Introducing a specific natural enemy can aid in regulating the reproduction and spread of bull thistle. This weevil feeds on the crown center of the rosette. Native to Europe, this weevil was extensively studied by the USDA to insure that it would not damage other desirable plants in the U.S. Figure 1. The adult rosette weevil. In mid-October, adult weevils awaken from summer aestivation and begin to feed and mate on thistle rosettes.
Females lay eggs along the midrib on the underside of leaves.
Each female can lay up to 800 eggs. Eggs hatch in 10- 12 days. The young larvae burrow into the midrib of the leaves and proceed to eat their way to the crown bud, or growing center of the thistle. There the larvae feed during the winter and early spring, causing tissue damage. Larval feeding on the crown bud changes the growth of the thistle, if not killing it.