Artificial light at night has increased strongly in recent decades and is now affecting moths, a key contributor to pollination networks. The global shift to light-emitting diode (LED) streetlights changes the nocturnal light environment even further, especially because of their high variability in spectrum and intensity. To date, the impact on mating success of moths is only little  known, making it essential to investigate their behavioural responses.

We recorded the flight behaviour of male moths (Sphinx ligustri) using a symmetrical flight tunnel. Two different light environments with a female positioned on one side of the tunnel were used to test the effect of different LEDs (1800K Amber, 2200K Warm white, 3900K Neutral white) and intensities (0.05 lux, 150 lux, 370 lux, 590 lux) on arrival location, flight duration, and direction changes of males.

We have created two different light habitats within the tunnel. A homogeneous light environment, with equal light on both sides of the tunnel, and a heterogeneous light environment, with light only on one side. In both habitats, the flight behavior of male moths was tested in the presence of females. To test whether mating behavior was impaired, a female was placed on only one side of the tunnel. In the heterogeneous habitat, the opposite side of the light was used to create a conflict.

We used a flight tunnel to observe and analyse the response of male moths (Sphinx ligustri L., Lepidoptera, Sphingidae) to different light stimuli. This flight tunnel consisted of plexiglass and had a total length of 3.50 m with an inner edge length of 30 x 30 cm. A sliding door enabled us to position the moth in the middle of the tunnel. On top of each end, opaque boxes were positioned that contained the different light sources as well as a hook to add a cage with a female. These boxes had an opening of 20 x 20 cm to connect them with the flight tunnel. To prevent males from entering these boxes, the opening was covered with Gauze (Nobamull, Danz GmbH u. Co KG). To exclude any impact of the room, all non-white or reflective surfaces were covered with white cloths, creating a uniform environment around the flight tunnel.

We tested three different light-emitting diodes (LEDs) provided by Osram (TRILUX GmbH & Co. KG): 1. Amber 1800 K (λ max = 596 nm) with almost no blue light components, 2. Warm white 2200 K (λ max = 615 nm) with a small proportion of blue in the light spectrum, specially developed to be particularly insect-friendly streetlight and still achieve a good colour effect, and 3. Neutral white 3900 K (λ max = 444 nm) with a high proportion of blue. The spectra of all LEDs were measured with a high-sensitivity spectrometer (Ocean Optics Maya2000 PRO; Figure 1b). Light intensity was measured in a distance of 50 cm directly below the light source using a luxmeter (RS Pro ILM Light Meter 1337).

For the experiments, we use a species from the Sphingidae family, which are primarily nocturnal or crepuscular and are among the most important pollinators among moths (Winfree et al., 2011). Sphinx ligustri are robust, good fliers that have been reliably attracted to females in other experiments (Storms et al., 2022). The pupae were stored during winter in the refrigerator at a temperature of 5°C without any light and removed between four and six weeks before the experiment. For hatching, the pupae were exposed to indirect daylight in a natural day-night rhythm. To prevent mating, the light pollution naïve males and females were kept separately in a uniquely labelled plastic bottle. During the day, the plastic bottles were not covered lightproof to maintain with daylight the natural rhythm. All experiments were carried out at night. Before experiments, all moths were fed under red light (Storm 400 Headlamp, Black Diamond Equipment Europe GmbH) with sugar solution (2M) to ensure they were not hungry. Afterwards, males were placed beneath separate metal tins standing on small wooden boards to enable easy transportation to the release platform within the flight tunnel. Furthermore, the tins ensured that the moths could rest for at least one hour before experiments in complete darkness and the experimenter was unaware of their body alignment in relation to the flight tunnel at the beginning of a trial. A virgin female older than two but younger than seven days was placed in a small cage covered with gauze. This cage was positioned in one of the two opaque boxes that could be easily switched from one side to the other. During all experiments, the position of the female was swapped after half of the males were tested. To ensure that all pheromones were removed before the next trials started, the entire tunnel was cleaned with ethyl alcohol and both, the tunnel as well as the room were ventilated. We performed a series of experiments to investigate the behaviour of male moths responding to various stimuli in two different light habitats.