||Light emission from street lighting or other light sources alters the living conditions for organisms in urban areas. Nowadays, the impact of light at night (ALAN) on urban plants and their trophic environment is not well understood. To gain more insight about herbivore plant’s interaction when exposed to ALAN, outdoor and greenhouse tests were conducted using the horse-chestnut leafminer, Cameraria ohridella, as a test organism due to its adaptive behavior. At the end of the season, the development of chestnut tree leaves and the leafminer were measured at illuminated versus non-illuminated sites in the city of Berlin and the rural area of Brandenburg. Illuminated leaves were larger than those grown in darker rural areas and, extended larval activity was recorded. Additionally, in the greenhouse, infested chestnut seedlings were exposed to two different light regimes; one treatment provided continuous illumination and the other short daylight conditions. After only one week, the mine size was lower on illuminated seedlings, presumably due to reduced leaf senescence. The leafminer developed a lower proportion of diapausing pupae and a higher proportion of free pupae, which leads to a further generation within the season. The results indicate a strong impact of ALAN on plant metabolism, a secondary effect on leafminer development and its larval activity. For urban trees, the consequence might be an increased herbivore / parasite pressure. For herbivores and parasites less adapted to winter damages than the invasive leafminer a reduced dormancy due to direct or indirect effects of ALAN could even threat the population.