The root-crown feeding weevil Trichosirocalus horridus (Panzer) exhibited a classical biological control pattern on its host plant, nodding thistle (Carduus nutans L.), by increasing in number after its release, causing an effect on its host, and decreasing in number with the decline of the plant. T. horridus did not readily spread from its points of release, and thereby concentrated its feeding. Nodding thistle rosette diameters decreased by 38%, bolted plant densities decreased approximately 50%, stem numbers increased 5-fold, while stem heights showed a gradual decline. The additional biocontrol agent on-site, Rhinocyllus conicus, contributed to the attack on the nodding thistle.
Susan C. Turner, Reg F. Newman, Sandy C. Cesselli. 2021. Trichosirocalus horridus Effect on Nodding Thistle (Carduus nutans). FLNRORD. Technical Report (FLNRORD). TR135