When nocturnal insects make their high-flying journeys through the darkness of night, they may have more than an internal compass to guide them on their way. Researchers reporting in the Cell Press journal Current Biology on August 31 show that Silver Y moths (Autographa gamma) also rely on turbulence cues to keep themselves from drifting off course in the wind.
“Insects cannot sense the mean airflow directly because they are carried along by it, and as a result they do not feel the air flowing past them, just as a man at sea far from land cannot tell if the boat is drifting or not,” explains Andy Reynolds of Rothamsted Research. “But they can sense turbulent fluctuations in the airstream because they get buffeted from side to side by them.”
The ride gets bumpiest when insects cross the mean airflow and smoothest when they head directly downwind. The turbulence therefore allows the insects to determine which way the wind is blowing. That’s something migratory songbirds apparently don’t do, the researchers say.
Reynolds, along with Jason Chapman, Cecilia Nilsson, and their colleagues, knew that insects and birds must have some way to cope with the wind when they travel at high altitudes. What they didn’t know was how: Do insects and/or birds rely directly on the wind currents or do they depend on visual cues from the ground?
To find out, the researchers used radar technology to track migrating birds and moths. The difference between the insects’ and the birds’ behaviors under various wind conditions was clear: “We could see that moths respond differently to winds from the right or the left, while birds do not seem to do so,” Reynolds says.
The story does have a twist: On cool nights, the mean wind direction veers with increasing height–a phenomenon known as the Ekman spiral. As a result, insects could be misled at times by turbulence cues to respond as though the wind were blowing them farther to the left or the right than it really is. (Right offsets are expected for insects flying in the Northern Hemisphere; left offsets are expected in the Southern Hemisphere).
“[We realized that] these ‘right offsets’ are a smoking gun–if present, they would provide near-irrefutable evidence that the migrants are relying on turbulence to detect the mean wind direction,” Reynolds says. “This is what we looked for and this is what we found for insects. It was not found in the songbirds.”
The findings show that migratory moths and birds solve the problem of wind displacement in different ways. The researchers say that more studies are needed to determine exactly how the moths monitor turbulence and how widespread this ability is among insects.