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Herbivores may significantly reduce plant populations by reducing seed set; however, we know little of their impact on seed movement. We show for the first time that the receptacle-feeding weevil Rhinocyllus conicus not only reduces seed production by the invasive thistle Carduus nutans but also inhibits release and subsequent wind dispersal of seeds. These effects generate large, though different, impacts on spatial spread and local abundance in two populations with differing demography, located in the United States and New Zealand. Furthermore, the mechanism is context dependent, with the largest effects through increased terminal velocity in the United States but through reduced seed production in New Zealand. Our results show that the benefit of biocontrol programs may have been underestimated; screenings of potential biocontrol agents should examine effects on pest dispersal and spread, as well as on abundance. [ABSTRACT FROM AUTHOR]

The article studies the amplitude of the invasive biocontrol weevil, Rhinocyllus conicus, on a newly adopted host plant, Cirsium canescens. The study found that Rhinocyllus conicus infests across the host plant's distributional range compared with the peripheral sand patches closer to areas where exotic thistle is common. The number of flower heads per Cirsium canescens plant also serves as another consistent predictor of weevil densities. The study further indicates that exotic weevil abundance on Cirsium canescens is associated to the local availability of native floral resources. Furthermore, the study suggests that the tenacity of a native species may need preservation of population in environments outside the best part of the native species' distribution.

We monitored populations of Canada thistle biocontrol agents Cassida rubiginosa, Ceutorhynchus litura, Larinus (= Hadroplantus) planus, Urophora cardui, Orellia (= Terellia) ruficauda, and Rhinocyllus conicus on Canada thistle (Cirsium arvense) at two national wildlife refuges in South Dakota from 1999 through 2003. C. litura, U. cardui, O. ruficauda, and R. conicus were present on both refuges. Agent populations were low except for C. litura, which was present in up to 90% of stems in some plots. C. litura infestation did not reduce thistle flowering, stem length, or over-winter survival. There was no change in thistle stem numbers over the study period and no difference in stem numbers in areas of high C. litura populations compared to areas of low C. litura populations. Our results suggest that insect biological control agents are inadequate for reduction of Canada thistle in southern South Dakota. [ABSTRACT FROM AUTHOR]