Introduction

Excessive Artificial Light at Night (ALAN), also called light pollution, is a growing environmental concern. Besides well documented negative effects on humans and many other species, it is also assumed to be one of several anthropogenic stressors responsible for insect declines, with potential knock-on effects on ecosystem functions. Particularly, the fatal attraction of many nocturnal insects towards light represents a fundamental negative effect in environments that would naturally be dark. While different mitigation strategies for this issue have been suggested, the focus of the two applied research projects behind our paper was to reduce the light emissions of streetlights within the trajectory of flying insects. Specifically, our objective was to develop a streetlight design that (A), would be nearly invisible at the light source and, (B), that would produce a well-defined and homogeneously illuminated path. In other words, the idea was to drastically reduce the amount of “spill light” beyond the area that is required to be illuminated for human safety (Figure 1).

Figure 1: The initial idea was to reduce the high amount of spill light typical for conventional luminaires (left panel) by optimizing the lighting geometry (right panel; Graphic by Dr. Catherine Perez Vega)

Two research projects

Our publication is based on results from two independently funded research projects: (1) Artenschutz durch umweltfreundliche Beleuchtung (AuBe, literally ‘Species protection through environmentally friendly lighting’), running from 2019 to 2025, where we have been sampling flying insects around street lighting at an established experimental field site  that includes a dark, unlit control, and in four communities in North-eastern and central Germany (Figure 2). The opportunity for lighting companies to collaborate with us in this research included an official invitation to tender at the start of the project. In addition to the sites covered in AuBe, we conducted similar samplings of flying insects in a related project, (2) NaturLicht (‘Nature Light’), in the South-western part of Germany from 2021 to 2022 (Figure 2). Together, the eight localities in the two projects cover a gradient from urban to rural settings, including variable levels of background light pollution, as visualized in the skyglow map in Figure 2.

Figure 2: The study sites involved in the two research projects in Germany represented with black stars; insect abundance data for the current paper comes from the four localities inside the red rectangles, in four other locations we changed the luminaires one year later and sampling is still ongoing. The color code in the map indicates background light pollution derived from skyglow maps, where red colors represent the highest levels of light pollution and blue areas are relatively dark.

Out of the six lighting manufacturing companies that took part in our invitation to tender, Selux GmbH was awarded the contract because it was the only applicant to offer the required light distribution curve. In cooperation with the Department of Lighting Technology at the Technical University Berlin, we then developed a lighting geometry that was (A) shielded, as it did not spill light at the luminaires head, and (B) tailored, as we customized the light emission to fit the different path widths at the experimental field and at the community sites (ranging from 2.5 to 6 meters). We further reduced glare by adding a shutter technology. Before we tested the novel luminaires, the illuminance at the experimental field was adapted from an average of 30 lux to 5 lux, to achieve comparable conditions with residential streets and along green spaces and freshwater bodies. This meant we had sampled data with a higher illumination at the experimental test field in 2019 and 2020 that we could then also compare with the lower light conditions in 2021 (conventional luminaire) and 2022 (tailored and shielded novel luminaires). In late 2021 those novel luminaires were ready to be installed at the experimental field site in and in the three communities in the South-west. Furthermore, we are currently also testing this tailored lighting approach in the four other communities to monitor the effectiveness under variable conditions. There the novel luminaires have been installed in 2023 (Figure 2).  

Results

The results we got from our samplings across the project locations were very consistent: we sampled around half the numbers of flying insects with the novel tailored and shielded luminaires compared to the conventional ones in three communities (Figure 3). Particularly impressive were results from the experimental field site. There, the attraction of flying insects to the shielded and tailored luminaires did not differ to results from the unlit, dark control field compared within the respective sampling dates (Figure 3). The effects we measured comparing high to low illuminance was less pronounced than those of the shielded and tailored light geometry. The importance of measures can therefore be ranked: shielding and tailoring of the light geometry is more important for the protection of flying insects than the reduction of illuminance. But when tailoring the light emission to the needs, we can recommend to use the lowest illuminance needed for the purpose, as dimming did also reduce insect attraction.

Figure 3: Numbers of trapped insects per trap and per night in different light treatment in the Westhavelland experimental field. Numbers indicate respective sampling years (1. Seasons 2019 and 2020, 2. 2021, 3. 2022). Letters above represent results from statistical tests, where same letter indicates no significant difference.

Figure 4: Numbers of trapped insects per trap and per night in different light treatments in the three communities in Southern Germany (sampling seasons 2021 and 2022). Stars indicate significant differences.

Implications for how to mitigate light pollution in the future

Our results suggest that a spatially optimized lighting provides the best mitigation for negative consequences of light pollution on flying insects. It means that particularly in the vicinity of protected areas like nature conservation reserves, but also near unprotected hotspots of biodiversity, like water bodies or wetlands, we should go the extra mile to customize illumination of public and private spaces. However, using spatially optimized lighting is always the second-best solution compared to no artificial light at all, which should, whenever and wherever possible, be the preferred solution.

Conclusions

To be able to find and implement viable solutions, light pollution that affects a wide range of organisms needs to be addressed in an inter- and transdisciplinary way. Without involving the stakeholders responsible for creation and implementation of lighting solutions, we will not succeed in reducing light pollution and making it less harmful. With our research we have shown that tailoring the lighting geometry and reducing light emission into flying insect trajectories can be very effective, and it may as well reduce adverse ALAN effects on other organisms such as plants, bats and birds. Simultaneously, this approach also reduces light spill into adjacent housing and can prevent inadvertent light emission from street-lighting into sleeping rooms thereby also improving human health.

Funding

Funding was provided through two projects. “Species protection through environmental friendly lighting” (AuBe) is funded within the Federal Programme for Biological Diversity by the Federal Agency for Nature Conservation (BfN) with resources from the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV; FKZ: 3518685A-H08) and “NaturLicht” funded by the “Sonderprogramm zur Stärkung der biologischen Vielfalt” in the federal state of Baden-Württemberg and funded by the regional council (Regierungspräsidium) Karlsruhe.

Read the full paper:

Dietenberger, M., Jechow, A., Kalinkat, G., Schroer, S., Saathoff, B., Hölker, F.: Reducing the fatal attraction of nocturnal insects using tailored and shielded road lights. Communications Biology 7, 671 (2024). https://doi.org/10.1038/s42003-024-06304-4