Vanessa cardui

Vanessa cardui is a well-known colourful butterfly, known as the painted lady, or formerly in North America as the cosmopolitan.
Distribution.
"V. cardui" is one of the most widespread of all butterflies, found on every continent except Antarctica and South America. In Australia, "V. cardui" has a limited range around Bunbury, Fremantle, and Rottnest Island. However, its close relative, the Australian painted lady ("V. kershawi", sometimes considered a subspecies) ranges over half the continent. Other closely related species are the American painted lady ("V. virginiensis") and the West Coast lady ("V. annabella").
Migration.
"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, 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 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. 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.
Mating behaviour in relation to migration.
"V. cardui" displays a unique system of continuous mating, throughout all seasons, including the winter. This may be attributed to its migratory patterns, thus significantly affecting its mating behaviour. During European migrations, the butterflies immediately begin to mate and lay eggs upon arrival in the Mediterranean in the spring, starting in late May. In the United States, painted lady butterflies migrating towards the north experience poor mating conditions, and many butterflies have limited breeding capabilities. The "local adult generation" develops during this time, roughly from the middle of May through early June in conjunction with the butterfly progression throughout their flight.
During its migratory process, these painted lady butterflies start breeding, and reproduce entirely throughout their migration. Scientists have not been able to find evidence of their overwintering; this may be because they migrate to warmer locations to survive and reproduce. Female painted lady butterflies may suspend their flight temporarily when they are "ready to oviposit"; this allows them the opportunity to continually reproduce throughout their migrations. Because these butterflies are constantly migrating, male butterflies are thought to lack consistent territory. Instead of requiring territory to mate with females and developing evolutionary behaviour to defend this territory, the mating butterflies appear to establish a particular "time and place" in certain locations that they find to be suitable for reproduction. More specifically, they locate certain perches, hilltops, forest-meadow edges, or other landmarks where they will stay until, presumably, a female arrives to mate.
Equally important for the reproduction of the painted lady butterflies is the males' exhibition of polygynous mating behaviour, in which they often mate with more than one female. This is important for painted lady butterflies because the benefits may supersede the costs of polygyny since no permanent breeding ground is used. Upon mating, which typically occurs in the afternoon, female painted lady butterflies lay eggs one by one in their desired breeding locations. The variety of eclosion locations ultimately dictates the male painted lady behaviour.
Female painted lady butterflies have been observed to have a relatively "high biotic potential", meaning they each produce large numbers of offspring. This perpetual influx of reproduction may be a reason why these painted lady butterflies have propagated so successfully. One interesting aspect that scientists have observed is that these butterflies like to fly towards rain. Further studies have suggested that the large amounts of rainfall may somehow "activate more eggs or induce better larval development". 
Inhabited locations begin to observe a large influx of new generations of painted lady butterflies in the fall, particularly in September and October. Their reproductive success declines relatively throughout the winter, primarily through November. However, they still continue to reproduce—an aspect of butterfly behaviour that is quite unique. Scientists hypothesize that these extensive migratory patterns help the painted lady butterflies find suitable conditions for breeding, thus offering a possible reason as to why these butterflies mate continuously.
Oviposition.
Adult butterflies feed on flower nectar and aphid honeydew. Females oviposit on plants with nectar immediately available for the adults even if it leads to high mortality of the larvae. This lack of discrimination indicates they do not take into account volatile chemicals released from potential host plants when searching for oviposition choices.
The availability of adult resources dictates a preference for specific areas of flowers. Flowers with more available nectar result in a larger number of eggs deposited on the plants. This reinforces the idea that the painted lady butterfly does not discriminate host plants and chooses mainly on the availability of adult food sources even if it increases the mortality rate of the offspring. The data also suggest that the painted lady butterfly favors quantity of offspring over quality.
Vision.
Painted lady butterflies have a visual system that resembles that of a honey bee. Adult "V. cardui" eyes contain ultraviolet, blue, and green photoreceptors. Unlike other butterflies, such as the monarch or red postman butterflies, painted ladies lack red receptors, which means that they are likely red-green colour-blind. Behavioral studies on the related species, "Vanessa atalanta", have demonstrated that "V. atalanta" cannot discriminate yellow light from orange light or orange light from red light.
Roosting behaviour and territory.
Groups of two to eight painted lady butterflies have been observed to fly in circles around each other for about one to five seconds before separating, symbolizing courtship. Groups of butterflies usually will not fly more than 4.5 m away from the starting point. To establish and defend their territories, adult males perch in the late afternoon in areas where females are most likely to appear. Once the male spots a female of the same species, he begins pursuit of her. If the foreign butterfly is a male, the original male will give chase, flying vertically for a few feet before returning to his perch.
"V. cardui" establishes territories within areas sheltered by hedgerows. "Vanessa cardui" tend to inhabit sunny, brightly lit, open environments and are often attracted to open areas of flowers and clovers. Adults spend time in small depressions in the ground on overcast days.
Host plants.
Larvae feed on Asteraceae species, including "Cirsium", "Carduus", "Centaurea", "Arctium", "Onopordum", "Helianthus", and "Artemisia".
The painted lady uses over 300 recorded host plants according to the HOSTS database.
Defence mechanisms.
The main defence mechanisms of the painted lady butterflies include flight and camouflage. The caterpillars hide in small silk nests on top of leaves from main predators that include wasps, spiders, ants, and birds.
Human interaction.
"Vanessa cardui" and other painted lady species are bred in schools for educational purposes and used for butterfly releases at hospices, memorial events, and weddings.
In popular culture.
After heavy rain produced an abundance of vegetation in the deserts, Southern California saw these butterflies migrating by the millions across the state in March 2019.