Establishment, dispersal, and prevalence of Rhinocyllus conicus (Coleoptera: Curculionidae), a biological control agent of thistles, Carduus species (Asteraceae), in Argentina, with experimental information on its damage

• We surveyed central Argentina for the establishment of Rhinocyllus conicus.
• We recorded the presence and prevalence of the weevil.
• The weevil is firmly established and spread in Argentina.
• These results encourage the implementation of the IPM approach in South America.

Use of the weevil, Rhinocyllus conicus Froelich, for classical biological control of Carduus species in Argentina has not been controversial, as it has been in Canada and the United States, because there are no native thistle species in the South American release areas. In this study, 30 years after its first field release in Argentina, the Pampas region was surveyed to confirm the establishment of R. conicus, covering the original release sites and most of the overall area of distribution of its host thistles, Carduus species. Sites (n = 121) were systematically selected and natural populations of thistles were examined to confirm the presence of the weevil. Twenty four sites with R. conicus were selected to record the weevil’s prevalence on Carduus thoermeri Weinman and Carduus acanthoides L. Additional opportunistic surveys were conducted in other regions of Argentina and Uruguay, more distant from the original release sites. Rhinocyllus conicus was found at 81% of the original release sites and at 69% of the total sites surveyed, an area of approximately 370.000 km2. On average, 92% of the examined heads had damage in areas where the weevil was present, showing its high prevalence. A pilot study showed that the oviposition period of the weevils lasted 119 days and seed production of C. acanthoides was reduced by 15.5%. These results encourage the implementation of an integrated management of thistles in Argentina, and maybe in other parts of South America.

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