Short-fringed or tyrol knapweed, Centaurea dubia (Asteraceae) is an invasive plant of fields and roadsides in the eastern United States. Insects from the plant's native Eurasian range have been introduced into North America in an attempt to control the spread of related Centaurea species. One of these biological control agents, Urophora quadrifasciata (Diptera: Tephritidae), a picture-winged seed-head fly, has dispersed throughout northeastern North America. This study examined the response of this seed-head fly to a nontarget host's (C. dubia) patch characteristics and seed production in an old field. Ninety percent of all stems contained at least one seed-head attacked by U. quadrifasciata and 59% of C. dubia seed-heads were infested. Neither location in the field, plant height, nor number of stems per patch influenced U. quadrifasciata numbers within seed-heads. Number of seeds per seed-head was influenced by location and patch size but not stem height.