Moths catch the wind to speed migration
Understanding how insects travel might help to predict pest invasions.
Far from being at the whim of winds, migrating insects can hitch a ride on favourable jet streams and adjust their flight direction to get to warmer climes, a UK study shows.
"Insect migration was always thought to be a rather chancy process," says study author Jason Chapman of Rothamsted Research, the agricultural research institute in Harpenden, UK. "We show evidence that a wide range of insects exert quite a lot of control on their pathway and are not, in fact, at the wind's mercy."
The study, published today in Science1, used radar to track the movement of more than 100,000 noctuid moths, hawkmoths and butterflies as they migrated to northern Europe in the spring and south to the Mediterranean in autumn every year between 2000 and 2007.
Chapman and his colleagues report that the insects can control the direction in which they fly, selecting the most favourable winds to maximize the distance they travel.
The team also found that insects tend to fly higher up, where the wind is fastest. The silver Y noctuid moth, for example, flies 425 metres above ground — higher than the top floor of the Empire State Building in New York. How the moths detect these altitudes is still being studied, but sophisticated mechanisms that determine wind speed while they are flying help them to reach average speeds of 54 kilometres an hour. With an additional jet stream push from behind, they can achieve top speeds of up to 90 kilometres an hour.
The moths seem to be able to detect the wind's direction and, using some sort of internal compass, correct their flight path. These moths only take flight when winds are at least somewhat favourable — never flying across the wind or into it. If winds don't completely align towards their destination, migrants can partially correct for that drift, keeping themselves on their preferred trajectory rather than simply being propelled downwind. In spring, migration corresponded with the northward wind. But in the autumn, migrants still arrived south even though the prevailing wind tended to blow eastward.
It is well known that migratory birds are able to control their flight direction, sometimes flying across the wind. But because insects have a shorter lifespan than birds, they can't afford to waste time getting to their breeding grounds.
"Because insects fly slower than birds, they had to evolve a way to increase their speed," says Chapman. "The way they've done this is to really exploit the wind." With favourable winds, moths are faster than birds, Chapman says. The insects' flight behaviour can add 4–6 metres per second on top of the push provided by the wind, allowing them to travel a distance of up to 700 kilometres during 8 hours of flight.
"This finding is remarkable. People thought that once these migrants flew up to the fast-moving air stream, they'd be transported to the 'correct' habitat," says entomologist Ring Carde of the University of California, Riverside. "We assumed that nocturnal moths didn't actually know their cardinal trajectory, that their transport was passive."
Chapman and his team plugged their observations into a computer model, called NAME, used by the UK Meteorological Office for mapping the spread of airborne pollutants. They ran two simulations: the first used inert particles blown passively by the wind, and the second included moth-like flight behaviours, such as the ability to orientate themselves towards their destination and climb to altitudes with the fastest winds.
The researchers show that these moth-like behaviours increase the distances migrated by 40%, but also decrease drift away from the destination. The simulated moths travelled nearly 100 kilometres farther than the passive wind-borne particles during the 8-hour periods simulated in the model.
At least 2.3 billion insects were involved in the high-altitude mass migrations observed by the team. Some are agricultural pests, "shuttling back and forth, breeding all the time", Chapman says. With the effects of climate change, we could soon see greater numbers of them arriving earlier, and expanding farther across northern regions.
Chapman anticipates that the work could result in a way of predicting where and when pests are likely to arrive. Crop growers could be informed of pending pest invasions, giving them more time to implement mitigation methods that would prevent the pests from eating crops or laying eggs that hatch into voracious larvae.
"It seems a shame to kill these insects. They've evolved an amazing capacity to move around the world, but that's part of what makes them dangerous pests," Chapman says. "They are hugely successful, migrating fast and breeding all along the route, and being a successful migrant means eating whatever crop is growing when they arrive."
Alma Solis from the Systematic Entomology Lab at the US Department of Agriculture in Beltsville, Maryland, says that the work is a "great leap forward" for uncovering the processes that speed insect migrations. "No one has been able to predict the mass movement of insects," she adds.
- Chapman, J. W. et al. Science 327, 682-685 (2010).
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