A new study has tracked migrating death’s-head hawk-moths by radio for up to 80km – the longest distance an insect has been continuously monitored in the wild.
By closely tracking individuals during migration, the world’s first study reveals a century-old mystery about what insects do on their long journeys.
The results reveal that the insects are capable of precise navigation, confirming that an internal compass guides them on their long journeys.
The study, published in the journal Scienceconfirms that sphinxes can accurately maintain straight paths over long distances, using sophisticated strategies to counter and correct adverse wind conditions.
Marked death’s-head hawk-moths have been tracked in flight up to 80 km in a straight line (Jon Evans).
With trillions of individuals migrating each year, insects are among the most common migratory animals on Earth. But even though migratory insects far outnumber better-known migrants, such as birds or mammals, their migrations are the least understood form of long-distance animal movement. The problem, for the most part, has been methodological.
“Studying insects in motion is a formidable challenge,” said first author Myles Menz, who conducted the research at the Max Planck Institute of Animal Behavior (MPI-AB) and is now a lecturer at James Cook University in Australia. “They are usually too numerous to be marked and found, and too small to carry tracking devices.”
Much of what we know about insect migration comes from studies that sample insects at a given time, such as by radar or direct observation, which has left vast gaps in our knowledge.
“Understanding what insects do during migration and how they respond to weather conditions is the final frontier of migration science,” Menz said.
The current study, which tracked radiolabeled individuals in a light aircraft, is the first to continuously study nocturnal migrating insects in the wild and represents the longest distance an insect has been continuously tracked in the field.
The Death’s Head Sphinx is a great nocturnal migrant that travels up to 4,000 km between Europe and Africa each year. Like many insects, the species is a multigenerational migrant, meaning that no individual knows the entire journey.
At MPI-AB, the team bred caterpillars to adulthood in the lab to make sure the individuals were naïve. When the moths emerged as adults, they were affixed with radio beacons weighing 0.2 grams, or less than 15% of adult body weight.
“Butterflies would probably eat more weight than that in one night, so these tags are extremely light on insects,” Menz said.
The researchers released the tagged butterflies and waited for flight to begin, after which they chose a single individual to track at a time.
They tracked 14 moths each for up to 80km or four hours – a stretch long enough to qualify as a migratory flight – using antennae mounted on a Cessna aircraft to detect precise locations every five to 15 minutes. The insects were tracked in a south-southwest direction from Konstanz in the Alps, which follows the route taken by the sphinxes to the Mediterranean and northwest Africa.
The Death’s Head Hawkmoth migrates annually between Africa and Europe (Doug Kelson).
Due to the practical constraints of flying in airplanes, the scientists tracked the moths continuously until the insects stopped en route.
“When you’re on a plane, it becomes extremely difficult to wait for the bugs to start migrating again because you would have to be in the air when it happens, which could be any time of the night,” the author said. principal Martin Wikelski, a movement ecologist from MPI-AB and the University of Konstanz, who piloted the plane during the study.
The results show that the butterflies maintained perfectly straight trajectories over long distances during flight. It wasn’t because they were expecting favorable winds. Instead, they used a range of flight strategies to shield themselves from prevailing winds, allowing them to maintain their course throughout the night. When the winds were favorable, they flew high and slow, allowing the air to carry them. But in strong headwinds or crosswinds, they would fly low to the ground and increase their speed to maintain control of their path.
“For years it was assumed that insect migration was mainly about getting swept away. But we show that insects are capable of being great navigators, on par with birds, and are much less vulnerable to windy conditions. than we thought,” Menz said.
“By showing that it is technically possible to continuously monitor individual insects as they migrate and observe their flight behavior in detail, we hope to inspire further studies to answer many other big questions in this field. .”
Co-author Dr Jason Chapman, an insect migration expert at the University of Exeter, commented: “We’ve known for some years that migrating insects have preferred migration directions that change with the season, But what was really surprising about this new study was how straight the hawkmoths’ tracks were for very long distances – they really stayed straight towards the winter breeding grounds for tens of kilometres.
“This means they are constantly adjusting their flight heading to correct for crosswind drift in real time, just like bees returning to their hives do, but over much greater distances – and this was a total surprise. .”
For the study authors, the next step is to answer the question of how butterflies are able to maintain such straight lines. “Based on previous lab work, it’s possible that insects use internal compasses, both visual and magnetic, to navigate their way around the world,” Menz suggested.
Menz, MHM, Scacco, M, & 5 others. Individual tracking reveals long-range flight path control in a migrating moth. Science, volume 377, pages 764-768. DOI: 10.1126/science.abn1663