Shedding light with harmonic radar: Unveiling the hidden impacts of streetlights on moth flight behavior

One of the most dramatic changes occurring on our planet is the ever-increasing extensive use of artificial light at night, which drastically altered the environment to which nocturnal animals are adapted. Such light pollution has been identified as a driver in the dramatic insect decline of the past years. One nocturnal species group experiencing marked declines is moths, which play a key role in food webs and ecosystem services such as plant pollination. Moths can be easily monitored within the illuminated area of a streetlight, where they typically exhibit disoriented behavior. Yet, little is known about their behavior beyond the illuminated area. Harmonic radar tracking enabled us to close this knowledge gap. We found a significant change in flight behavior beyond the illuminated area of a streetlight. A detailed analysis of the recorded trajectories revealed a barrier effect of streetlights on lappet moths whenever the moon was not available as a natural celestial cue. Furthermore, streetlights increased the tortuosity of flights for both hawk moths and lappet moths. Surprisingly, we had to reject our fundamental hypothesis that most individuals would fly towards a streetlight. Instead, this was true for only 4% of the tested individuals, indicating that the impact of light pollution might be more severe than assumed to date. Our results provide the first experimental evidence for the fragmentation of landscapes by streetlights and demonstrate that light pollution affects the movement patterns of moths beyond what was previously assumed, potentially affecting their reproductive success and hampering a vital ecosystem service.

Methods
Description of methods used for collection/generation of data:
Moth males of different species were released individually in the center of six circularly arranged high-pressure sodium streetlights (radius: 85 m).

A similar number of individuals was tested each day with the streetlights either turned on or off to compensate for daily fluctuations in weather and ambient light conditions.

A harmonic radar (Raytheon Marine GmbH, Kiel, NSC 2525/7 XU) was used to track the flight paths of individual moths.

All-sky photometry was used to measure spatially resolved sky brightness utilizing a commercial digital single-lens reflex camera (Canon EOS 6D) with a full-frame CMOS sensor (20.2 Megapixel) operating with a 180° circular fisheye lens (Sigma 8 mm f/3.5 EX DG).

Methods for processing the data:
The CR2 images retrieved by all-sky photometry were processed with the software “Sky Quality Camera” to get luminance values for each pixel.

Standards and calibration information: We used a calibrated digital all-sky camera.

Environmental/experimental conditions:
Experiments were only performed during warm summer nights without rain or strong wind.