Adults emerge in early spring and feed on new Canada thistle foliage. Adults are oval, up to 7½ mm long, and have a hard, green protective covering and black undersides. Eggs are laid in spring and summer. Young larvae feed on the undersides of new leaves, leaving behind a thin layer of leaf tissue (“window feeding”), while older larvae make irregular feeding holes from the top sides of leaves. Larvae are green and up to 6 mm long. They have spines on their margins and a forked tail spine on which they accumulate molted skins and waste to serve as a protective parasol. Larvae develop through five instars and then pupate in late summer. Pupae are brown, oval-shaped, and have black spikes along their margins. Emerging adults feed on young Canada thistle foliage until late fall and then overwinter in soil litter. There is one generation per year.