USDA, APHIS, PPQ has distributed the seedhead gall flies Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) as classical biological agents of the introduced weeds spotted and diffuse knapweed (Centaurea maculosa and C. diffusa, respectively) (Asteraceae) in the United States. From 1987 to 1996, Urophora spp. have been released in 97 counties in 14 midwestern and western states. Established populations of U. affinis and U. quadrifasciata are confirmed in 85 and 95 counties, respectively, among all 14 states. These include the first reports of successful establishment of Urophora spp. in Arizona (two counties), Colorado (eight counties), Michigan (one county), Minnesota (six counties), Nebraska (four counties), Nevada (two counties), North Dakota (one county), South Dakota (four counties), Utah (three counties), and Wisconsin (two counties). The first confirmed establishment of U. quadrifasciata in Indiana and Michigan is also reported.
Lang, R. F.; Richard, R. D.; and Hansen, R. W.
"Urophora Affinis and U. Quadrifasciata (Diptera: Tephritidae) Released and Monitored by USDA, APHIS, PPQ as Biological Control Agents of Spotted and Diffuse Knapweed,"
The Great Lakes Entomologist, vol 30
Available at: https://scholar.valpo.edu/tgle/vol30/iss2/7