Europe
Musk thistle (Carduus nutans), plumeless thistle (Carduus acanthoides), Italian thistle (Carduus pycnocephalus), slenderflower thistle (Carduus tenuiflorus), milk thistle (Silybum marianum), bull thistle (Cirsium vulgare), Canada thistle (Cirsium arvense), and marsh thistle (Cirsium palustre)
When eggs are deposited in spring, they are covered over with chewed plant material that becomes tan-colored with age, appearing as part of the plant. Larvae are white with brown head capsules, C-shaped, and reach up to 4 mm long. Adults are dark brown or black with yellowish tufts of hair, giving them a mottled appearance in spring. As they age, adults shed some of these hairs and turn brownish-black. They can be up to 6 mm long and have short snouts.
Overwintering adults emerge in early spring and deposit eggs on bracts of thistle seed heads and stems. Larvae hatch in late spring and early summer and develop through four instars. Larvae burrow through bracts or mine plant stems until they reach the seed head where they feed on receptacle tissue and developing seeds. Pupation occurs within seed heads in late summer as seeds mature. Adults emerge for a brief time before overwintering in sheltered areas. There is one generation per year.
Adults feed on foliage and leave signature rounded feeding holes over the entire leaf surface, though this damage is typically minor. Larval mining and feeding on receptacle and seed tissue reduces viable seed production. This does not kill existing plants but does reduce the rate of thistle spread.
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