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Urophora cardui
Biological agent type
Stem gall fly.
 
Plant species attacked
Canada thistle (Cirsium arvense)
 
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Urophora cardui adult
Urophora cardui larvae
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
Stem
 
Mode of attack
The larvae cause galls on the stems that act as nutrient sinks, stressing the plants and reducing seed production and plant height.
 
Destructive stage
Larval-internal.
 
Collection and release information
This agent is widespread and can be moved by collecting adults with a sweep net or the galls from the plants in the fall. Consult with ODA for availability for your area.
 
History in Oregon
The stem gall fly Urophora cardui introduced in 1980 is prevalent throughout the Willamette Valley. It is well established west of the Cascades. Releases for Eastern Oregon should be obtained from the cold hardy population established at the Ladd Marsh Wildlife Refuge in Union County.
 
Release status in Oregon
 
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distribution map