Europe
Bull thistle (Cirsium vulgare)
Larvae are barrel-shaped, off-white, have dark anal plates, and can be up to 5 mm long. Adults are brownish-gray with a yellow head and brown legs. Wings are clear with gray-brown “VI” or “IV” markings, the “V” occurring near the wing tip. Adult males have rounded abdomens and can be up to 5 mm long while females have a long, pointed ovipositor for laying eggs and can be up to 7 mm long.
As bull thistle plants are bolting in spring, overwintering larvae pupate inside galls produced the previous year. Adults emerge in early summer and deposit eggs on maturing bull thistle buds. Hatching larvae burrow into seed heads and feed on seed-producing tissue, which induces the formation of galls. There may be multiple larvae per seed head. Third (final) instars overwinter within galls. 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
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