Nocturnal light pollution from street lamps, spotlights and the like disrupts the natural day-night rhythms of humans, but also of many animals, including insects. So far, however, there is only little data on whether and to what extent artificial light is partly responsible for the decline in insects. That is why researchers have now formulated recommendations on how the effects of light pollution on insects could be better investigated in the future.
80 percent of humanity live under skies that are no longer really dark, even at night – and the trend is rising. Because our artificial lighting turns night into day almost everywhere today. This has serious consequences for our health and also for many animals. For example, the lighting affects the natural day-night rhythm, can delay the reproduction of some animals and interferes with the pollination of plants.
What does light do to insects?
In addition, light pollution could also be partly responsible for the loss of insects, suspect researchers working with Gregor Kalinkat from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB). About half of all insect species are nocturnal and rely on darkness and natural light from the moon and stars to orient themselves and move around or to avoid predators. The natural day-night rhythm is also important in order to look for food and reproduce at night. An artificially lit night disrupts this natural behavior – and could therefore reduce the chances of survival.
“We suspect that artificial light at night is one of the main reasons for this negative trend, which is well documented in Germany in particular,” says Kalinkat. According to the scientists, flying insects in particular are attracted to lamps, sky lights and neon signs and then die there from exhaustion or as easy prey for predators. Earlier studies already suggested that the artificial light sources act like a “vacuum cleaner”: On a test field in Westhavelland, up to 260 times as many insects buzzed around illuminated street lamps as in the dark surroundings. “Therefore, light pollution could be an important reason for the worldwide decline in insects,” states Kalinkat.
Sparse evidence
The problem: So far, there have been almost no studies on the effects of light pollution on insects that investigate the long-term relationship over a period of more than one to two years. Kalinkat and his team have therefore now collected all the available data and, based on this, created a checklist with the key points that should be taken into account in future long-term experiments. “Direct evidence that artificial light plays a role in insect decline is still sparse, and we propose a number of steps on how future studies can be improved to fill these gaps,” said the researchers.
“Our literature search revealed only eleven studies that tracked the development of insect populations with explicit consideration of light pollution over more than one season,” said the research team. “Almost all of these studies were carried out in Europe and North America, with only one study taking place in Africa.” In addition, mainly butterflies (Lepidoptera) and hardly any other groups of insects were examined, and only one type of light source, such as exclusively white LED lamps .
Optimize time, space and measuring devices
That is why the researchers are now making recommendations for more meaningful insect monitoring in the future. Accordingly, the duration of the study should be as long as possible – even over several seasons – in order to record natural fluctuations in insect populations. “The longer a time series, the better, so as not to draw wrong conclusions,” explain the scientists. The investigations should not only refer to the area directly around the light source, but also to the entire surrounding landscape. In this way, other influences of the decline in insects such as increased land use could be better excluded.
In addition, Kalinkat and his colleagues advise using different types of traps and trapping methods in order to capture the greatest possible diversity of insects. Not only the type of individual insects, but also their body and eye size should be documented. Since butterflies, beetles and the like perceive colors and intensities of light with their complex eyes very differently than humans, common lighting technology specifications such as the unit of human light perception “Lux” are often not sufficient for such studies. Accurate, spectral measuring devices are required for more precise measurements, the team said.
Pressing problem
Studies based on these recommendations are urgently needed, according to the scientists. “The global ‘insect extinction’ – in the form of species and population declines – is one of the most pressing problems in biology and environmental sciences with possible consequences for food production, to name just one example,” emphasizes Kalinkat. On the occasion of the current international dark sky week, the “Dark Sky Week”, the first insect monitoring studies are already being carried out to draw attention to the effects of light pollution on insects. For example, in model regions in northern and western Brandenburg, southern Mecklenburg and Fulda in Hesse, where it is still very dark at night, insects are collected on lamps, assigned to their species and counted.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB), specialist article: Insect Conservation and Diversity, doi: 10.1111 / icad.12482