Biodiversity is globally under pressure, and the current decline in insect biomass and diversity is likely caused by human activities. Key drivers of biodiversity loss include agricultural intensification and anthropogenic climate change. Nevertheless, a thorough understanding of potential interactions between both factors and the mechanisms underlying insect declines, in general, is still lacking. Here, the authors investigate the combined effects of nitrogen fertilisation and drought, as applied to host plants, on the preference and performance of the butterfly Lycaena tityrus. Individuals performed best on plants having received medium nitrogen levels, while performance was reduced by either a lack of or strong fertilisation, the former potentially caused by nitrogen limitation and the latter by increased concentrations of toxic allelochemicals. Female oviposition preference though was positively related to nitrogen fertilisation, resulting in a mismatch between preference and offspring performance at high nitrogen levels. Plant drought stress additionally reduced herbivore performance, and females appeared to suffer more from low‐quality food than males. Our results indicate that increasing nitrogen fertilisation, as applied in intensive agriculture, may substantially reduce host‐plant quality for insect herbivores, which may be exaggerated in the course of climate change due to the more frequent occurrence of droughts. Our study thus contributes to a better understanding of the mechanisms underlying human‐driven insect declines in agricultural landscapes and beyond.