.
Painted Lady moves south from semi-arid savannahs to reach more humid areas in the African subtropics December and February (Chris Teague).
With no knowledge of the foodplants used by the adults and caterpillars in Africa, the team could only go on their analysis-based suspicion that the subtropics were a likely bet and to focus on open spaces, as Painted Ladies are seldom found within woodlands in Europe. The analysis involved modelling based on recorded observations of eggs, larvae and pupae from 646 sites across 30 countries.
It turned out that the reasoning behind focusing on the subtropics was sound, with nearly 2,000 adult butterflies and more than 2,700 caterpillars located from Ivory Coast in the west to Ethiopia in the east.
Sightings revealed that Painted Ladies take up residence in semi-arid savannahs on their arrival between September and November, when these areas become drier and the butterflies move south to more humid savannahs and highlands between December and February. During these months, the warmer weather seems to allow the butterflies to breed rapidly, producing between three and five generations in sub-Saharan Africa before surviving adults move north towards Europe in February.
It is thought that the tropical forests of central Africa act as a barrier to Painted Ladies moving any further south, as they prefer drier, more open areas.
It is hoped that the new understanding of the annual movements of Painted Lady will help scientists to understand why we sometimes see huge arrivals of the species, with some years seeing 100 times the normal number of butterflies appearing in Europe. A 2021 study suggested a link between these influx years with rains in sub-Saharan Africa, which are thought to have boosted plant growth, but the newly identified winter breeding grounds were not considered as part of that research.
Talavera hopes his team can go on to explain the bigger question of why Painted Lady makes such a long annual migration at all. It is the longest round-trip made by any butterfly in the world, but the evolutionary benefits of the 15,000-km migration, played out over several generations, are not fully understood.
Talavera, G, García-Berro, A, Talla, V N K, & Vila, Ro. 2023. The Afrotropical breeding grounds of the Palearctic-African migratory painted lady butterflies (Vanessa cardui), Proceedings of the National Academy of Sciences. DOI: https://doi.org/10.1073/pnas.2218280120