Insects are the smallest flying migratory animals. Although the number of migrating animals is far higher than that of migratory birds or bats, the migratory behavior of insects is much less researched.
“Migratory insects are usually too numerous to tag and find, and too small to carry tracking devices,” Myles Menz, first author of the study published in the journal Science, is quoted as saying in a statement from the Max Planck Institute.
Previous findings mostly come from studies in which the insects and their locations were recorded as snapshots, for example by radar or through direct observation. “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 Menz.
In the current study, the researchers tracked skeleton hawk moths equipped with radio transmitters in a light aircraft over a distance of up to 80 kilometers. This is the longest distance so far over which the trajectories of insects have been tracked in the field.
With a weight of up to 3.5 grams, squirrel hawk moths are comparatively large, nocturnal moths. On their migration between Europe and Africa, they cover up to 4000 kilometers every year. However, the entire distance is not covered by individual individuals, but by generations. This means that no single animal knows the entire route.
The researchers raised caterpillars in the laboratory. Hatched moths were fitted with miniaturized radio transmitters that weigh 0.2 grams – less than 15 percent of the body weight of an adult squirrel hawk moth. “The food that a moth eats every night is probably more than this weight,” explains Menz. So the transmitters were probably easy to carry as flight luggage.
The moths were released and 14 individuals were tracked for up to four hours and distances of up to 80 kilometers. The animals flew from Constance in a south-southwest direction to the Alps, which corresponds to the migratory route towards the Mediterranean and Northwest Africa.
The night pursuits show that they kept perfectly straight trajectories over long distances. To do this, they did not necessarily wait for a tailwind, but used flight strategies to stay on course throughout the night, even when the wind conditions were less favourable: if the wind was favourable, they flew high and slowly and let the air carry them. On the other hand, in strong headwinds or crosswinds, they flew low and increased their speed to maintain control of the course.
“For years it was assumed that insects were mainly driven by the wind during long-distance migration,” says Menz. However, the study shows that the navigation of insects is on a par with that of birds.
In further investigations, the team wants to investigate 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.