Cirsium arvense (L.) Scop. is a perennial herb indigenous to Eurasia that is now present throughout temperate regions of the world where it is considered one of the worst weeds of pastoral and agricultural systems. Classical biological control has been attempted in both North America (NA) and New Zealand (NZ). However, nearly 50 years after the first agent releases there are no indications of successful control. We review the status of the five agents deliberately released for control of C. arvense in NA and NZ, plus the species unintentionally introduced, and the occurrence of insects native to NA on C. arvense. We retrospectively evaluate C. arvense as a target weed, critique the agents selected for release, and contrast the different situations in NA and NZ. In retrospect, we see justification for the agents released in NA, but it is evident that these agents would not meet the more stringent host specificity requirements necessary to be released today. The failure of the program in NA is attributed to compromised safety, and lack of impact. Non-target impacts by one of the released agents, Rhinocyllus conicus, have raised safety concerns for native thistle plants. The other released agents either failed to establish, or if established, had no impact on the weed. In contrast, the situation in NZ is quite different because there are no related native thistles (Cardueae), and thus little chance of non-target impacts. Thus far, failure in NZ is attributed to lack of effectiveness due to non-establishment, or no impact, of released agents. In the past, the same agents that were released in NA were subsequently released in NZ, without considering whether or not these were the best choices. Thus, the past failure in NZ might be due to the previous lack of a NZ-specific approach to biocontrol of thistles in general and C. arvense in particular. A new approach taking into consideration the absence of native Cardueae has resulted in the release of agents more likely to be effective, and has potentially set NZ on track towards successful biological control of C. arvense, and other thistles.
