Effect of introduced biological control organisms on the density of Chondrilla juncea in California.
Following the intentional introduction of the exotic biological control organisms Puccinia chondrillina (skeletonweed rust), Cystiphora schmidti (skeletonweed gall midge) and Aceria chondrillae (skeletonweed gall mite) into N. California, reductions in the density of Chondrilla juncea at 3 locations varied from 56.3 to 87.2%. P. chondrillina on rosettes in early spring gave the best correlation between change in plant density and the number of plants attacked by control agents. As in Europe and Australia, P. chondrillina appeared to be the organism most damaging to populations of C. juncea in California. While Cystiphora schmidti provided a local impact on plant density, there were no significant correlations between attack by A. chondrillae and plant decline.