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Biocontrol: Weed Projects

Biological Control of Bull Thistle In California

Bull thistle, Cirsium vulgare (Savi) (Asteraceae), is a widespread exotic biennial weed that is associated with a high degree of disturbance, such as overgrazed permanent pasture and woodland clearings. The bull thistle seedhead gall fly, Urophora stylata (Fabricius) (Diptera: Tephritidae), is a host specific biological control agent that has been released into California from Europe for the biological control of bull thistle. The bull thistle seedhead gall fly has one generation per year. The adult flies emerge from overwintering seedhead galls from late May through early July and may live up to two months. Oviposition occurs on top of developing flower buds, and the eggs hatch after about one week. After hatching, the larvae migrate to the receptacle where they induce gall tissue formation which the larvae feed on. A multi-chambered gall forms on the receptacle of the seedhead with individual larvae occupying separate chambers of the gall. Each gall may contain from five to 20 larvae. Seedheads infested with the gall flies produce less seed due to the limited amount of receptacle area for seed production.

The seedhead fly was released at ten sites in El Dorado, Marin, Mendocino, Modoc, San Luis Obispo, Shasta, and Tulare Counties by the USDA-ARS, Biological Control of Weeds Laboratory in cooperation with the Biological Control Program from 1993-1995. In 1997, new releases were made at three sites in Marin and Humboldt counties. The release information is summarized in Table 1. Post-release surveys by Biological Control Program staff and county agricultural biologists have failed to find established populations at any release sites. Galls with larvae were found at the El Dorado, Shasta, and Mendocino County sites at the end of the season that they were released but no evidence of U. stylata was found the following year at the same sites. Encouraging recoveries were made at two of the 1997 releases sites in Marin County. The two sites will be monitored closely in 1998 for establishment. Establishment in Oregon has been successful only at sites below 1,000 feet elevation.