Insects are the world’s smallest flying migrants, but they can maintain perfectly straight trajectories even in unfavorable wind conditions, according to a new study from the Max Planck Institute for Animal Behavior (MPI-AB) and the University of Konstanz. Researchers radio-tracked migrating hawkmoths up to 80 kilometers away – the longest distance an insect has continuously monitored in the wild. By closely tracking individuals as they migrate, the world’s first study unveils a centuries-old mystery about what insects do on their long journeys. The study, published in Science, confirms that hawk moths can maintain a straight flight path over long distances by employing sophisticated strategies to counteract and correct for adverse wind conditions. The results show that insects are capable of precise navigation, confirming that an internal compass guides them on their long journeys.
With trillions of people migrating every year, insects are among the most migratory animals on earth. These include well-known species such as the monarch butterfly, as well as species of enormous social and ecological importance such as grasshoppers, mosquitoes and bees. But although insect migrants are far more well-known migrants like birds or mammals, their migration is the least understood form of long-distance movement of animals.
The problem was largely methodological. “Studying insects on the go is a daunting challenge,” says first author Myles Menz, who conducted the research at the MPI-AB and is now a senior lecturer at James Cook University in Australia. “They are usually too numerous to tag and locate and too small to carry tracking devices.”
Much of what we know about insect migration comes from studies that looked at insects at a specific point in time, e.g. B. by radar or direct observation, which has left large gaps in our knowledge. “Understanding what insects do during migration and how they react to the weather is a final frontier in migration science,” says Menz.
The current study, which radio-tracked tagged individuals in a light aircraft, is the first to continuously study nocturnal migratory insects in the wild and represents the longest distance an insect has been tracked continuously in the field. The team, which includes researchers from MPI-AB and the University of Konstanz in Germany and the University of Exeter in the UK, focused on the squirrel hawkmoth – a large, nocturnal migrant that travels up to 4000 kilometers between Europe and Africa each year. Like many insects, the species is a multigenerational migrant, meaning no individual knows the entire route.
At the MPI-AB in Konstanz, Germany, the team bred caterpillars to adulthood in the laboratory to ensure that the individuals were naïve. When adult moths emerged, they were fixed with radio tags that weighed 0.2 grams – less than 15% of adult body weight. “The moths would probably eat more weight in one night, so these trailers are extremely light for the insects,” says Menz.
The researchers released the tagged moths and waited for the flight to begin, after which they each chose a single individual to follow. The team tracked 14 moths for up to 80 kilometers or 4 hours each – a distance long enough to be considered a migratory flight – using antennas mounted on a Cessna aircraft to track every five to 15 Minutes to determine exact positions. Insects were tracked in a south-southwest direction from Constance to the Alps, following the route of hawkmoths towards the Mediterranean and north-west Africa.
Due to practical limitations of flying on an airplane, the scientists tracked moths continuously until the insects stopped along the way. “When you’re on an airplane, it becomes extremely difficult to wait for the insects to start migrating again because you would have to be airborne, which could be any time during the night,” says senior author Martin Wikelski. a movement ecologist from the MPI-AB and the University of Konstanz, who piloted the aircraft during the study.
The results show that moths maintained perfectly straight trajectories while flying over long distances. It wasn’t because they were waiting for a favorable tailwind. Rather, they employed a variety of flight strategies to protect themselves against the prevailing winds, allowing them to stay on course throughout the night. When the wind was favourable, they flew high and low, letting the air carry them. But in strong headwinds or crosswinds, they would fly low to the ground and increase speed to maintain control of their path.
Menz: “For years it was assumed that insect migration was primarily about being 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 the migration of individual insects and observe their flight behavior in detail, we hope to stimulate further studies to answer many more big questions in this area.”
The next step for the study authors is to answer the question of how moths are able to maintain such straight lines. “Based on previous laboratory work, it’s possible that the insects use internal compasses, both visual and magnetic, to map their path around the world,” says Menz.
