A first-of-its-kind study reveals that migrant insects can travel in perfectly straight flight paths
iStock/ Pawel Gaul
- The world-first study unlocked a century-old mystery of what insects are up to during migration
- The current study followed radio-tagged insects in a light aircraft
- It revealed that the hawkmoths are excellent navigators
Insects are some of the most common migrating animals on Earth- a fact that is often forgotten. Insect migrants such as monarch butterflies, locusts, mosquitoes, and bees, far outnumber 'popular' migrants such as birds and mammals. Yet their migration is the least understood form of long-range animal movement.
Now, in a fascinating world-first study, researchers from the Max Planck Institute of Animal Behavior (MPI-AB) and the University of Konstanz radio-tracked migrating hawkmoths for up to 80 kilometers—the longest distance that any insect has been continuously monitored in the wild. Their results revealed that the world's smallest flying migrants are excellent navigators and can maintain perfectly straight flight paths even in unfavorable wind conditions.
Published in Science, the report unlocks a "century-old mystery" of what insects do over their long-range journeys. The study confirmed that hawkmoths could accurately maintain straight trajectories over long distances, employing sophisticated strategies to counter and correct unfavorable wind conditions, revealing that an internal compass accompanies them on their journeys.
"Studying insects on the move is a formidable challenge," first author Myles Menz, who conducted the research at MPI-AB and is now a lecturer at James Cook University in Australia, said in a statement. "They’re usually too numerous to mark and find again and too small to carry tracking devices."
Much of what we know about insect migration is from studies that sample insects through radar or direct observation, which is only at a single moment in time. As a result, this has left a gap in our knowledge.
"Understanding what insects do during migration and how they respond to weather is the last frontier in migration science," said Menz.
The current study, which followed radio-tagged individuals in a light aircraft, is the first to continuously study nocturnal migrating insects in the wild and represents the longest distance over which any insect has constantly been tracked. The team zeroed in on the death's head hawkmoth—a large, nocturnal migrant that travels up to 4000 kilometers between Europe and Africa every year. Like several insects, the species is a multi-generational migrant, and no one knows the entire route.
At the MPI-AB in Konstanz, Germany, the team reared caterpillars until adulthood in the laboratory. When these moths emerged as adults, they were branded with radio tags weighing 0.2 grams. "The moths would probably eat more weight than that in a night, so these tags are extremely light for the insects," said Menz.
According to the release, the tagged moths were then released, and the researchers waited for the flight to begin, after which they chose a single insect to follow. They followed 14 moths each for up to 80 kilometers or 4 hours—a stretch long enough to be considered migratory flight—using antennas mounted on a Cessna airplane to detect precise locations every five to 15 minutes. Insects were followed in the south-south-west direction from Konstanz into the Alps, which follows the route taken by hawkmoths towards the Mediterranean and Northwest Africa.
Insects are superb navigators
Flying in an aircraft does involve constraints, so the scientists tracked the moths continuously until the insects stopped. "When you’re in an airplane, it becomes challenging to wait for the insects to begin migrating again because you would have to be in the air when this happens, which could be anytime in the night," says senior author Martin Wikelski, a movement ecologist from the MPI-AB and the University of Konstanz, who piloted the plane during the study.
The moths maintained straight trajectories for long distances during flight. Rather cleverly, they employed flight strategies to buffer against winds, which allowed them to hold their course throughout the night.
"For years, it was assumed that insect migration was mostly about getting blown around. But we show that insects are capable of being great navigators, on par with birds, and are far less vulnerable to wind conditions than we thought. By showing that it is technically possible to continuously monitor individual insects over migration and to observe their flight behavior in detail, we hope to inspire more studies to answer many more big questions in this area," said Menz.
The authors intend to answer the question of how moths can maintain such straight lines. "Based on past lab work, it’s possible that the insects are using internal compasses, both visual and magnetic, to chart their way around the world," added Menz.
Each year, trillions of insects make long-range seasonal migrations. These movements are relatively well understood at a population level, but how individual insects achieve them remains elusive. Behavioral responses to conditions en route are little studied, primarily due to the challenges of tracking individual insects. Using a light aircraft and individual radio-tracking we show that nocturnally-migrating death’s-head hawkmoths maintain control of their flight trajectories over long distances. The moths did not just fly with favorable tailwinds, but during a given night adjusted for head and crosswinds to precisely hold course. This behavior indicates that the moths employ a sophisticated internal compass to maintain seasonally beneficial migratory trajectories independent of wind conditions, shedding light on how insects traverse long distances to take advantage of seasonal resources.
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