The new adults emerge, feed briefly, and pass the summer and winter in a period of inactivity, under the thistle rosettes, in litter, or in adjacent forest areas. The more insects present inside the head, the fewer seeds produced. Fifteen weevil grubs per head will prevent all seeds from forming in that head. Seasonal activity of the head weevil is closely synchronized with the terminal heads in spring. Lateral flower heads, produced over the course of the summer, may escape attack by this insect.
Infested musk thistle flower heads.
The rosette weevil, also called the crown weevil, occurs on musk thistle, bull thistle, and plumeless thistle. The weevils are not able to complete their life cycle in other thistles found in Alabama. They are about two-thirds the size of thistle head weevils and have a rough texture. Adults emerge from summer resting places in the fall. They lay eggs in the midrib of thistle leaves, and complete egg-laying in the spring.