||The current decline in insect numbers and biomass is likely due to several factors  and one of the lesser studied factors is the increased artificial light at night (ALAN). Several negative impacts of ALAN on insects have been described  but evidence that it ultimately results in population declines has been circumstantial due to a lack of emperical data [3,4]. Here, we experimentally exposed natural habitats to three colours of artificial light, and a dark control, and studied the impact on moth population numbers during five consecutive years. With this experimental, multi-year study, we can isolate the effects of artificial light from other anthropogenic factors that are often confounded in correlative studies. Furthermore, we can study long-term effects that only become apparent after several years. In the first two years, the number of moths in the illuminated and dark treatments did not differ, but after the second year, the number of moths in the illuminated treatments was lower than in the dark control (Figure 1). This first implies a causal relationship between ALAN and local population declines and thus a contribution of ALAN to insect declines.