Artificial light at night (ALAN) is a widespread, human-induced alteration of the landscape that affects insect dispersal and potentially contributes to insect decline. We used an experimental area with streetlights installed adjacent to an agricultural drainage ditch to experimentally assess the abundance and diversity of emerging and flying aquatic Diptera over a period of six months in summer and autumn. Emergence was two-fold lower in the lit site, while flying adults were eight-fold more abundant at traps under lights. Results were taxon- and sex-specific. Males of nine taxa were less abundant in lit emergence traps, and females of most taxa were more abundant in lit air-eclector traps than in controls. We developed an empirical model based on emergence and capture rates and used this model to estimate that the majority of the 54 flying Diptera taxa we identified were attracted to light from the adjacent water body, and that a few taxa were attracted from a distance of up to 1800 m. This work provides evidence that artificial light in riparian areas can reduce emergence in aquatic Diptera and hinder dispersal, with effects that vary depending on the taxon. Because many riparian predators rely on adult aquatic insects as prey, these changes can cascade across aquatic-terrestrial ecosystem boundaries. Given the large number of streetlights that are installed along freshwater shorelines, the observed effects are likely to be of relevance to freshwater bodies around the globe.
