Finding your way home in the dark can be tricky. But migrating moths manage to cover huge distances at night by riding high-speed gusts of wind toward their breeding site. And they don’t just catch some air and hope for the best, a new study suggests: Even when the wind strays off-course, the nocturnal commuters use an internal compass to stay on track.
Some migrating insects, such as dragonflies and butterflies, were already known to tweak their flight paths to compensate for wind drift. But these low-flying species travel during the day, when the sun and other visual cues can guide their journey (ScienceNOW, 8 January). In contrast, migrants such as the crop-gobbling silver Y moth (Autographa gamma) prefer to cruise on faster, high-altitude air currents that mainly occur at night. This risky strategy has puzzled scientists, who wondered how the moths avoid being blown away from their seasonal breeding grounds.
To investigate, an international team led by entomologist Jason Chapman of Rothamsted Research, an agricultural science center in Harpenden, U.K., tracked swarms of silver Y moths leaving the United Kingdom for their winter breeding site in the Mediterranean. The researchers used vertical radar beams to pick up the movements of moths whizzing more than 150 meters overhead. After analyzing 42 mass migration events over two summers, they found that more than 90% happened when the wind was blowing roughly southward. This could mean the moths test the wind each night before deciding to catch a ride, the researchers suggest in a study reported online this week in Current Biology.
The team then looked for evidence of a biological compass that would explain the moths’ pickiness. They found that during most of the mass migrations, a significant proportion of the moths pointed their bodies in the same direction, suggesting that they shared a genetic instinct telling them where to go. When the wind direction was off by more than 20 degrees, the moths changed their flight angle to nudge themselves back on the right path.
Together, these results suggest “an extremely sophisticated set of behaviors,” says Chapman, who believes a magnetic compass is the most likely explanation. “They’re not just randomly being blown wherever the wind happens to be blowing.” Chapman says this is the first study to demonstrate a compass sense in nocturnal migrating insects, which are more difficult to observe because they fly at great heights.
Understanding the moths’ sense of direction could help scientists predict future insect migrations, which might increase as global warming makes northern countries more hospitable to pests, says ecologist Robert Srygley of the University of Oxford, U.K. “It makes our job a lot easier,” he says. The next challenge is to figure out how the compass works, says animal navigation biologist Richard Holland of the University of Leeds in the U.K., who wasn't involved in the research. That will require tracking individual moths, which may become possible when radio transmitters currently used for following larger insects get small and light enough to be mounted on a moth.