Bull thistle has one approved biological control agent in California: the gall fly, Urophora stylata (Diptera: Tephritidae). Adult female flies deposit their eggs in young flower buds (Fig. 1). Larvae burrow into the flower head and induce the formation of a hard, woody gall which reduces seed production. Adult flies emerge in late May and June, mate, and females visit flower heads and deposit eggs in July and August. Many larvae can develop inside one head (Fig. 2). When the larvae have finished their development, they hibernate in the head during winter and early spring. This fly has one generation per year. On a population level, this gall fly has been estimated to reduce seed production by 60%. Adults are about the size of house flies. The body is light gray and the wings are clear with a dark “IV” marking (Fig. 1).

In California, U. stylata was released at 24 locations statewide, but it formed permanent populations at only three locations: San Simeon, San Luis Obispo County; Tomales, Marin County, and Eureka, Humboldt County (Fig. 3). All three locations are within 10 miles of the coastline. While the fly is able to build up high populations (over 50% of heads attacked) where it established, its limited distribution severely reduces the benefits this beneficial organism can provide statewide. Urophora stylatahas not been found attacking any other plant species.

Biological Control Agents


Common name




Urophora stylata

gall fly



First released in CA in 1993

How the Technique Is Employed

Look for signs of the gall fly by examining old flower heads. Using a leather glove, un-infested heads will give or collapse when squeezed with fingers, infested heads with galls are hard and resist being squeezed. Adult flies can be found on open flower heads in June through August. The bull thistle gall fly may be particularly useful in areas where populations of bull thistle persist from year to year. Many bull thistle populations are transitory, colonizing recently disturbed sites then slowly die out as more permanent vegetation establishes.

Adult flies can be collected in the field by sweep net. Alternatively, galled seedheads can be collected in the fall and winter and placed in large-mesh fruit bags, such as orange bags, at field sites where adult flies will emerge in the spring. The bags protect the heads from being eaten by rodents and the large mesh allows the adult flies to escape.

The gall fly should be released where bull thistle populations are large and immediate eradication is not the primary objective.

Special Tips

The bull thistle gall fly is present in plants throughout the year. Adult gall flies emerge in June and are active visiting flower heads in July and August while larvae are present inside seedheads during the other ten months. Dead seedheads laying on the ground may contain live gall fly larvae, so it is best not to remove them until adults have finished emergence in the spring.


Herbicides that kill bull thistle rosettes during the winter or early spring, before adult flies emerge, may reduce plant populations and not affect adult emergence.

Where Can I Get These?

The gall fly is available on a very limited basis because of its restricted geographic distribution and because the transitory nature of bull thistle populations prevent establishment of field nursery sites.

Insects may be available from your county Agricultural Commissioner or the CDFA Biological Control Program located in Sacramento.

There are no known commercial vendors of this beneficial insect.


Harris, P. and A.T.S. Wilkinson. 1984. Cirsium vulgare (Savi) Ten., Bull Thistle (Compositae). In: Kelleher, J.S. and M.A. Hulme (eds), Biological Control Programmes against Insects and Weeds in Canada 1969-1980, Commonwealth Agricultural Bureaux, England, p. 147-153.

Piper, G. L. and E.M. Coombs. 2004. Urophora stylata. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.), Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis, pp. 375-376.

Villegas, B. 2000. Releases of the bull thistle gall fly, Urophora stylata, for the biological control of bull thistle in California. In: Woods, D.M. (ed.), Biological Control Program Annual Summary, 1999. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, California, pp. 44-46.

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA.