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Biological Control : A Guide to Natural Enemies in North America Anthony Shelton, Ph.D., Professor of Entomology, Cornell University

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Urophora affinis
(Diptera: Tephritidae)

Ronald Lang, USDA-APHIS-PPQ, Forestry Sciences Lab, Montana State University, Bozeman, MT 59717-0278.

Urophora affinis, a seedhead fly, was first released in the United States in 1971. Its release is part of a program to introduce a complex of spotted and diffuse knapweed enemies to help control these natives of Eastern Europe and Asia. In North America spotted and diffuse knapweed are efficient competitors and since being introduced around the turn of the century have spread throughout the northern tier of states and as far south as Nebraska and Virginia. They favor disturbed sites and overgrazed rangelands and cause a reduction of forage for cattle of up to 100% in infested areas. Spotted knapweed and diffuse knapweed also prosper on grasslands and riparian sites and displace native vegetation in these habitats.

Although knapweed can be controlled by extensive cultivation or herbicide application, its existence on stream banks and rough terrain and the typically expansive areas of infestation make these methods impractical and/or economically unfeasible. Therefore, an integrated pest management approach is being sought to help in controlling this weed.

U. affinis has been released in Arizona, California, Colorado, Idaho, Michigan, Minnesota, Montana, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Utah, Washington, Wisconsin, and Wyoming. Populations have been established in all of these states but Nevada. U. affinis may be found in the spring on the unopened buds of spotted or diffuse knapweeds.


U. affinis is a small (3-4 mm) fly with lightly barred wings and a dark body. The female ovipositor is prominent and nonretractable. The fly larva causes the plant to form a gall in which the larva lives, feeds, and eventually emerges as an adult. The gall on the receptacle is pointed at the tip and is made of lignified material.


Spotted and diffuse knapweed-infested areas.

Pests Attacked

Extensive host specificity testing found that U. affinis is host specific to spotted and diffuse knapweed. The field and laboratory tests found that flies collected from these two weeds in various locations in Europe were host specific to the plant on which the flies were reared. Economically important plants such as artichoke and safflower have physical and visual barriers that prevent U. affinis from being attracted.

Life Cycle

U. affinis overwinters as larvae in the seedheads. Pupation is in the spring, and the adults emerge in mid to late spring. After mating takes place the eggs are laid between the bracts on closed flower buds. Developing larvae cause the plant to form a fusiform gall from tissues of the ovary and receptacle. The gall becomes lignified and hard. The females recognize the proper host bud by size and by probing to check the thickness and hardness of the bud exterior.

Pesticide Susceptibility

Not yet known.

Commercial Availability

In some states U. affinis adults may be obtained at no cost from state weed management agencies. Several commercial suppliers can also provide U. affinis adults.


U. affinis creates metabolic sinks, drawing nutrients from the plant that extend beyond the attacked seedhead. New flower buds may abort and not develop due to the lack of nutrients. Unattacked seedheads produce an average of 12.6 viable seeds. Each gall is estimated to reduce seed production by 2.4 seeds per seedhead. U. affinis needs to be supplemented with other seedhead agents and root borers.
It is anticipated that this fly will be an effective control agent in conjunction with other root boring and seed feeding insects. Spotted and diffuse knapweed patches are hard to find in Europe because they are kept under control with the native insects and diseases.


Release sites should be free from disturbance and from development or pesticide use for at least ten years. U. affinis likes full sun in open areas.

For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, Feature Article on conservation in Volume II, No. 1 of Midwest Biological Control News.


Harris, P. (1980) Effects of Urophora affinis Frfld. and U. quadrifasciata (Meig.) (Diptera: Tephritidae) pm Centaurea diffusa Lam. and C. maculosa Lam. (Compositae). Z. ang. Ent. 90: 190-201.

Story, J. M., Boggs, K.W., and Nowierski, R.M. (1989) The effect of two introduced seedhead flies on spotted knapweed. Montana Agriculture Research, Montana, Montana State University, Bozeman, MT 59717-0278. Winter: 14-17.

Zwolfer, H. (1970) Investigations on the Host-Specificity of Urophora affinis Frfld. (Diptera: Trypetidae) Weed Projects For Canada Progress Report. C.I.B.C. Report: European Station, Delemont, Switzerland:1-28.

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Top: Urophora affinis female. V.Farquhar.

Center: U. affinis galls. R.Richard

Bottom: U. affinis adult collection cages. R.Richard

Top: Urophora affinis female.
Photo: V.Farquhar.

Center: U. affinis galls.
Photo: R.Richard

Bottom: U. affinis adult collection cages. Photo: R.Richard

Spotted knapweedrosette.

Spotted knapweedrosette.

Photo: V. Farquhar


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