Many populations of Lepidoptera (butterflies or moths) migrate, sometimes long distances, to and from areas which are only suitable for part of the year. Lepidopterans migrate on all continents except Antarctica, including from or within subtropical and tropical areas. By migrating, these species can avoid unfavorable circumstances, including weather, food shortage or over-population. In some lepidopteran species, all individuals migrate; in others, only some migrate.
The best-known lepidopteran migration is that of the eastern population of the monarch butterfly which migrates from southern Canada to wintering sites in central Mexico. In late winter/early spring, the adult monarchs leave the Transvolcanic mountain range in Mexico for a more northern climate. Mating occurs and the females begin seeking out milkweed to lay their eggs, usually first in northern Mexico and southern Texas. The caterpillars hatch and develop into adults that move north, where more offspring can go as far as central Canada until next migratory cycle.
The Danaids in South India are prominent migrants, between the Eastern Ghats and Western Ghats. Three species will be involved in this, namely Tirumala septentrionis, Euploea core and Euploea sylvester. Sometimes they are joined by lemon pansy (Junonia lemonias), common emigrant (Catopsilia pomona), tawny coster (Acraea terpsicore) and blue tiger (Tirumala limniace).
Definition
Migration in Lepidoptera means a regular, predictable movement of a population from one place to another, determined by the seasons.Scoble, MJ. (1995) "Migration" in The Lepidoptera: form, function and diversity. 68-71. Previewed in Google Books on 09 Oct 2009. There is no unambiguous definition of migratory butterfly or migratory moth, and this also applies to proposals to divide them into classes.Meerman, J.C. (1987) "" Wet. meded. KNNV 180. Migration means different things to behavioral scientists and ecologists. The former emphasize the act of moving whereas the latter discriminate between whether the movement has been ecologically significant or not. Migration may be viewed as "a behavioural process with ecological consequences".
Migration in Lepidoptera takes place in two of the three modes of migration identified by Johnson (Johnson, 1969). In the first mode (also Johnson's first), the Lepidoptera move in one direction in their short life-span and do not return. An example is the pierid butterfly, Ascia monuste, which breeds in Florida but sometimes migrates along the coast up to 160 kilometers to breed in more suitable areas.
In the second mode (Johnson's third), migration takes place to a place of hibernation or aestivation where they undergo diapause and the same generation survives to return. The classic example is that of the nymphalid monarch butterfly (Danuas plexippus).
Adventive
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.
Seasonal migration
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.
Difference with bird migration
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. 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.Common, I. F. B. (1954). "A study of the ecology of the adult bogong moth, Agrotis infusa (Boisd) (Lepidoptera: Noctuidae), with special reference to its behaviour during migration and aestivation". Australian Journal of Zoology. 2 (2): 223–263.
Flight behaviour
Migratory Lepidoptera are, in most cases, excellent flyers. Species like Vanessa atalanta are capable of managing a fierce headwind. In case of headwind, they usually fly low and are more goal-oriented.J. Blab, T. Ruckstuhl, T. Esche en R. Holzberger (1989) Actie voor Vlinders, zo kunnen we ze redden, Weert: M&P. During migration, some species can be found at high altitudes, ranging to up to two kilometers. This is especially noteworthy for day-flying species like Vanessa atalanta, since the temperatures on these altitudes are low and day-flying species depend on the outside temperature to stay warm. It is thought that Vanessa atalanta produces enough body warmth during flight since it has also been recorded migrating at night.
In the case of transcontinental migration where distances are large, the flying speed of the butterfly (of the order of 3 metres per second or less) is inadequate for timely completion of journey. The migration is carried out by relying on heavy winds; a persistent wind speed of 10 metres per second being able to provide a displacement of 300 to 400 kilometers in a single day. For example, the painted lady (Vanessa cardui) migrates from Africa to Spain aided by tail winds.
That the migratory species are good flyers, is not the same as saying they are robust flyers. The small diamondback moth is also a migratory species that migrates 3,000 kilometers and can be found up to altitudes of 100 meters or more.Yau I-Chu (1986) "The Migration of Diamondback Moth " in: Diamondback Moth Management, Proceedings of the First International Workshop, Tainan, Taiwan, 11–14 March 1985. Shanhua: The Asian Vegetable Research and Development Center. Inhoud .
Navigation
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.
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. 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. 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. 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 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.Scientists make compass discovery in migrating moths , University of Greenwich. 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.
Multigeneration migration Some species of Lepidoptera are able to migrate over multiple generations. Painted ladies (Vanessa cardui) are known for the longest migration on Earth, approximately 9000 miles round trip (nearly double that of the Monarch butterfly). This journey may take up to six generations of Painted Ladies, though some individuals have been recorded to fly a staggering 4200km Suchan, T., Bataille, C.P., Reich, M.S. et al. A trans-oceanic flight of over 4,200 km by painted lady butterflies. Nat Commun 15, 5205 (2024). https://doi.org/10.1038/s41467-024-49079-2 . Genetic analysis of pollen grains transported by hundreds of butterflies collected across ten countries allowed researchers to track down the origin of a migration event that took place over 9 months from 2018 to 2019. Most likely, the butterfly outbreak originated in northern Arabia and the Middle East, when vegetation was unusually abundant in late 2018 to early 2019. Their migration to eastern Europe, apparently spanned two or three generations and finally reached western Europe during the summer of 2019.
Areas where migratory butterflies and moths can be found
Migratory Lepidoptera can be found on all continents, migrating within or from the Tropics and Subtropics. North, they can be found up to Spitsbergen, above the Arctic Circle. Some migratory Lepidoptera have spread over most of the world. Some of these are pest insects, such as the diamondback moth, Helicoverpa armigera, and Trichoplusia ni.
Examples of migratory Lepidoptera
In the Indian subcontinent, migrating Lepidoptera are common just before the monsoon season. Over 250 species are known to migrate during this period.C.B. Williams (1930) The Migration of Butterflies Oliver & Boyd: Edinburgh. 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. 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.