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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 (Vanessa kershawi, sometimes considered a subspecies) ranges over half the continent. Other closely related species are the American Painted Lady (Vanessa virginiensis) and the West Coast Lady (Vanessa annabella).
Vanessa 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 in May and June, but for decades naturalists have debated whether the offspring of these immigrants ever make a southwards return migration. Recent research suggests that British painted ladies do undertake an autumn migration. Using an entomological radar, scientists at Rothamsted Research provided evidence that autumn migrations take place at high altitude, which could be why these migrations are seldom witnessed.
The Vanessa cardui are known for their distinct migratory behavior. In California, they are usually seen flying from north to northwest. These migrations appear to be partially initiated by heavy winter rains in the desert where rainfall controls the growth of larval food plants. Painted Lady migration patterns are highly erratic and they do not migrate every year. There is evidence to suggest that global climatic events, such as El Nino, may affect the migratory behavior of the Painted Lady butterflies, causing large-scale migrations.
Based on experimental data, it appears that the Painted Lady’s migration pattern in Northern Europe does not follow a strict northwest 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 who did have access to the sun. This suggests that Vanessa cardui require a direct view of the sky, implying the use of a solar compass to orient their migratory direction and maintain a straight flight path.
Relationship with humans
Vanessa cardui butterflies are raised in many preschool and elementary classrooms to demonstrate the life cycle of a butterfly. Naturally, this is one reason they are so popular amongst children. They are also often found in science fair projects.
Life cycle with notes for rearing in classrooms
As these animals are cold blooded and their life cycle does not depend on a certain number of day/night cycles, temperature can greatly affect the times presented here.
At 90 °F (32 °C) the entire life cycles will take roughly 16 days. At 65 °F (18 °C) the life cycles will take months. At such extreme temperatures one can expect some deaths. At room temperatures the egg takes three to five days to hatch. The eggs are tiny, as tiny as a sugar crystal. They are green and ribbed and can be observed best with a magnifying glass. It is possible to view the cap at the top of the egg where the caterpillar will emerge.
The embryo can be viewed growing inside the egg using a hand lens or dissecting scope. A high powered dissecting scope allows for watching hatching quite clearly. If eggs turn deep green, or become dented and wrinkled, the eggs do not contain living embryos. Just before hatching the embryos fill the whole egg and make the eggs look black or brown. As protection against disease, newly laid eggs may be knocked off the leaf, or left attached to the leaf, and dipped in dilute household bleach solution (1 part household bleach to 200 parts water) for 1–2 minutes and swished about. Afterwards, the eggs are left on a paper towel to dry. This will kill disease on the surface of the eggs and increase caterpillar survival.
The caterpillars will emerge as small and black and will begin to eat immediately. As they grow they will shed their skins three times, called instars. At each instar the caterpillar will need much more food as it has expanded in size. It will also become more spiky. These spikes do not contain poison and are not sharp. The moulted skin appears as a black speck, what looks like dirt, near the caterpillar. Many people believe this to be the excretion of the caterpillar. Occasionally the moult will look like an entire, dead, caterpillar, as snake's skin does. If under stress they will sometimes shed into a fifth instar, which is a very large caterpillar. A fifth instar is a sign that care is incorrect in some way, typically due to diet.
The four instars take 7–11 days to turn into a chrysalis. The caterpillar will spin a patch of silk and attach its hind end to the silk. At this point it begins changing internally, forming a "j" shape. Once the caterpillar forms a J, it should not be disturbed as it can no longer reattach itself to the silk pad. A fallen "J" caterpillar can be laid on its side on a flat piece of cotton and may shed successfully. The chrysalis is very soft at first and will dent if resting on a hard surface. After hardening, the chrysalis will crack if dropped or struck. The chrysalis can be dark or light colored depending on conditions during development of the caterpillar. It takes 7–11 days for the chrysalis to turn into a butterfly.
When emerging from the chrysalis the butterfly pumps its wings with fluid to expand them. This happens within a few minutes of emerging or cannot happen at all. Once the wings are expanded they are still soft for up to a day. Initially the butterfly prefers not to move as its wings harden, but after the wings harden for a few hours the painted lady will become incredibly sensitive to movement and will damage its still soft wings when frightened. It is best to wait a full day after emergence from the chrysalis to handle a painted lady. Its wing span is 2 inches (50 mm).
Mating behavior in relation to migration
Vanessa cardui displays a unique system of continuous mating, throughout all seasons, including the winter. This may be attributed to the migratory patterns of the painted lady butterfly, thus significantly affecting its mating behavior. During European migrations, the butterflies will immediately begin to mate and lay eggs upon arrival in the Mediterranean in the spring, starting 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 will start breeding, and reproduce entirely throughout their migration. Scientists have not been able to find evidence of the Vanessa cardui overwintering; this may be attributed to the fact that they migrate to warmer locations in order to survive and reproduce. It is suspected that female painted lady butterflies 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, it is believed that male butterflies lack consistent territory. Instead of requiring territory to mate with females and developing evolutionary behavior 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 will locate certain perches, hilltops, forest-meadow edges, or other landmarks that they will stay on until, presumably, a female painted lady arrives to mate.
Equally important for the reproduction of the painted lady butterflies is the male butterflies’ exhibition of polygynous mating behavior, in which the males often mate with more than one female butterfly. This is important for painted lady butterflies because the benefits may supersede the costs of polygyny since there is no permanent breeding ground. Upon mating, which typically occurs in the afternoon, female painted lady butterflies lays her eggs one by one in her desired breeding locations. The variety of eclosion locations ultimately dictates the male painted lady behavior.
Female painted lady butterflies have been observed to have a relatively “high biotic potential”, meaning they produce large numbers of offspring per female. 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 behavior 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.
Adult butterflies will feed on flower nectar and aphid honeydew. Female Vanessa cardui will 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 the Vanessa cardui do not take into account volatile chemicals released from potential host plants when searching for oviposition choices.
It has been found that the availability of adult resources dictates preference for specific areas of flowers. A study in the Journal of Insect Behavior found that flowers with more available nectar resulted in a larger number of eggs deposited on the pants. This reinforces the idea that the Painted Lady butterfly does not discriminate host plants and chooses mainly on availability of adult food sources even if it increases mortality rate of the offspring. The data also suggests that the Painted Lady butterfly favors quantity of offspring over quality.
Roosting behavior 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 meters away from the starting point. In order to establish and defend their territory, adult males will perch in the late afternoon in areas where females are most likely to appear. Once the male spots a female of the same species, they will begin 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 its perch.
The Vanessa cardui establish 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. Adult Vanessa cardui will spend time in small depressions in the ground on overcast days.
The painted lady uses over 300 recorded host plants according to the HOSTS database. For raising in the classroom one need only sprout a bed of black oil sunflower seeds, as are used for bird seed. The caterpillars will eat the true adult leave (not the sprouts) and in this way one may inexpensively produce many host plants. Soak the seeds for eight hours in 10% diluted household bleach (1 pt bleach, 9 pts water) to ensure a disease free, even sprouting. Place the seeds upon the surface of the soil and keep moist until they grow roots and can be watered normally.
The main defense mechanisms of the Painted Lady butterflies include flight and camouflage. The Venessa cardui caterpillars will hide in small silk nests on top of leaves from main predators that include wasps, spiders, ants, and birds.
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