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Urophora stylata
Biological agent type
Seed head gall fly.
Plant species attacked
bull thistle (Cirsium vulgare)
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Urophora stylata adult
Urophora stylata damage
Images by Eric Coombs, Oregon Department of Agriculture.

If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Site of attack
Mode of attack
The larvae form a hard multi-chambered gall in the flower receptacle that interferes with seed production. Many seeds become lodged in the gall tissues, rendering them incapable of dissemination.
Destructive stage
Collection and release information
This bio-agent is available for mass redistribution. Consult with ODA for availability for your area.
History in Oregon
The seed head gall fly Urophora stylata was introduced in 1988. It is now collectible in Benton and Douglas Counties. It has been widely introduced throughout Oregon, but has only established east of the Cascades in Morrow County. It is unlikely to control the weed because seeds can originate from distant infestations. 
Release status in Oregon
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distribution map