Phenological mismatches due to climate change may have important ecological consequences. In a three-year study, phenological shifts due to experimental warming markedly altered trophic relationships between plants and insect herbivores, causing a dramatic decline of reproductive capacity for one of the plant species. In a Tibetan meadow, the gentian (Gentiana formosa) typically flowers after the peak larva density of a noctuid moth (Melanchra pisi) that primarily feeds on a dominant forb (anemone, Anemone trullifolia var. linearis). However, artificial warming of approximately 1.5 degrees C advanced gentian flower phenology and anemone vegetative phenology by a week, but delayed moth larvae emergence by two weeks. The warming increased larval density 10-fold, but decreased anemone density by 30%. The phenological and density shifts under warmed conditions resulted in the insect larvae feeding substantially on the gentian flowers and ovules; there was approximately 100-fold more damage in warmed than in unwarmed chambers. This radically increased trophic connection reduced gentian plant reproduction and likely contributed to its reduced abundance in the warmed chambers.