Night, light and flight: light attraction in Trichoptera

Artificial light is an important and necessary part of urban environments, but light can have substantial direct and indirect effects on populations of various organisms. Urban areas are often situated close to water and thus organisms dependent on water could be especially vulnerable. Trichoptera is one of the most abundant insect orders in freshwater, but its attraction to light has not been analysed in detail. We contrasted catches in light traps and passive traps at three locations in Sweden. The results showed that artificial light can affect Trichoptera populations. Attraction to light varied between Trichoptera species and females were more attracted than males. Day-, evening- and especially night-active species were all attracted to light. Light catches of day- and evening-active Trichoptera could partly be a consequence of atypical flight activity, i.e. they are deceived to take flight when a lamp is lit during night. In all, artificial light can alter Trichoptera populations, sex ratios and species composition. This impact should be considered when erecting and managing light sources near waterways