Urophora stylata 
<<back to bull thistle
Common name
Bull thistle seed head gall fly
Weed impacted
Bull thistle seed production is impacted by fly larvae. Larvae induce and feed on gall tissue* in the developing seed head, reducing seed production by up 60%.
*galls are an abnormal swelling or deformation of plant tissue caused by insects, mites, microorganisms or injury
Insect description
Adults are roughly the size of house flies. The body is light gray in color with a light brown to yellow scutellum (located on the back behind the head). The wings are clear with a "IV" marking. Larvae are pale colored and 3 to 5 mm long. Five to over 20 larvae can be found in each gall.
Life cycle
Adults emerge from the previous year's seed head in late spring to mid summer. Females lay eggs on developing flower buds. After one week, larvae hatch and burrow into seed producing tissue and induce gall tissue production. Larvae and gall tissues grow throughout the summer and overwinter as third-stage larvae, emerging as adults the following year.
| Jan | Feb | March | April | May | June | July | Aug | Sept | Oct | Nov | Dec |
| overwintering stage - dispersal | adults emerge | larval development in seed head | overwintering stage - collection and dispersal | ||||||||
Field identification 
Adults can be found on bolting, bud-forming bull thistle plants from late May through July. Galled flower heads, containing developing larvae, are present beginning in summer and persisting through winter. Seed heads containing larvae can be identified by their fluffy appearance and by the presence of a hard, walnut-sized gall.
Availability
This insect is present in Washington but is available on a VERY limited basis, in part due to the transient nature of bull thistle infestations. When available, bull thistle galls are collected in autumn and distributed near bull thistle rosettes in 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
Not appropriate for small infestations, heavily grazed areas or areas that flood.
Using the Agent: Integrated Weed Management (IWM)
Urophora stylata may significantly reduce seed production if bull thistle populations are sustained for many years. However, bull thistle infestations tend to be transitory, colonizing recently disturbed sites only until more permanent vegetation establishes, and thus impacting fly densities. Bull thistle may persist longer when land is continually overgrazed or disturbed. Insects should be released when weed populations are large and immediate eradication is not the primary objective. Most treatments are not directly compatible with the use of this biocontrol agent. Any control methods that prevent flowering and seed production 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 bull thistle populations can either be hand-pulled, cut just below the root crown 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.
Repeated mowing can help to control some thistle species, particularly when done in summer (before flowers have gone to seed) prior to fall herbicide applications. However, mowing bull thistle, especially from late summer to mid-spring, will likely destroy U. stylata developing in seed heads.
Prescribed burning may control certain thistles by facilitating the growth of competitive, desirable plants, but the success of this strategy often depends on a complex combination of ecological factors. The disturbance caused by burning is likely to greatly increase the bull thistle infestation rather than control it. It will also inhibit establishment of U. stylata 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.
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
Urophora stylata adult - Peter Harris, Agriculture and Agri-Food Canada, bugwood.org http://www.ipmimages.org/images/768x512/0580003.jpg
Urophora stylata larvae - Peter Harris, Agriculture and Agri-Food Canada, bugwood.org (insectimages.org), http://www.insectimages.org/images/768x512/0580004.jpg
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