Insects in the subfamily Rhynchitinae are commonly known as tooth-nosed snout weevils. Adult weevils girdle flower peduncles and leaf petioles, leaving partially severed flowers or leaves hanging on the plant. It is thought only the female weevils engage in girdling, but both genders then congregate inside the now-upside down, partially opened flowers, to feed on pollen, mate, and lay eggs.

Weevils expend a lot of time and energy in this clipping behavior. It may function to reduce the suitability of the plant for other flower-feeding insects that otherwise would compete with the weevil and its larvae for resources.

The eggs hatch only after the flower heads drop to the ground, where the larvae feed on the decomposing plant tissue before overwintering in the soil. Pupation and adult emergence occurs the following summer, timed to correspond to the sunflower's entering its flowering, reproductive phase[1].  I have seen vast swaths of plants thus affected, and not only sunflowers.