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INSECTS:
- Leafy Spurge
- Knapweeds
- St. Johnswort
- Yellow Starthistle
- Musk Thistle
- Canada Thistle
- Tansy Ragwort
- Toadflax
- Poison Hemlock
- Purple Loosestrife
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(insect photographs)
Canada Thistle Insects
Thistle Stem Gall Fly and Galls
Urophora cardui

The Thistle Stem Gall Fly, Urophora cardui, attacks the primary and lateral stems of Canada thistle. Adults will lay their eggs on the thistle plant in the early summer when bolting plants are 12 inches tall or more. The developing larvae stimulate the plant to form a hard, woody, stem gall. Gall formation directs nutrients away from the normal metabolic and reproductive functions of this aggressive weed. Abnormally developed flower heads frequently occur above the gall, often reducing seed production. Multiple "cardui" galls are possible on a single Canada thistle plant. Releases of 105 adult flies are available during June to mid-July for $100.00.
Stem Gall Cross Section


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Biological Control of Weeds, Inc.
1418 Maple Drive • Bozeman, MT 59715
Phone 406-586-5111

Email: bugs@bio-control.com