[2] The adult fly is light greenish brown with brown-banded wings and iridescent green eyes.

It is 3 to 5 millimeters long.external image The female lays about 70 eggs beneath the bracts on immature flower heads.

When the larva emerges from the egg a few days later it burrows into the flower head and feeds on the developing florets.

As the larva grows it begins to feed on the developing seeds, often consuming the entire contents of the flower head during its two-week larval stage.

The fly is often outcompeted by other species that attack knapweed flower heads, such as certain weevils, so it does best where there are no competitors.