The use of artificial light at night (ALAN) is increasing exponentially worldwide and there is growing evidence that ALAN contributes to the decline of insect populations. One of the most conspicuous ecological effects is the strong attraction of ALAN to flying insects. In several studies, light sources with strong short-wavelength emissions have been shown to attract the highest numbers of flying insects. Furthermore, flying stages of aquatic insects are reported to be more vulnerable to ALAN than flying stages of terrestrial insects. This is concerning because freshwater habitats are likely affected by ALAN that originates from human activity centers, which are typically close to sources of freshwater. However, the effects of ALAN on aquatic insects, which spend their larval phase (amphibiotic insects) or their whole life cycle (fully aquatic insects) in freshwaters, are entirely understudied. Here, we investigated the phototaxis of aquatic insects to ALAN at different wavelengths and intensities. We used floating light traps and compared four, near-monochromatic, lights (blue, green, red, and yellow) at two different photopic light intensities in a ditch system, which was not exposed to ALAN previously. Similar to flying stages of (aquatic and terrestrial) insects, we found a strong positive phototaxis of aquatic life stages. However, in contrast to the flying stages, there is no clear preference for short-wavelength light. Overall, responsivity to wavelengths in the center of the visible range (green, yellow; 500-600 nm) was significant for all orders of aquatic insects studied, and the nymphs of Ephemeroptera did not respond to blue light at all. This is likely an adaption to how light is attenuated in freshwater systems, where not only the water itself but also a variety of optical constituents act as a color filter, often like in our case filtering out short-wavelength light. Therefore, insects living in freshwater bodies often live in longer wavelength-dominated environments and might therefore be especially sensitive to green/yellow light. In conclusion, the different spectral sensitivities of both aquatic and flying insects should be taken into account when planning lighting near freshwater.
