Europe and Western Asia
Spotted knapweed (Centaurea stoebe ssp. micranthos), diffuse knapweed (C. diffusa), and squarrose knapweed (C. virgata subsp. squarrosa)
Eggs are white and elongate. Larvae are creamy white, barrel-shaped, and with heads that retract slightly. Larvae of flies do not have head capsules but do develop dark brown anal plates by the end of the feeding period. The pupa is brown, barrel-shaped, and 3 mm long. Adults can be up to 4 mm long. They have dark bodies and clear wings marked with faint horizontal bars. Females have long, pointed, black ovipositors.
There is usually one generation per year, though two may occur in warm climates. Overwintering as third-instar larvae, flies pupate for approximately 14 days in the spring. Adults emerge when knapweed is in bud. Females lay up to 120 eggs in groups of 1–5 among immature florets inside closed flower heads. After 3–4 days, larvae hatch and tunnel into the base of the flower head to feed on receptacle tissue. Larval feeding triggers the formation of a hard, woody gall that surrounds the larva. Typically 2–4 galls are found per flower head, though the number depends on flower head size and knapweed species. Most larvae require a cold period to induce pupation, and thus overwinter in flower heads; 10–25% of larvae may pupate early in suitable climates, with second generation adults emerging in early fall.
Larval feeding directly destroys some seeds, possibly helping to reduce the rate of knapweed spread. In addition, galls drain nutrients from other parts of the plant, which causes stunting and reduces the number of flower heads produced.
Randall, C.B., J.E., Andreas, and J. Milan. 2023. Knapweed Biocontrol Agents: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2023-34-KNAPWEEDS-A. https://bugwoodcloud.org/resource/files/27451.pdf