Light pollution disturbs moths even in the dark

Scientists have identified light pollution as one of the causes of the sharp decline in insects in recent years: many nocturnal insects fly to artificial light sources and circle around them incessantly. There they become easy prey for bats and other predators or eventually fall to the ground exhausted and die.

Moths are one group of nocturnal insects that are in significant decline. Their disappearance is also problematic because they play a key role in food webs and in the pollination of plants.

A new study now shows that the behaviour of moths changes not only in the cone of light from street lamps, but also outside the illuminated area. The experiments were carried out by a group from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany, in cooperation with researchers from Berlin and Providence (USA). The results have been published in the scientific journal PNAS.

"Using radar tracking, we found that the orientation of moths is also disturbed outside the cone of light: the flight paths of the three species of hawkmoths and one species of lappet moths, for which we were even able to demonstrate a barrier effect from streetlights, were significantly more curvy than normal," says Dr Jacqueline Degen, head of a junior research group at the JMU Biocentre.

"Surprisingly, we had to reject our basic assumption that most individuals would fly to one of the streetlights," explains the Würzburg researcher. This only applied to four per cent of the individuals studied: "This suggests that the effects of light pollution are not limited to direct attraction to light sources, but are much more far-reaching and complex than previously assumed."

Flight tracking using a Radar System near Marburg

The experiments took place at a radar system in Großseelheim near Marburg in Germany. The harmonic radar is currently the only radar system that can track small insects over several hundred metres. The research group observed the flight behaviour of a total of 95 moths up to one kilometre away from the release point, which was surrounded by a total of six street lamps at a distance of 85 metres.

In order to record the insects by radar, a transponder had to be attached to each individual moth. This small antenna is 10.5 milligrams light and twelve millimetres long. It does not change the flight behaviour of the moths in any way -- the researchers had previously clarified this in elaborate control experiments.

Interaction With the Moon

What also emerged from the experiments: There is an interaction between the disorientation of the moths caused by artificial light and the moon. This depends on whether the moon is above or below the horizon. "We don't yet have a precise understanding of this interaction," says Jacqueline Degen. But this is likely to change in the course of further research.