Monarchs and locusts - champion insect migrants

Many of you who read last week's article on bird migration in this page may have been amazed at the ability our feathered friends have to fly such long distances. As we explained, animal migration is indeed a fascinating aspect of life many scientists are trying to understand.

In today's article, we will examine yet another segment of creatures from the animal kingdom that go globetrotting...

Like our feathered friends, many species of insects too migrate for various reasons. You may find it hard to believe that insects, though small in size and fragile-looking, undertake some of the toughest migrations on record. Among insects, the best known migrants are the butterflies and locusts.

Much studies have been carried out on insect migrations and many discoveries have been made. Migrating insects provide researchers with a living biological barometer; they provide clues about the state of the environment over the range of migration.

According to them, three types of migration are done; they are one way journeys to breed, journeys from breeding area to feeding area, and journeys from breeding area to hibernation.

Of all the insect migrations that have been studied, the most amount of research has been done on the spectacular migration of Monarch butterflies, which are also known as milkweeds, because it is their host plant.

These large, slow moving, orange hued butterflies found across much of North America in the summer make a long, long journey to their over wintering grounds in the south in enormous flocks amounting to millions. At the onset of autumn, the butterflies could be seen flying to Florida, Southern California, Mexico, the West Indies and Central America.

The Monarchs that live in South America fly even further south. Despite their fragile form, Monarchs can attain flying speeds of up to 15 km (9 miles) per hour and cover immense distances. They are believed to make a journey that covers a distance of 3,400 km or more.

How do these butterflies know when to migrate? Many factors such as the change in the number of daylight hours and fluctuating temperature are cues for them to move to warmer areas. The changes that take place in their host plant, the milkweed, also signals them that it's time to move. (The milkweed begins to turn yellow and dries up as the temperature gets colder).

At their winter destinations, Monarch butterflies settle on vast numbers on the branches of evergreen trees. Here they remain in a torpid (slow moving or motionless) state throughout the winter. In the spring, the butterflies that have survived the winter, once again begin their journey back north, to their summer grounds.

What's interesting about the migration of monarch butterflies is that on the way they breed, sometimes passing through as many as three generations. Individuals make the trip once; their children's grandchildren return south in the following winter.

Amazingly, the butterflies fly in masses to the same winter roosts, often to the exact same tree. How do they know where to go and which tree to roost on? No one really understands how their homing system works, but it is obvious that it is a completely inherited pattern of responses, because there are no opportunities for learned behaviour.

Researchers have noted that migrating insects, or in this case the butterflies, generally fly in a straight line. When meeting obstacles, rather than deviate to the left or right, they fly over it.

On what do these migrant butterflies survive? It is believed that the fat stored in their abdomen is a critical element for survival in winter.

This fat must not only fuel their flight, but also must last until the next spring, when they begin their flight back. Most Monarchs migrating southwards, stop to sip nectar and gain weight on the way.

Destructive migrants

Another major group of insects that migrate are the desert locusts. Unlike the beautiful Monarch butterflies, locusts cause a lot of damage to crops when migrating. In fact, they have become a pest to many nations who are now looking for ways to control the damage caused by swarms of locusts.

Even though a locust weighs only about two grammes, it eats its own weight in food everyday. Imagine millions of them congregated in a grain field, and the damage such a huge swarm could cause.

According to Food and Agriculture Organisation (FAO) officials, one ton of locusts can eat as much food as 2,500 people! So, desert locusts are a threat to the agriculture of many countries in Africa, Asia Minor and Western Asia.

What you would be surprised to learn is that most of the time, this destructive desert locust lives as a harmless grasshopper. It lives a solitary life doing no harm, laying eggs that hatch into fresh generations of normal grasshoppers; wingless at first and gradually developing wings as they mature into adults.

What makes them destructive desert locusts then? Confinement to limited amount of breeding grounds due to environmental conditions. When this happens, the juveniles (young) begin to form groups.

These groups form even larger ones and eventually a huge army! What's more, by this time the harmless, little grasshoppers have become gregarious (living in flocks) yellow grasshoppers and even developed longer than normal wings which enable them to fly farther and faster.

These huge swarms of locusts fly long distances, even across continents in search of food. In fact, the longest insect migration recorded is that of a desert locust amounting to about 4,500 km, wayback in 1988.

Once these locusts are airborne, they fly swiftly through the skies in vast clouds. They are generally guided by the wind and the Sun.

However, these swarms of locusts, which could contain millions of individuals, could break up due to weather fluctuations. And when eggs from these locusts hatch, they could be normal grasshoppers, once again.

It is not only Monarch butterflies and grasshoppers that migrate. Many other species of butterflies and insects also do. For instance, the Green Darner dragonfly makes a journey that can exceed 3,000 kilometres from Manitoba to Texas. It takes two generations of dragonflies to complete the journey.

The migration of dragonflies is less understood when compared with that of monarchs and locusts. However, it is an established fact that they commence their journey to warmer areas in late summer.

They find their way using natural landscape features such as sea coasts and large rivers. Migration takes place in large swarms, with no actual leader. It seems these insects generally follow the passage of cold fronts when migrating.

One thing that becomes very clear when looking into migration patterns of animals, especially insects and birds, is that the weather plays a key role. Climate change therefore could have a drastic impact on migration.

This could force them to either change their over wintering sites or to adapt to the changes in their own environment. The ultimate fate would be their total destruction. So, as all kinds of animals are needed to maintain the ecological balance, we must pay more attention to the impact our careless actions could have on the climate.

Find out more interesting facts about insect migration.

We too will enlighten you on this subject in our future issues, as we still have not dealt with fish, mammal and other animal migrations.

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Fact File

* There are four kinds of insect migration and two are known as dynamic migration and homeostatic migration. The desert locust is a dynamic migrator while the monarch falls into the latter category Dynamic migration is directed movement controlled by tides or wind, with navigational abilities not essential.

Homeostatic migration is a two way movement with migrant or offspring returning to breeding areas, hence the need for navigational abilities.