Rhinocyllus conicus is also found on Canada thistle, but it is smaller and has a short rostrum that it barely visible from above. In Larinus planus the rostrum long, held forward and has conspicuous antennae about the mid point. The bracts around the egg hole turn brown. This differs from Rh. There is no preference between male and female buds.
Single larvae develop in buds where they consume the soft tissues.
Infested buds fail to open fully, are distorted and do not produce seed. It differs from Rh. The adults emerge in the spring to feed on the young foliage near the shoot tips. Mating and oviposition starts with the appearance of the first flower buds. One generation a year.