Migrating insects observed on their journey for the first time

Many flying insects migrate seasonally, sometimes covering enormous distances.

Common swifts and golden orioles have already left. And the black redstarts will follow soon. We know that many bird species head south in winter. But the fact that trillions of insects are migrating at the same time was less well known until now.

Butterflies, dragonflies, and hoverflies also seek out warmer winter quarters. Research is gradually painting an ever more detailed picture. Yesterday, a study in the scientific journal “Science” showed that death's-head hawkmoths from the Lake Constance region can cross the Alps in a single night.

Insects are thus the smallest flying migratory animals on Earth, and yet they are capable of maintaining perfectly straight flight paths even in unfavorable wind conditions – this is the finding of a new study by the Max Planck Institute of Animal Behavior (MPI-AB) and the University of Konstanz.

Tracking radio-tagged moths from an airplane, the researchers followed them over distances of up to 80 kilometres – the longest distance over which an insect has ever been continuously observed in the wild. By precisely tracking the moths during their migration, this pioneering study, now published in the journal Science, has lifted the age-old mystery of how insects behave on their long-distance migrations. The study demonstrates that death's-head hawkmoths employ sophisticated flight strategies to adapt to prevailing wind conditions, thereby maintaining their flight direction with precision over long distances. The results also suggest that insects are capable of navigating with great overall accuracy on their long journeys by following an internal compass.

Insects are among the most numerous migratory animals on Earth — with trillions of individuals embarking on journeys each year, sometimes over enormous distances. They include well-known species such as the death's-head hawk-moth, as well as species of great social and ecological significance, such as locusts, mosquitoes, and bees. Yet although the number of migrating insects far exceeds that of better-known migratory animals — such as migratory birds or bats — their migratory behavior is considerably less well studied.

The problem is largely methodological in nature. «Studying migratory insects is a major challenge«, explains Dr. Myles Menz, lead author of the current study, who conducted the research at the MPI-AB and now works as a lecturer at James Cook University in Australia. «They are usually too numerous to mark and recapture, and too small to carry tracking devices.“

Much of what we know today about insect migration comes from studies in which insects or their locations were recorded as individual snapshots — for example by radar or through direct observation — which is why significant knowledge gaps remain. «Understanding what individual insects do during their migration and how they respond to weather conditions is one of the great challenges in animal migration research«, says Menz.

In the current study, researchers tracked insects fitted with radio transmitters from a light aircraft, following themevery step of the way" have tracked is the first in which the migration of free-living, nocturnal flying insects could be continuously observed over an extended period. Accordingly, the movement data recorded in the process set a record for the longest distance over which the continuous flight paths of insects in the field could be tracked. The team of researchers from the MPI-AB and the University of Konstanz, as well as the University of Exeter (United Kingdom), focused on the death's-head hawkmoth — a large, nocturnal moth that travels up to 4,000 kilometres between Europe and Africa each year during its migrations. As is common with many insects, however, this distance is not covered by individual animals but across generations. This means that no single animal knows the entire route.

For its study, the research team at the MPI-AB in Konstanz raised death's-head hawkmoth caterpillars through to adulthood in the laboratory to ensure that the individuals were naive — that is, without any prior knowledge. As adults, the animals were then fitted with miniaturised radio transmitters weighing just 0.2 grams — equivalent to less than 15 percent of the body weight of an adult death's-head hawkmoth. «The food a moth consumes each night likely exceeds this weight. The transmitters are therefore very light for the insects«, says Menz.

After attaching the transmitters, the researchers released the moths and waited for them to take off. They focused on observing a single animal at a time. In total, the team tracked 14 nocturnal moths for periods of up to four hours and over distances of up to 80 kilometres — distances that represent individual nightly migratory flights for these animals. They used antennas attached to a Cessna to determine the precise location of the animals from the aircraft every five to fifteen minutes. The insects were tracked in a south-southwesterly direction from Konstanz into — and in some cases over — the Alps, which corresponds to the route of the death's-head hawkmoths towards the Mediterranean and northwest Africa.

Due to entirely practical constraints that arise when flying in an aircraft, the researchers tracked the nocturnal moths until the insects made a stopover along their route. «When you're in an airplane, it's nearly impossible to wait for the insects to continue their migration. You would have to already be in the air at that moment — which could be at any point in the middle of the night», explains Prof. Dr. Martin Wikelski about the problem. Wikelski is an ecologist at the MPI-AB and the University of Konstanz and piloted the aircraft during the measurements.

The results of the current study were able to show that the moths maintained completely straight flight paths over long distances during their journey. This was not, however, because they waited for favorable tailwinds. Rather, they employed a range of flight strategies to counter the prevailing winds and thus maintain their course throughout the night: when the wind was actually favorable, they flew high and slow, allowing themselves to be carried by the air. In strong headwinds or crosswinds, on the other hand, they flew low and increased their speed in order to retain control of their course.

Menz says: "For years it was assumed that insects primarily let themselves be carried by the wind during long-distance migration. However, we were able to show that insects can be true navigation experts, comparable to birds for example, and that they are far less susceptible to adverse wind conditions than we thought." He continues on the research methodology: "By proving that it is technically possible to continuously track individual insects during their migration and observe their flight behavior in detail, we hope to inspire further similar studies to answer the many remaining open questions in this field.«

For the authors of the study, the next step will be to investigate how death's-head hawkmoths determine the direction to their destinations in order to fly toward them in a straight line. "Based on earlier laboratory work, there is a certain likelihood that the insects use internal compasses — both visual and magnetic — to establish their global flight paths", says Menz.