The ability of weeds to proliferate into nonindigenous habitats has been attributed to escape from their native natural enemies, allowing reallocation of resources from chemical defense into growth and reproduction. Many invasive weeds, however, eventually encounter their native, coevolved enemies in areas of introduction. Examination of herbarium specimens of an invasive phototoxic European weed, Pastinaca sativa, through 152 years reveals phytochemical shifts coincident in time with the accidental introduction of a major herbivore, the parsnip webworm, Depressaria pastinacella. Plants collected before the introduction of webworms in North America and during the earliest stages of establishment (1850-1889) are lower in toxic furanocoumarins than all plants subsequently collected in North America and lower than European plant samples collected before 1889. Thus, introduction of a major specialist herbivore can increase noxiousness of a species in its area of introduction, illuminating a potential consequence of classical biocontrol programs involving insect herbivores and poisonous weeds.