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Trichosirocalus horridus
Biological agent type
Musk thistle crown weevil..
Plant species attacked
musk thistle (Carduus nutans)
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Trichosirocalus horridus damage
Image by Eric Coombs, Oregon Department of Agriculture.
Trichosirocalus horridus adult
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Site of attack
Root and root crown.
Mode of attack
The larvae feed in the root/crown and the adults feed on the leaves. The central stem often dies out after severe larval damage, causing the development of several smaller stems to arise.
Destructive stage
Collection and release information
Limited in distribution at this time. Consult with ODA for availability for your area.
History in Oregon
The root/crown weevil Trichosirocalus horridus was released in Malheur, Klamath, and Umatilla Counties in 1994. The weevil was only recovered in Klamath County at the Devils Garden site, where stand density declined by 80%. The weevil does not do well in moist pastures. This insect, or a closely related species, has been reported to attack Scotch and Italian thistles.
Release status in Oregon
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distribution map