For a high-flying insect, long-distance migration must be easy. The bug just ascends to a height where the air is zipping along — about 500 to 2,500 feet — and heads whichever way the wind blows.
That is what many scientists had assumed. But a study out of Britain shows that some moths and other high-fliers are active, not passive, migrators. They won’t ride just any wind, choosing high-speed tailwinds that are generally headed north or south depending on time of year and adjusting their own flight heading if necessary to compensate for drift.
Jason Chapman of Rothamsted Research in Harpenden, England, and colleagues used a vertical, narrow-beam radar to determine the headings of individual insects flying on their migration to or from southern Europe. They obtained data from more than 100,000 insects over eight years.
“What our radar tells us is that when the winds are in a seasonably favorable direction, moths are up there in large numbers,” said Dr. Chapman, lead author of a paper describing the work in the journal Science. “But the next night, if the wind isn’t in the right direction, we don’t see any.”
How moths detect wind speed is a mystery, Dr. Chapman said, but they probably use a visual mechanism to tell direction. And they must have some kind of internal compass as well as an inherited sense of direction.
Dr. Chapman said the data showed that when the wind direction was very close to north or south, the insects flew with it, reaching speeds up to 55 miles an hour. But if the wind deviated by more than 20 degrees, the bugs would change their heading. “They compromise,” he said. “They can’t afford to lose too much speed, but they want to stay as close as possible to north or south.”
Understanding how these creatures make use of winds may eventually help scientists who try to forecast the movements of large numbers of insect pests, Dr. Chapman said.