We study whether butterfly diet predicts butterfly distribution along an urban gradient. Data come from a large scale participatory scheme involving the general public reporting butterfly abundance in their gardens (more than 6000 gardens), completed by a special survey on the relationship between butterfly species and flower species based on amateur pictures (more than 3500 pictures). Many studies show that urbanization promotes the loss of native plant species and their replacement by non-native species, so in this context we addressed the question of butterfly diet impacts provided by exotic and native plants in urban landscapes and we quantified diet diversity to identify specialist and generalist butterflies. Diet specialists had a longer proboscis and both specialization and proboscis length were positively correlated to preferences for exotic flowers. Nevertheless, diet specialist butterflies tended to avoid urban areas more strongly than diet generalists, while preference for exotic flowers was surprisingly not correlated to preferences for urban life in butterflies. All together this suggests that diet preferences do not play a strong part in determining butterfly distribution along urban gradients but also that diet specialization must be correlated to other life history traits such as dispersal ability or flexibility in habitat selection.