Europe
Canada thistle (Cirsium arvensis)
Larvae are white, C-shaped, and may appear pointed in the front end. They grow up to 3 mm long and have brown head capsules. Adults are mottled black and white with a “t”-shaped marking on their backs. The adults also have thick bodies, very long, curved snouts, and can be up to 4 mm long.
Overwintering adults emerge from plant litter early in the spring and feed on Canada thistle leaf and stem tissue. Eggs are laid in spring in the midvein on the underside of new rosette leaves. Emerging larvae mine leaf veins, stems, and root crowns of Canada thistle throughout spring and summer as they develop through three larval instars. Pupation occurs in the soil, and emerging adults overwinter in plant litter. There is one generation per year.
Larval mining and adult feeding do not significantly impact weed populations directly because only non-essential tissues are typically consumed. Feeding does cause secondary damage, however, as pathogens and other organisms are able to enter the stems of targeted plants via holes made by exiting larvae. One pathogen, Puccinia punctiformis (listed below), may opportunistically enter Canada thistle stems in this manner, causing a synergistic negative effect on attacked Canada thistle stems; however, this has not been explicitly studied.
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