There are no biological control agents that have been specifically released for cotton thistle control in the United States. A thistle head weevil "Rhinocyllus conicus" that feeds on "Carduus pycnocephalus" has also been shown to feed on cotton thistle. This insect was the object of imprudent biological control introduction, and it became an invasive species that has threatened endangered native thistles in North America (Strong 1997). Establishment of this thistle head weevil as a biological control agent for cotton thistle has been unsuccessful in the Pacific Northwest. A thistle crown weevil ("Trichosirocalus horridus") that feeds on musk, bull, plumeless, Italian, and creeping thistles will also feed on cotton thistle. In Australia, this insect has been shown to kill cotton thistle rosettes. The related "Trichosirocalus briese" is also being tested as a control agent. The Australian painted lady butterfly has been known to use this invasive species as a host plant, but the larvae do not do enough damage to the plant for this butterfly to be used as biological control agent.