Population density influences various life history parameters of a wide range of animal species. However, especially in invertebrates it is virtually unexplored in how far also behavioural traits are affected by population density and in which stage during ontogeny such effects may be shaped. We investigated the influence of larval versus adult population density on the behavioural phenotype of a non-social insect, the mustard leaf beetle Phaedon cochleariae (Coleoptera: Chrysomelidae). Individuals were kept either separated or in groups throughout life or their rearing condition was switched after metamorphosis. In the adult stage, several behavioural traits were recorded, which clustered in three behavioural dimensions, boldness, activity and exploration. Female boldness and male activity were shaped by the adult population density experiences. In contrast, female activity was mainly affected by the population density experienced as larvae, and female exploration was influenced by the interaction between the larval and the adult population density experiences. Generally, continuously separated reared beetles were boldest, as has been similarly found for vertebrates, pointing to a possible cross taxa similarity. The present study highlights that larval versus adult experiences of population density alter various behavioural traits to different degrees even in a non-social insect, demonstrating a high phenotypic plasticity within this holometabolous species.