Vanessa cardui is the most widespread of all butterfly species. It is commonly called the painted lady, or formerly in North America the cosmopolitan.V. cardui occurs in any temperate zone, including mountains in the tropics. The species is resident only in warmer areas, but migrates in spring, and sometimes again in autumn. It migrates from North Africa and the Mediterranean to Britain and Europe in May and June, occasionally reaching Iceland, and from the Red Sea basin, via Israel and Cyprus, to Turkey in March and April. The occasional autumn migration made by V. cardui is likely for the inspection of resource changes; it consists of a round trip from Europe to Africa.
For decades, naturalists have debated whether the offspring of these immigrants ever make a southwards return migration. Research suggests that British painted ladies do undertake an autumn migration, making 14,500 km (9,000 mi) round trip from tropical Africa to the Arctic Circle in a series of steps by up to six successive generations. The Radar Entomology Unit at Rothamsted Research provided evidence that autumn migrations take place at high altitude, which explains why these migrations are seldom witnessed. In recent years, thanks to the activity of The Worldwide Painted Lady Migration citizen science project, led by the Barcelona-based Institute of Evolutionary Biology (Catalan: Institut de Biologia Evolutiva), the huge range of migration has begun to be revealed. For example, some butterflies migrated from Iceland to the Sahara desert, and even further south.
V. cardui is known for its distinct migratory behaviour. In California, they are usually seen flying from north to north-west. These migrations appear to be partially initiated by heavy winter rains in the desert where rainfall controls the growth of larval food plants. In March 2019, after heavy rain produced an abundance of vegetation in the deserts, Southern California saw these butterflies migrating by the millions across the state.
Similarly, heavier than usual rain during the 2018-2019 winter seems to have been the cause of the extraordinarily large migration observed in Israel at the end of March, estimated at a billion individual butterflies. Painted lady migration patterns are highly erratic and they do not migrate every year. Some evidence suggests that global climatic events, such as el Niño, may affect the migratory behaviour of the painted lady butterflies, causing large-scale migrations. The first noticeable wave of migration in eastern Ukraine was noted in the 20s of April 2019. From May 15, numbers began to grow and it was possible to observe hundreds of this species in the Kharkiv region of Ukraine, including in the city streets of Kharkiv.
Based on experimental data, the painted lady's migration pattern in northern Europe apparently does not follow a strict north-west heading. The range of headings suggests that migrating butterflies may adjust their migration patterns in response to local topographical features and weather, such as strong wind patterns. Laboratory-raised autumn-generation painted lady butterflies were able to distinguish a southern orientation for a return migration path. According to the same laboratory-based study, when butterflies were isolated from the sun, they were unable to orient themselves in a specific direction, opposed to those that did have access to the sun. This suggests that V. cardui requires a direct view of the sky, implying the use of a solar compass to orient its migratory direction and maintain a straight flight path.
