Lepidoptera migration

Species that are recorded in unexpected areas (adventive species) are not considered to be migratory species, because these did not leave their habitat on their own strength. Examples are species that are imported as egg or caterpillar alongside of their host plants or individuals that were reared by a collector but have escaped. An example of an introduced species is Galleria mellonella, which is found all over the world, because it is reared as food for captive birds and reptiles. At times it is difficult to decide if a species is adventive or migratory. Migratory species like Chrysodeixis chalcites and Helicoverpa armigera would be able to reach western Europe on their own, but are also common in greenhouses.

Australian species Agrotis infusa spends the summer as an imago in the Australian Alps, and is sometimes found in buildings in large numbers.

Lepidoptera migration is often seasonal. With species of which all individuals migrate, the population moves between areas in the summer and winter season or the dry and wet season.

For species of which only part of the population migrates, seasonal migration is hard to determine. They can maintain themselves in part of their habitat but also reach areas where they cannot establish a permanent population. They only live there in the season that is most favorable for the species. Some of the species have the habit of returning to their permanent residence at the end of the season.

An important difference with bird migration is that an individual butterfly or moth usually migrates in one direction, while birds migrate back and forth multiple times within their lifespan. This is due to the short lifespan as an imago. Amazingly the monarch receives no navigation instruction for the migration from their parents, unlike birds.[4][5] Species that migrate back and forth, usually do so in different generations. There are however, some exceptions:

• The famous migration of the monarch butterfly in North America. This species migrates back and forth in one generation, though it completes only part of the journey in both directions in that generation. No individual completes the entire journey which is spread over a number of generations. The imago of the last summer generation is born in North America, migrates to Mexico, Florida, or California and stays there for the winter. After the winter it migrates back to the north to reproduce. In a couple of generations, the monarch migrates north to Canada.
• The migration of the bogong moth in Australia. This species migrates from south-eastern Australia to the Australian Alps in the summer to avoid the heat. After the summer it returns to reproduce.[6]

To be able to migrate over long distances, species must be able to navigate. There are several ways they do this.

Landscape: Lepidoptera use coastal lines, mountains, but also roads to orient themselves. Above sea it has been observed that the flight direction is much more accurate if the landscape on the coast is still visible.[7]

Celestial navigation: Butterflies are known to be capable of navigation with the help of the Sun. They can also navigate by using polarized light. The polarization of the Sun's light changes with the angle of the rays, hence they can also navigate with cloudy weather. There are indications that they can even make corrections depending on the time on a day. Diamondback moths are known to fly in a straight trajectory which is not dependent on the angle of the Sun's rays.[14] Tests have been performed to interfere with the biological clock of certain species by keeping them in the dark and then observing if they would choose for other flight paths. The conclusion was that some species did, and others did not.[15] Research on monarchs demonstrates that with removal of antennae, the location of the circadian clock, individuals do not localize in any one direction during flight as they do with antennae intact.[16] Night flyers cannot use sun light for navigation. Most of these species rely on the Moon and stars instead.

Earth's magnetic field: A number of moths use the Earth's magnetic field to navigate, as a study of the stray heart and dart[17] suggests. Another study, this time of the migratory behaviour of the silver Y, showed that this species, even at high altitudes, can correct its course with changing winds, and prefers flying with favourable winds, which suggests a great sense of direction.[18] A study in Panama of Aphrissa statira, which migrate in large groups annually, found that this species loses its navigational capacity when exposed to a magnetic field, suggesting they use the Earth's magnetic field to determine their direction of flight in migration.[19]

Distribution map of Acherontia atropos. Red, all year. orange, summer

• In the Indian subcontinent, migrating Lepidoptera are common just before the monsoon season. Over 250 species are known to migrate during this period.[20][21]
• On Madagascar, Chrysiridia rhipheus migrates between populations of four plant species of the genus Omphalea, the host plant of this species. The three western Omphalea species live in dry coniferous forests, the eastern species is found in the rainforest and is the only species that is green year round.[22]
• Rhodometra sacraria is normally found in Africa, large parts of Asia and southern Europe. At times, it migrates north and can reach central and northern Europe.
• Vanessa cardui is found all over the world, except South America, at altitudes of up to 3,000 meters above sea level. Its home, however, are subtropical steppe areas. Its relative Vanessa atalanta is also migratory.[23]
• Several South and Central American species of Uraniidae display a great peak in migratory behavior in certain years. In the years with a great number of migrating individuals, there are "population explosions". Individuals migrate to the south and east. There is no real re-migration to speak off. These moths feed on Omphalea species. These can be found in scattered populations all across South and Central America, but only part of the areas where they are found are a permanent habitat for these moths. Research suggests that the cause of the migration peak is an increase in toxicity when much of a single plant is eaten and decreasing toxicity when only small amounts of a certain plant are eaten.[24]
• The migration of Euploea core, Euploea sylvester, Tirumala septentrionis, and Tirumala limniace between the Western Ghats and Eastern Ghats in India cover up to 350 to 400 km. This migration happens twice a year. The most probable reasons seem to be the heavy monsoons, reproductive diapause, and a lack of host plants, nectar and alkaloidal resource availability.
• The Australian painted lady periodically migrates down the coast of Australia.[25] Occasionally, in periods of strong migration in Australia, these butterflies have been known to migrate to New Zealand.[25] The bogong moth also migrates biannually towards and from the Australian Alps.[6]
• The gatekeeper butterfly (Pyronia tithonus) from southern and eastern Britain has been expanding to northern Britain over the past three decades, despite their preference for warmer conditions.[26]
• The clouded yellow (Colias croceus) migrates between Europe and Siberia to Northern Africa and Southern Asia.[27][28][29][30]
• In North America, the true armyworm moth (Mythimna unipuncta) and the black cutworm moth (Agrotis ipsilon) migrate north in the spring to escape dangerously warm temperatures and migrate south in the fall to avoid extremely cold weather. The reproductive systems in both females and males are less developed during the south-bound migration and calling behavior increases in the north-bound migration, indicating that mating is more favored in the summer.[31][32]
• Anagrapha falcifera (a celery looper) is another migratory moth in the United States.[33]

• 
  Tirumala septentrionis
• 
  Vanessa cardui
• 
  Appias albina
• 
  Catopsilia pomona
• 
  Euploea core
• 
  Euploea sylvester

The green areas are the areas where Macroglossum stellatarum can survive year round. In the blue areas it can be found in summer. In the yellow area it is found in winter

Macroglossum stellatarum is a moth that is recorded in the subtropical part of the Palearctic realm year round. In summer, the species disperses north up to Scandinavia and Iceland. In winter it migrates further south, deeper into Africa and to the Indian subcontinent.

In the Netherlands and Belgium, there are 100 to 200 records per year in an average year. In warm summers however, like 2005 and 2006, several thousand are recorded.[34] In mild winters, small numbers can survive this far north, but these numbers are insufficient to call it a real population.[3]

A phenomenon like migration is an evolutionary development. By migrating, the species has survived the process of natural selection. There are a number of advantages and disadvantages to migration. An example of this is the protozoan Ophryocystis elektroscirrha a parasite of the monarch. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.[35] Infection with this parasite creates an effect known as culling whereby migrating monarchs that are infected are less likely to complete the migration. This results in overwintering populations with lower parasite loads.[36]