How Do Butterflies Migrate?
Most butterflies are light enough to drift on a breeze, yet millions of these fragile insects migrate across continents each year. With the possible exception of the famous monarch (Danaus plexippus plexippus), migratory butterflies are less well-known than migratory birds but no less remarkable. Scientists have identified nearly 600 migratory butterfly species, found in both tropical and temperate regions and across every major butterfly family. Their journeys follow the changing seasons, the availability of flowers rich with nectar, and the growth of host plants for their caterpillars. Some butterflies undertake long-distance migrationsâfrom deserts to coastlines to mountain passesâthat rival those of birds in scale.
The recapture of marked monarch butterflies has revealed that they can travel as far as 130 km (80 miles) in one day. The longest distance recorded thus far for the complete flight of a single migrant monarch butterfly is 3,010 km (1,870 miles).
Unlike migratory birds, however, most butterflies do not complete these epic routes in a single lifetime. The majority of lepidopteran migrations are multigenerational, unfolding as a relay of life. One generation flies part of the way, lays eggs, and dies; the next generation matures from larva to adult and continues the trip. Each leg of the migration thus represents an individual insectâs first and only voyage. In North America, migratory monarchs move between Mexico and Canada over about four generations, with one especially long-lived generation making the majority of the trip south to overwinter. The painted lady (Vanessa cardui) crosses from North Africa to northern Europe and back in several generations. In Asia and parts of Australia, swarms of lemon emigrants (Catopsilia pomona) move with the monsoon winds, whereas the nearly cosmopolitan red admiral (Vanessa atalanta) tracks the seasons across North Africa and Europe and in North America. The Australian caper whites (Belenois java) form great seasonal swarms as they move, possibly seeking larval food sources and mild temperatures. Although not a butterfly, the nocturnal bogong moth (Agrotis infusa) is an Australian migratory species known for its 1,000-km (621-mile) springtime journey to mountain caves in New South Wales and Victoria.
The exact mechanisms that migratory butterflies use to navigate are still being uncovered. Instinct is clearly a key factor, because parents do not show their progeny the way. The well-studied migratory monarchs use a combination of the Sunâs position in the sky and their own biological clocks to create a time-compensated solar compass, meaning the compass adjusts with the shifts in the Sunâs position throughout the day. They also take directional cues from the Earthâs magnetic field. The night-flying bogong moth is also well researched and, incredibly, navigates using the stars as well as geomagnetic information correlated with visual landmarks. Painted ladies and the ubiquitous red admirals utilize a time-independent solar compass, meaning the Sun provides only a simple heading indicator, and they do not seem sensitive to magnetic cues. Other migratory species, such as lemon emigrants, may rely primarily on such visual landmarks as rivers and valleys to guide them.
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