A survey determined the extent and severity of the infestations of four species ofCarduusthistles in the United States. In the mainland United States, 360 counties of 3068 have economically important infestations of musk thistle (Carduus nutansL.), the Appalachian and midwest regions harboring the greatest infestations. Plumeless thistle (Carduus acanthoidesL.) and Italian thistle (Carduus pycnocephalusL.) are of economic importance, but have limited distribution. Welted thistle (Carduus crispusL.) is approaching pest status only in Virginia.
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Paul H. Dunn (1976) studied this question.
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