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Thistle Stem Gall Fly (Urophora cardui)

Generations per year:  One

Growth:  4 stages of growth

Egg—up to 30 eggs can be laid in the vegetative shoots at any time during the plants growing season.

Larval—larvae mature inside of the stems of Canada thistle causing the plant to form a gall.  When the plant dies or freezes, the gall becomes brown/gray and woody.  It is very hard and protects the larvae until spring.

Pupal—activity resumes in the gall in early spring as the larvae pupates. 

Adult—flies emerge in late spring to early summer.  Adults are 0.2 to 0.32 inches long with distinct black W-shaped markings on the wings.

Overwintering stage—larval at 98% of total body weight—located in woody galls

Destructive stage—larval

Impact on the Canada thistle—the insect does not kill the plant, however, it reduces the plant’s ability to compete and resist pathogens.  The stems above the galls are usually non-productive with abnormal seed heads.


  • Disturbed areas where Canada thistle is thick.
  • Surface water nearby or high relative humidity
  • Semi-shaded areas slightly preferred over those in full sun

Field subject to flooding, grazing, mowing and chemical treatments are not conducive to fly survival.

Release Date:  Thistle Stem Gall flies are typically released in June to mid July. 

Cost:  A carton of adult gall flies = approximately 105 insects

$100.00 plus overnight shipping costs to apply

If you are interesting in ordering Thistle Stem Gall Fly, please submit the following form or call (406) 485-2744 x100.

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