Europe
Musk thistle (Carduus nutans), plumeless thistle (C. acanthoides)
Larvae are barrel-shaped, off-white, have dark anal plates, and can reach lengths of 4 mm at maturity. Adults are brownish-black with a yellow head and legs. Wings are clear and with two black “VII” or “IIV” markings with the “V” occurring near the tip of the wing. Adults are 3–5 mm long; males have a rounded abdomen, and females have a long, pointed ovipositor for laying eggs.
Overwintering larvae pupate inside galls in early spring as plants start bolting. Adults emerge in late spring and early summer when new Canada thistle stems are bolting and forming buds. Eggs are deposited on plant shoots in axillary buds throughout summer. Hatching larvae burrow into stems and cause galls to form. There are often multiple larvae in one gall. Larvae overwinter in the third (final) instar. There is one generation per year.
Larval feeding reduces seed production, and seeds in infested seed heads are physically stuck to gall tissue, thereby reducing dispersal distance. Galls from larval feeding act as metabolic sinks, diverting
Randall, C.B., J.E., Andreas, J. Milan, and K. Gladem. 2024. Introduced Thistle 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-2024-8-INTRODUCED THISTLES-A. https://bugwoodcloud.org/resource/files/29169.pdf