On the trail of “celebrities” insects: Researchers have equipped squirrel hawk moths with small transmitters, thereby showing that these seasonally migrating migratory butterflies use highly developed flight strategies similar to those of migratory birds. You can even adapt to unfavorable wind conditions in order to stay on course over long distances. The results of the pioneering study also show that migratory insects can be guided by a complex navigation system.
In spring they move north to their summer quarters – in autumn they return to the warm south. Not only migratory birds follow this concept: Many flying insects also migrate seasonally, sometimes covering enormous distances. The numerous species from different families thus represent the smallest and numerically most widespread migratory animals on earth. However, their migratory behavior has been much less researched than that of birds. Above all, more detailed information is missing: “Understanding what individual insects do during their migration and how they react to the weather is one of the major challenges for research on the migratory behavior of animals,” says lead author Myles Menz from the Max Planck Institute for behavioral biology in Radolfzell.
Insect celebrities equipped with transmitters
In order to provide new information in the research field, Menz and his colleagues carried out studies on a special representative of migratory moths: the squirrel hawk moth (Acherontia atropos) is known for its distinctive markings on its back, which resemble a human skull. This trait earned him an appearance in the film The Silence of the Lambs. However, the reason for using the squirrel hawk moth as a test animal was its considerable size and complex migration behavior. The moths have a wingspan of over ten centimeters and cover enormous distances on their annual migrations from Africa to Central Europe. However, as is usual with many insects, the entire migration cycle is covered over several generations. But the individual specimens also cover long distances.
For the study, the researchers reared squirrel hawkmoth caterpillars to adulthood in the laboratory. Then the moths were equipped with miniaturized radio transmitters that weighed only 0.2 grams. As the scientists explain, this corresponds to less than 15 percent of their body weight and is therefore a reasonable burden for the moths: “The food they eat is probably often more than this weight,” says Menz. He and his colleagues then let the test animals, equipped with “backpacks”, fly off from Constance.
In order to be able to record the information from the transmitters from relatively close range, the scientists accompanied the moths migrating south with a light aircraft. In total, the team tracked 14 moths over a maximum period of four hours and distances of up to 80 kilometers. In this way, the researchers were able to at least record the course of individual nocturnal flight stages. As they point out, this is the longest distance to date that an insect has been continuously observed in the wild.
Not just with a tailwind
The analysis of the data showed that the moths followed perfectly straight flight paths over long distances during their nocturnal flight. However, as the weather data recorded in parallel showed, this was not due to the fact that they waited until the wind was favorable at their backs. Rather, they apparently employed a variety of flight strategies to counteract the prevailing winds and thus stay on course. When there was a strong headwind or crosswind, they flew particularly low and increased their speed to maintain control of the course, the researchers report.
However, the moths also take advantage of favorable winds: when there is a tailwind, they fly high and slowly and can therefore be carried particularly effectively by the air. “For years, it was assumed that insects mainly let themselves be driven in this way during long-distance migration. However, we were able to show that insects can be true navigation experts on a par with birds and that they are far less vulnerable to adverse wind conditions than we thought,” says Menz.
The study also shows once again that migratory insects can orientate themselves in complex ways. In further investigations, the scientists will now investigate this aspect more closely: They want to clarify how the squirrel hawk moths determine the direction to their destinations in order to fly to them in a straight line. “Based on previous laboratory work, there is some likelihood that the insects use internal compasses, both visual and magnetic, to set their global flight paths,” says Menz. In general, the team now sees the success of the research methodology as a basis for further research. “By proving that it is technically possible to continuously track individual insects during their migration and to observe their flight behavior in detail, we hope to stimulate further similar studies in order to answer the many other open questions in this area,” says Menz to lock.
Source: Max Planck Institute for Animal Behavior, Article: Science, doi: 10.1126/science.abn1663