Individual and combined effects of Trichosirocalus horridus and Rhinocyllus conicus (Coleoptera: Curculionidae) on musk thistle

Field experiments were conducted in Northeast Kansas under conditions of limited plant competition to evaluate the individual and combined impact of the imported weevils Rhinocyllus conicus Froelich and Trichosirocalus horridus (Panzer) on the introduced weed, musk thistle (Carduus nutans L.). No effects on seed production occurred when low larval densities of T. horridus (<20 per plant) fed in rosettes, with or without the later-arriving, head-feeding weevil, R. conicus, present. In contrast, under high T. horridus larval densities (66 per plant), production of flower heads was reduced compared to uninfested plants. This indirect effect resulted in a 30% loss in viable seed per plant (with R. conicus present). Plant survival was not affected by T. horridus within the range of larval densities tested. R. conicus, when present alone, reduced viable seed by approximately 45%, whereas high densities of both weevils reduced viable seed by 59%. Thus, under conditions of low plant competition, neither weevil substantially limited seed production by musk thistle. This suggests the need for a multi-faceted management program for musk thistle involving additional forms of stress, such as interspecific plant competition.