At any moment there is a flock of millions of insects flitting around. The insects do not twitter gaily like the birds flying across. They cannot be seen. Several are crop pests covering thousands of kilometres to make their way towards an unsuspecting field, aided by strong winds and a highly efficient inbuilt compass.
How creatures, barely a few centimetres in length, have evolved to undertake such long journeys has fascinated scientists for ages; some of the distances match those of migratory birds.
“The insects, even butterflies and moths, are believed to rely on fast-moving, high-altitude winds for migration,” said Jane Hill, biology professor from the University of York in the UK. But observing such migrations is difficult owing to the high-altitude flight, small body sizes and the fact that several species are nocturnal.
Jane Hill and scientists from Rothamsted Research in the UK have for the first time used radars to unveil the sophisticated flight behaviour of these insects. It was published by Science on February 5, 2010. These radars are not the ordinary air or sea traffic sensors. Specialized in studying insect flight, they emit a stationary beam vertically upwards that can detect individual insects as they migrate between heights of 200m to a kilometre above the radar. The radars measure the size of the insect, its height of flight and its speed and direction of movement.
Using radar data for more than 100,000 individual insects, collected from 569 mass migrations in spring and fall, the scientists have examined the flight behaviour of four groups of migratory insects: moths (Autographa gamma and Noctua pronuba), hawkmoths (Sphingids) and butterflies (Hedylids). These insects dislike the English weather to the extent that they flock all the way to the Mediterranean basin in winters and return to the UK during spring. A total distance of 4,000 km is covered in just 40 hours; some of the tiny butterflies are barely 15 mm in length and live for just a few days.
Another remarkable feature about their journeys is the insects know how to identify the winds most helpful in moving them in the right direction. For example, in the northern hemisphere the insects use very fast-moving winds to achieve rapid, long-distance transport northwards in spring and southwards in autumn. The inbuilt compass helps them choose the right direction as they travel at a speed of up to 100 km per hour. Although the insects travel downwind, they do make subtle adjustments to their movements so as to not let the winds drift them away from their preferred direction.
“A significant number of these insects are likely to be pests of crops or vectors of diseases,” says David Reynolds, professor at the Natural Resources Institute of the University of Greenwich in the UK. “Following the recent warm winter and spring in the UK, many aphid species started their spring migration much earlier. Thus, a better understanding of their migration strategies will be crucial in terms of pest management,” suggested Jason Chapman, ecologist at the Rothamsted Research.
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