Increasing light pollution is a problem for many insects. But moths are hardly deterred when looking for a partner – at least as long as the light pollution is only moderate, as a study reveals. According to this, male privet hawk moths use the moon to find their female counterparts. A diffuse stray light on the horizon disturbs them only slightly.
In addition to classic pollinators such as bees and bumblebees, less well-known species are also affected by the decline in insects. These also include moths, which are important nocturnal pollinators for many ecosystems. The widespread light pollution is currently being treated as a possible reason for the decline in nocturnal insects, since these species often orientate themselves on the stars and could be distracted by artificial light sources. “Nocturnal insects evolved under natural nocturnal light conditions and can therefore use faint light, including starlight, for orientation. Dung beetles, for example, can use the Milky Way for orientation and can also perceive polarization patterns in the moonlight,” explains co-author Franz Hölker from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) in Berlin.
Sent privet hawk moths in search of a mate
A research team led by Mona Storms and Jacqueline Degen from the University of Würzburg, in cooperation with the IGB, has now investigated to what extent moths could be disturbed by artificial light sources. To this end, they investigated the reproductive behavior of male privet hawkmoths (Sphinx ligustri) under different light conditions. In order to work with as little light pollution as possible, they brought the moths to Großseelheim near Marburg in central Hesse for their investigations, where the Milky Way can even be seen on a clear night at night.
In order to be able to analyze the exact lighting conditions on site, the scientists used a so-called all-sky camera. “This makes it possible to completely capture the nocturnal light environment and, for example, to distinguish natural light sources from “skyglow” – a type of indirect light pollution,” explains co-author Andreas Jechow from IGB. Finally, after documenting the light conditions and the position of the moon, the male moths were sent one after the other in search of a mate. The scientists offered them several cages in different directions with female moths as a pheromone source.
In the moonlight, moths find their mates faster
The result of the study shows that when the moon is above the horizon, the male moths find their female conspecifics much faster and with greater reliability. According to the researchers, it does not matter whether the moon is covered by the clouds or is clearly visible. The phase of the moon also probably has no influence on the moth’s search for mating – in contrast to its position in the sky. The further south the moon was, the more likely the privet hawk moths flew to the females in the cages located in that direction – so they use it as a kind of compass.
To investigate a possible influence of man-made light pollution, the scientists offered the moths a bell of light on the horizon. But she seemed to leave them cold. “As soon as the moon appears above the horizon and contrasts with the general illumination from light pollution near the horizon, the male moths fly there and not away,” explains Jacqueline Degen. “We have yet to find out what that looks like when there is more light pollution from one or more strong light sources. But actually this is good news for now.”
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries; Specialist article: Communications Biology, doi: 10.1038/s42003-022-03331-x