The effects of artificial lighting on adult aquatic and terrestrial insects

1. There is a growing concern that artificial light might affect local insect populations and disrupt their dispersal across the landscape. In this study, we investigated experimentally the effect of artificial light on flying insects in the field, with an emphasis on aquatic insects. We asked whether lights prevented the ability of insects to disperse across the landscape, a process that is crucial in colonising restored habitats. 2. We set up six, c. 3.5m high downward facing high-pressure sodium streetlights along a permanently connected oxbow in the Spree River of eastern Germany. We collected insects using 12 flight intercept traps, each with trays at three different heights (0.5, 1.5 and 2.5m), placed at distances 0, 3, 40 and 75m from the lights and 5, 8 and 80m from water. The number of emerging aquatic insects in the study area was measured with six emergence traps. We emptied the traps 22 times between June and September 2010; the lights were on for 11 of these nights and off for the other 11. 3. In total, we caught almost 27 times as many insects at traps 0m from the lights when the lights were on than when they were off. Most insects caught when the lights were on were aquatic, with Diptera being the most common order. Furthermore, the proportion of aquatic insects caught at traps 0, 3 and 40m from the lights when they were on was significantly higher than when they were off. On lit nights, more aquatic insects were captured per hour and m(2) (area in which flying insects were intercepted) at traps 0m from the lights than emerged from per square metre per hour from the Spree River. 4. Our results suggest that adult aquatic insects can be negatively affected by artificial light and that city planners should take this into account when designing lighting systems along rivers.