Insect migrations

Among arthropods, insects are provided with wings, as are birds among vertebrates. Presence of wings gives these animals increased mobility. Insects show migrations, which, in some respects, remind one of the better known bird migrations.

Cases of insect migrations may be grouped under three categories:

(1) Seasonal migrations.

(2) Migrations to new lands/areas due to insect's special attributes.

(3) Migrations due to human activities.

Seasonal migrations of butterflies have been known to man quite long. Early explorers of the New World witnessed this phenomenon without clearly understanding it. Even Columbus, when his fleet was approaching Cuba, saw large swarms of butterflies almost darkening the sky. There were huge cricket migrations (Anabrus simplex) when the Mormons came to Utah in the USA. The Mormon cricket is a species endemic to western North America.

Seasonal migration of the monarch butterfly (Danaus plexippus) has been well studied by American entomologists. When winter is approaching, these butterflies from all over USA and Canada migrate southward, flying in swarms, and cover up to three thousand kilometers, and reach Mexico, Florida and Cuba, where they settle on trees, covering their trunks and branches almost fully. During winter, they remain almost motionless, crawling away at times to escape direct sun. Butterfly covered trees are a tourist attraction. Activities in the migrants return in spring, and now swarms start in a northward journey. During this return journey they often breed and lay eggs on milkweed. Parents die, and the offspring continue the northward migration. By the time the swarms reach their summer station, it is the third generation. In the next autumn the butterflies migrate again, and come to rest on the same trees as their forefathers in the southern destination (Akimushkin, 1973). The migration of the western populations of the monarch in the USA is more complicated. These populations overwinter in aggregations along the coast of southern California.

South American monarchs (a different species from the North American) also show weak seasonal migrations, towards the equator in autumn, and southward in spring (Akimushkin, 1973). Danausgilippus in South America is non-migratory, a feature which has allowed the evolution of several subspecies.

Northward and southward migrations, in spring and autumn respectively, are shown also by some European butterflies. The painted lady (Cynthia cardui), the red admiral (Vanessa atalanta) and the death's head hawk moth (Acherontia atropos) are some examples of this. The painted lady migrates in large swarms. After spending winter in Africa, they gather in swarms again, and fly towards Europe. PJ saw them crossing Morocco, in Rabat, once a year, and flying north during more than one day by millions. The whole town of Rabat was covered with the butterflies. C. cardui is widespread on most continents with the exception of South America, where it is rare, and of New Zealand. They fly over the Mediterranean, and fly high over the Alps. They reach north Germany, Britain and Russia. Later they are in Scandinavia. In their migration they cover thousands of kilometers. During summer the migrants, arriving from Africa, breed in Europe. After egg laying the parents die. Their progeny, which are larger and more brightly coloured than their parents, fly southward to North Africa to breed in winter there. The painted lady cannot survive the winter in Northern Europe and Britain.

In the migration pattern of the painted lady there is an indication that they follow warm air currents. For example, they reach Britain, before settling in the Western Europe, which is more southern. This is perhaps because the British coasts are warmed by the Gulf Stream.

There is an obvious similarity in the migration of birds and of these butterflies. In the northern hemisphere both migrate to a southern location in autumn, and return in spring to their northern station. But there is a significant difference between them. Most long range migratory birds have their breeding grounds in a northern station. When winter is approaching, they, along with their young ones, migrate to the warmer conditions of a southern station in search of new feeding grounds. In spring they return to the northern quarters. Thus the same generation performs both southward and northward journeys, whereas the butterfly generation, performing northward/southward journey, is separated by one or more generations from the generation in the previous journey in the opposite direction. A particular butterfly generation, if it migrates, migrates only once, either northward or southward. How do they follow the same route and reach the same station as their parents/grandparents? The only answer we have at present to this question is "instinctively". In fact study of butterfly migrations is in its initial stages. Studies in coming times may unfold some interesting details.

Some dragonfly species also show seasonal migrations. Swarms of dragon-flies have been seen flying southward across the Alps in autumn.

Lady bird beetles too are known to undertake seasonal migrations like some butterflies and dragonflies (Akimushkin, 1973). They are believed to swarm southward and northward in autumn and spring respectively. Swarms of lady birds have been seen flying across countries in Europe, Africa and America. In California they have been seen feeding on aphids and other phytophagous insects in fruit orchards in valleys. In autumn they move up on hills as swarms, and high up on the hills they enter into winter sleep or diapause under stones and dry leaves on the ground. When it is warmer in spring, they become active again, and move into valleys, where fruit trees are in blossom, and harbour a good supply of plant feeding insects. The great French entomologist J. H. Fabre saw a small chapel, built on a hill top, with all the walls and the roof covered with a continuous sheet of small red globules. Coming closer to the building he realized that the globules were actually overwintering lady birds.

Now let us turn to those insects which migrate to new lands due to their special features and establish themselves in new areas. The monarch butterfly of North America (Danausplexippus) is a great flier. It can cover thousands of kilometers. It has been able to cross even the Pacific Ocean.

