Bug FAQThistle Stem Gall Fly
Urophora cardui · Order: Diptera · Family: Tephritidae
Also called: Canada thistle gall fly, Fruit fly
The adult fly chews a tunnel to exit the gall it created in thistle stems. It is native to central Europe, ranging from the UK and Sweden down to the Mediterranean and east toward the Crimea. Females lay up to thirty eggs on thistle stems during the plant’s growing season; larvae form galls, overwinter, and pupate in early spring.
Source: Wikipedia
How to get rid of it
The fly has been introduced to North America to help control the invasive Canada thistle weed.
Source: Wikipedia
What it eats
Females lay one to thirty eggs on the stems of Canada or creeping thistle during the host’s growing season.
Source: Wikipedia
Life cycle
Eggs are deposited on thistle stems; larvae burrow into the stem, create a gall, and overwinter as third-instar larvae. In early spring the larvae pupate for about 24 to 35 days and emerge as reddish-brown adults.
Source: Wikipedia
When you see it
Egg laying and larval development occur during the thistle’s growing season.
Source: Wikipedia
Where it comes from
Its native range is central Europe, covering the same area as its habitat.
Source: Wikipedia
Where it is found in the US
Recorded in 26 of 51 states and DC. Most records: Colorado, Oregon, New Jersey, Wisconsin, Minnesota, Illinois, Washington, Maryland.
Darker = more records. Counts are museum and citizen-science records, not population size.
Source: GBIF
Status: introduced in United States; introduced in Canada. Source: iNaturalist
Months it is reported in the US
Source: GBIF
More photos
Same family (Tephritidae)
Goldenrod Gall FlyEurosta solidaginis
Goldenrod Brussels Sprout Gall FlyProcecidochares atra
Mediterranean Fruit FlyCeratitis capitataDamages trees, gardens or crops
Cotton-gall TephritidAciurina bigeloviae
Oriental Fruit FlyBactrocera dorsalisDamages trees, gardens or crops
Mottled Thistle FlyXyphosia miliariaSources
- Wikipedia: https://en.wikipedia.org/wiki/Urophora_cardui
Look it up elsewhere: GBIF · iNaturalist · BugGuide · ITIS · Wikipedia
Text written 2026-10-22 from the sources above and checked sentence by sentence against them. Data fetched 2026-10-22.