Canada thistle stem gall fly

Canada thistle stem gall fly is an exotic, small, fruit fly. It is native to Central Europe. It has been intentionally released in Canada beginning in 1974 and in the United States beginning in 1977 as a biocontrol agent against invasive Canada thistle. It now occurs across southern Canada and the northern half of the United States.

Adults are active from late May to mid-July. They are found wherever their host plant grows. The developing galls become visible in late July or early August, but they do not become large and obvious until late August to September.

Adults are up to 5⁄16″ (8 mm) in length, including the wings. The body is 3⁄16″ to ¼″ (5 to 6 mm) in length.

The thorax is black above, white on the sides. The plate between the abdomen and thorax (scutellum) is white.

The abdomen is black. It is rounded on the male, pointed on the female. The ovipositor on the female is robust, black, hard, and rigid.

The wings are translucent and whitish, with a bold, black, W-shaped, serpentine marking.

Total length: 3 ⁄16″ to 5⁄16″ (5 to 8 mm)

Canada thistle

One generation per year: Late May through mid-July

The female uses her tough ovipositor to insert eggs into the stem tissue at an axillary bud of a host plant. When the eggs hatch, the resulting larvae burrow into the stem. The plant responds by creating a protective swelling (gall) around the invaders. A single gall can contain one, ten, even up to 30 larvae, each one it its own chamber. Third stage (instar) larvae overwinter in the gall and pupate the following spring. In late spring and early summer of the following year, when the host plant begins flowering, the new adults emerge by chewing a tunnel through the gall. Adults are short-lived, mating, laying eggs, and dying within 10 to 20 days.

Canada Thistle Stem Gall Fly (Tephritidae: Urophora cardui) Male
Carl Barrentine

Akkerdistelgalboorvlieg Urophora cardui, poetsend
Roy Kleukers

Wyroślówka ostowa (Urophora cardui)
Arti Lopa