In 1850 these butterflies were first seen in the Hawaii. After a decade they appeared in New Zealand, and later in Australia.

Another striking example of a species, migrating to new lands and extending its range with help of its special attributes, is a tiny flea beetle, Chaetocnema confinis (Kalaichelvan et al, 2001). This species originally belongs to North America. But in recent years it has extended its range enormously. Now it is in Central and South America, Africa, Southeast Asia and in some Pacific islands. Outside N. America it was first seen in 1979 in the island of La Reunion by one of us (PJ). After that it has been collected from Mauritius, Madagascar and east Africa. In 1996, it was reported from the Palau Island, Ryukyu Archipelago of Japan, northern Thailand, Vietnam, Taiwan and the Hawaii. In 2001, the present authors (along with Serge Doguet and Mr. Kalaichelvan) reported presence of this insect in India. It is strongly suspected, though not yet confirmed, that it is present in China, and, perhaps, in Australia (Jolivet, 1998, 2000).

The following special attributes of this flea beetle seem to be helping the insect in spreading its range almost throughout the tropical and the subtropical world.

(a) Its light and small body (about 1.5 mm in length), such that it may be readily carried away by winds and air currents.

(b) Its excellent flight capacity.

(c) Its polyphagous habit, as it feeds on leaves of various plants, though basically it is a feeder on a number of Ipomoea species.

(d) Its female is facultatively parthenogenic. If a single female reaches a new area, through parthenogenesis she can establish a new population. It may be mentioned here that males of this insect are known only from the New World. Elsewhere there are colonies of only parthenogenetically reproducing females.

Locusts are grasshoppers, with short antennae (Acrididae), with an inherent property of swarm forming and migrating. They show polyphenism, i.e. they, under certain environmental conditions, produce a swarm forming and migratory phase. A locust swarm moves a long distance, destroying all vegetation in the way, and producing a famine like condition in the countries covered. Locust breeding grounds are oases in deserts. They live like any grasshopper species, but, due to continued breeding, the population density in the limited breeding ground increases, and, when the population density has reached a certain high, a shift in the direction of the migratory phase occurs. Eventually the migratory phase is produced, and a swarm leaves the breeding ground. The migratory phase differs from the nonmigratory phase both in structure and physiology.

Some species of seed weevils or bean weevils (the beetle family Bruchidae), which infest stored legumes, show a phenomenon with some resemblance with the migration of locusts. When the density of the seed weevil population in a store becomes quite high, a new phase develops, which has been referred to as the flight phase or the active phase. The active phase individuals have only partly developed reproductive organs, greater flight capacity and a migratory tendency. They are meant for reaching new stores, though they do not fly as a swarm. One of us (KKV), along with his students, has studied this phenomenon in Calloso-bruchus analis and C. maculaius (Tiwary ei al, 1989; George ei al, 1994).

Let us now discuss some cases of insects extending their range and reaching new countries and continents through human activity.

The vine louse or the vine Phylloxera is a tiny insect, similar to an aphid. It infests roots of grape vines, producing small swellings in the roots or root galls. It was originally in North America. In 1918, it suddenly appeared in France. In the new country, it soon became a serious pest of grape vines, making the vines dry up, and threatened end of the wine industry, though in N. America it was not doing appreciable damage to grape cultivation. While the French wine industry was trying to survive by importing grapes, the pest spread through the rest of Europe. It was realized that the American variety of grape plants was resistant to the vine louse. The French growers succeeded in saving their grape cultivation by importing grape plants from America, and by using them as stocks, on which they grafted their own variety of grapes (Akimushkin, 1973).

The spread and distribution of the grape Phylloxera is believed to have been through human commercial movements. The winged phase of the grape lice is a weak flier, and, therefore we cannot imagine that it crossed the Atlantic on its own.

Another particularly notable example of an insect migrating through man's commercial activities is of the Colorado potato beetle (Leptinotarsa decemlineata). This insect was harmless, originally confined to the eastern slopes of the Rocky mountains in North America, feeding on leaves of night-shade, a local weed of the Colorado region. When European, moving across the N. American continent, reached its western parts, they started potato cultivation. The Colorado beetles readily took to potato leaves, and they particularly liked the young and tender leaves of this new food plant. The voracious feeding on potato leaves seems to have greatly improved their fecundity. A single female lays about 700 to 2800 eggs. Larvae grow fast, feeding on young potato leaves, and soon develop into the next generation adults. After recovery from a winter diapause and before it is time for the next winter dormancy, three generations are produced. It has been estimated that a single female at the beginning of an active period theoretically leaves behind 80 million descendants at the end of that period. Predators and parasitoids regulate the numbers.

Soon the Colorado beetle spread through most of N. America. In 1871 it had reached the Atlantic coast of the N. American continent. In 1876 it appeared in Germany, and then quite quickly it moved to reach most of Europe. In 1990s it came to China (Jolivet, 1991a and b; Jolivet, 1994).

