Urophora cardui 
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Common name
Canada thistle stem gall fly
Weed impacted
Fly larvae impact plant vigor in Canada thistle by inducing the plant to divert energy away from root and flower production to produce gall* tissue in stems. Growth and flowering can be retarded, but this agent alone does not kill plants or prevent spread.
*galls are an abnormal swelling or deformation of plant tissue caused by insects, mites, microorganisms or injury
Insect description
Adult flies are roughly the size of house flies. The body is black with a dark, distinctive w-shaped banding pattern on the wings. Larvae are pale in color with dark anal plates and are enclosed in the gall structure.
Life cycle
Adults emerge from the previous year's galls in late spring. After mating, eggs are laid in the stems of growing Canada thistle plants throughout the growing season. Galls vary in size depending upon the number of larvae within, ranging between 3 and 10. In autumn, the gall turns brown and hard as the plant tissues die for the season. Flies overwinter as mature larvae in galls and emerge as adults in late spring.
Field identification 
Adult flies can be found in Canada thistle infestations from spring to early summer. Their dark wing bands distinguish them from most other flies in Canada thistle infested fields. Galls begin to develop on stems beginning in summer and persist through the winter. Ranging in size from a small stem swelling to a large crab-apple, galls can sometimes be difficult to spot when stems are green, but tend to become more apparent in autumn and winter as stems turn brown and leaves drop.
Availability
Galls are collected in winter and redistributed in Canada thistle infestations in late winter or early spring. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form by September (for priority) for redistribution the following spring.
Comments
This insect does best at moist sites but seems to establish in a wide variety of environmental conditions. It is not appropriate in areas where Canada thistle is flooded, grazed, mowed, or sprayed. Strong grass competition in conjunction with good fly densities may reduce Canada thistle infestations.
Using the Agent: Integrated Weed Management (IWM)
While it may help to slow stem growth and seed production, U. cardui by itself will not kill Canada thistle plants. Use of this agent is ideal for long-term management of Canada thistle when infestations are large, other treatments are difficult or costly, and immediate eradication of the weed is not the primary objective. Most control tools are not directly compatible with the use of this biocontrol agent. Any control methods that prevent summer stem growth will reduce or eliminate biocontrol populations. Biocontrol agents can be used within large patches and satellite patches (which lead to increased spread) can be controlled using other management tools.
Small or satellite populations of Canada thistle can either be repeatedly hand-pulled or sprayed. A selective herbicide should be used to avoid diminishing the effects of strong grass competition. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate.
Mowing is not recommended for Canada thistle control. In some cases, regular mowing (approximately every 3 weeks during the growing season) can reduce infestations after many years, however, results have been inconsistent. If mowing is not done regularly, it can increase the spread of Canada thistle by stimulating new growth from the rhizomatous root systems and reducing grass competition. Mowing thistles, especially from late summer to mid-spring, will likely destroy U. cardui developing in the stems.
Some methods of grazing (see Winston et al. below) can reduce Canada thistle and help to increase desirable plant productivity. However, combining grazing and biological control may be difficult, since grazing animals trample many of the plants. Successful integration of these two strategies would require careful timing of grazing events around key activity periods of the insects.
Prescribed late-season burning may help control Canada thistle by facilitating the growth of competitive, desirable plants, but success of this strategy often depends on a complex combination of ecological factors. The disturbance caused by burning, particularly during spring, may stimulate Canada thistle infestations rather than control it. Most burning events would likely inhibit establishment of U. cardui or destroy existing populations of the fly.
Any treatment regime, whether biological, chemical, or otherwise, should include revegetation with native or other desirable plants to promote plant competition. Competition stresses weeds and allows for more effective treatment. To learn more about revegetation and IWM, click here.
References
Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.
Jacobs, J., Sciejienka, J., and F. Menalled. 2006. Ecology and management of Canada thistle. USDA-NRCS Invasive Species Technical Note No. MT-5. 11 pp. URL www.msuextension.org/ruralliving/Dream/PDF/canada.pdf
Winston, R., Hansen, R., Schwarzlaander, M., Coombs, E., Randall, C. B., and R. Lym. 2008. Biology and Biological Control of Exotic True Thistles. USDA Forest Service, FHTET-2007-5.
Photo Credits
IWCP staff
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