Timing of attraction to light of nocturnal insects is spectrum and taxon dependent: implications for mitigating light pollution

Nocturnal insects provide major ecosystem services but are vulnerable to artificial light at night as it disrupts their nocturnal activity, mostly via positive phototaxis. Although the attraction of nocturnal insects to artificial light at night is known to vary by taxon, light intensity, and colour, the temporal distribution of spectrum specific phototaxis throughout the night remains largely uncharted. Understanding the interaction between taxon and light spectrum on temporal patterns in phototaxis is critical for developing time-scheduled, spectrally adjustable lighting systems in order to mitigate the negative effects of artificial light at night. We examined how spectrum-dependent phototaxis in nocturnal insects varies over time depending on the taxonomic group. We used custom camera-light traps equipped with LED strips capable of emitting four monochromatic spectra, and one dichromatic spectrum with a calibrated radiant output to assess phototactic activity. We recorded the timing of attraction for twelve non-consecutive nights at four forested locations in the Netherlands. Phototactic activity intensified from the early to the middle of the night but varied with taxon. For example, Diptera (midges) exhibited an earlier centre of phototactic activity than Lepidoptera (moths). Notably, Diptera were earlier attracted to especially ultraviolet and red light, and later to green and amber light. Interestingly, the timing of Lepidopteran phototaxis was unaffected by light colour. These findings reveal taxon and spectrum-dependent variation in the timing of insect phototaxis and highlight the potential of targeted spectral tuning and light curfews, or emission reductions especially in the early and middle parts of the night.