In Europe efforts to fight the potato pest was not yielding satisfying result. The governments of Germany and France made laws to prevent any further import of potatoes from America. Germany used its army to fight this severe menace to potato cultivation. The army men dug trenches around an infested field, and then, after sprinkling oil, the infested crop was burnt (Akimushkin, 1973). Next year only some potato plants were grown as a bait for the potato beetle, if any were still surviving. Very few beetles were attracted to the bait. Hence it was inferred that the pest could be eradicated. Then came the first world war. Soldiers had to be withdrawn from their agricultural assignment. In 1914 the pest again appeared in a serious form. Perhaps this second appearance was due to stages of the pest moving with provision and baggage of American soldiers.

Fight against the Colorado potato beetle continues, though with application of modern methods of control, presence of the pest in the field does not cause panic. It seems, at least in Europe, reasonably contained.

A recent insect migrant with help of human commercial activities is the flea beetle Epitrix hirtipennis (Jolivet, 1998). It feeds on tobacco. It is originally from Mexico, Central America and USA. In 1984 it reached Italy. In 1993 it was in Turkey. Perhaps it is present in most of the Mediterranean countries.

In fact there are many examples of insects moving with crops and plant products from one country to another. The maize root worm (Diabrotica virgifera virgifera) of the New World has arrived in Serbia, near Belgrad, in 1992, probably with American planes (Jolivet, 1998). It has then invaded most of western Europe, has reached Paris, France, and a few years ago, Italy, Hungary, and is probably now also in Turkey. The old world Aulacophora, another related galerucine, is specially attracted by cucurbits and cucurbutacins, which are toxic compounds present in cucurbit plants, but Diabrotica is far more dangerous on other crops, including maize. So far, it does not seem much virulent in Western Europe.

Often for biologically controlling an insect pest or a noxious weed, insects are imported into a country from another country. But this has to be done after a very careful and well planned study of interaction between the species proposed to be imported and the fauna and flora of the receiving country. The case of the Guam Island is well known. A number of parasitoids had been introduced to eradicate some pests, but it resulted in wiping out hundreds of endemic species of moths.

Hugh Dingle (in Resh and Cardé, 2003) has reviewed insect migrations. He has pointed out that juvenile hormone, a time-compensated sun compass, and some other mechanisms, still to be studied, are involved in regulating such migrations. Lepidoptera, namely Uraniidae, the day-flying moths, in tropical America, Madagascar or New Guinea, Pieridae, Nymphalidae, also dragonflies, and large Hymenoptera seem to maintain a constant direction during migration. PJ has seen, several times, the migrations of Urania in Panama and Nicaragua and those of Alades (Uraniidae) in New Britain, and it is certain that nothing can make the moths deviate from their course. Sometimes, in New Guinea, a mimetic butterfly (Papilio laglai%ei) matches the migrating flight and derives its protection from the toxicity of the model, the uranids. Migrations of Uraniidae have been specially studied by Smith (1983) and Lees and Smith

(1991). There are nocturnal, non migrating Uraniidae (e.g. Lyssa, syn. Nyctalemon) in South East Asia, but the brightly colored day flying ones, Urania (tropical America), Chrysiridia (East Africa and Madagascar) and Alcides (New Guinea and N. Australia) are all migratory, and as caterpillars, feeders on the toxic euphorbiaceous genus Omphalea and related plant genera (Smith, 1992). The biochemistry of these plants may be one of the driving forces in population regulation, migration and strategy of the physiology of these butterflies (Smith, 1983, 1992). Hundreds and thousands of Urania fly synchronously every year generally in an eastward or south-eastward direction through Central America from Mexico as far south as northern Columbia, always in unidirectional dispersions, probably in response to diminished food in their usual breeding territories (Hogue, 1993). Strangely, contrary to theoretical predictions, the speed of flight among Urania fulgens in Panama is independent of both body mass and abdominal lipid mass (Dudley et al, 2002).

Dingle (loc. cit.) thinks that migrating butterflies incorporate a sun-compass, as in case of movements of honey bees and ants, and, that for the nocturnal migrants, other mechanisms, still not fully understood, are involved. Migration is a trait of considerable complexity and uraniid and danaid migrations are far from being adequately understood.

— Fig. 6.1. Leptinotarsa decemlineata (Say) — Fig. 6.2. Timarcha (Metallotimarcha)

(Col. Chrysomelinae), the Colorado metalüca Laicharting (Col. Chrysomelinae).

potato beetle, a great migrant (after P. A non-migrant, but nocturnal and feeding

Jolivet). on Rubiaceae and Ericaceae.

— Fig. 6.3. Chaetocnema confinis Crotch (Col. Alticinae) from North America, a great migrant (after P. Jolivet, 2000).

— Fig. 6.4. Papilio laglai%ei Depuiset (Papilionidae), New Guinea, the mime (photo P. Jolivet).

— Fig. 6.5. Alcides agathyrsus Kirsh (Uraniidae), the model. A case of Batesian mimicry (photo P. Jolivet).


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