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Prediction of host plant range and ecological impact of exotic phytophagous insects, such as insects for classical biological control of weeds, represents a major challenge. Recently, the flowerhead weevil (Rhinocyllus conicus Fröl.), introduced from Europe into North America to control exotic thistles (Carduus spp.), has become invasive. It feeds heavily on some, but not all species of native North American thistles (Cirsium spp.). We hypothesized that such non-target use among native plants could be better predicted by knowledge of characteristic chemical profiles of secondary compounds to supplement the results of host specificity testing. To evaluate this hypothesis, we reviewed the literature on the chemistry of Cirsium and Carduus thistles. We asked what compounds are known to be present, what is known about their biological activity, and whether such information on chemical profiles would have better predicted realized host range and ecological effects of R. conicus in North America. We found an extensive, but incomplete literature on the chemistry of true thistles. Two main patterns emerged. First, consistent chemical similarities and interesting differences occur among species of thistles. Second, variation occurs in biologically active groups of characteristic compounds, specifically flavonoids, sterols, alkaloids and phenolic acids, that are known to influence host plant acceptance, selection, and feeding by phytophagous insects. Surprisingly, sesquiterpene lactones, which are characteristic in closely related Asteraceae, have not been extensively reported for Cirsium or Carduus. The minimal evidence on sesquiterpene lactones may reflect extraction methods vs. true absence. In summary, our review suggests further research on thistle chemistry in insect feeding is warranted. Also, since the exotic Canada thistle (Cirsium arvense) is an invasive thistle of current concern in North America, such research on mechanisms underlying host range expansion by exotic insects would be useful.