The musk thistle seed weevil, Rhinocyllus conicus, was released in Colorado over 30 years ago and can now be found throughout the state. In many areas it has significantly reduced the density of musk thistle. The department of Agriculture has discontinued widespread distribution of this insect since it is found almost everywhere in the state, plus there are concerns since this insect will feed on native thistles.
Another weevil, Trichosirocalus horridus, feeds in the crowns of musk thistle, killing the apical meristem and reducing the flowering potential of the plant. This insect is currently established in many areas around the state and is still being distributed by the Insectary.