The butterflies’ migration

Not everyone gets this, but butterflies migrate too! And oh, what a journey! Some of the migrating species are particularly well-known, like the monarch butterflies (Danaus plexippus), and they make for one of the most spectacular natural phenomena in the world.

Every year, during spring, millions of monarch butterflies begin their journey northward, starting in Central America and arriving in Canada, from where, once winter comes, the next generation - their offspring - leaves to return to Mexico. This journey is big! It covers about 5,000 km and it takes four generations to cover the entire route. Most butterflies have much shorter life expectancies, from a few weeks to a maximum of 2 months, but the fourth generation of monarchs, which is born at the end of summer, can survive up to 6 - 8 months. This last generation, moreover, cannot reproduce, and so avoids "useless" reproductions as these butterflies will live for the whole winter in the south.

But how do butterflies orient themselves? In order to decide when to leave, and where to go, monarch butterflies use a combination of internal stimuli (a metabolic "clock" that tells them when the migration season is approaching) and external stimuli (the position of the sun). Unfortunately, climate changes, habitat destruction, pollution, and other anthropogenic factors are creating enormous problems for them. In 1997 their "wintering grounds" occupied about 45 hectares of Mexican territory, while nowadays these lands cover just 5 hectares!

However, the monarch butterfly is not the only one migrating. In Europe, there are also species that make long journeys to move as seasons change. In spring, the thistle vanessa (Vanessa cardui) completes long migrations from West Africa to Scandinavia, traveling 12,000 to 14,000 km round trip, one of the longest seasonal journeys regularly undertaken by an insect. These long trips are also made possible by favorable winds regularly blowing between Africa and Western Europe, provided that the butterflies manage to fly at an altitude of 1-3 km above sea level, so as to be able to exploit their thrust. Indeed, with their strength alone and without the help of the winds, the vanessa butterflies would reach a flight speed of 6 meters per second, too little to cross the Sahara in flight. To succeed, however, the butterflies must fly non-stop during the day and can only rest at night, stopping from time to time to stock up on nectar. 

Vanesses are great travelers, in fact even the Vanessa atalanta is able to carry out long migrations: at the beginning of spring the adults wintering in central-southern Europe begin to migrate northward in the Scandinavian regions, sometimes pushing even to Iceland. From mid-August, instead, the return migration southwards to reach the warmer wintering environments begins. Also for this reason the Vanessa atalanta is one of the butterflies that can be observed in flight until late autumn. 

Lastly, we discovered that Cabbage flies also migrate! The adults of the Major Cabbage Fly, Pieris brassicae, leave North Africa in spring and after several stops, to reproduce, they reach Northern Europe where they spend the summer. In autumn, thanks to the warm air currents blowing between 1000 and 1500 meters, they cross Europe and the Mediterranean, reaching North Africa. It has been calculated that the butterflies can fly for 8-10 hours a day at a speed of about 15 Km/h.

A curiosity, in conclusion: the cabbage moths (butterflies of the genus Pieris) are named this way because they grow from larvae that during winter, develop and feed on cultivated and spontaneous Brassicaceae (cabbage plants).

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