The Impact of Biological Control on the Distribution and Abundance of Chondrilla juncea in South-Eastern Australia

SUMMARY (1) Changes in the distribution, frequency and abundance of three forms of Chondrilla juncea occurring in south-eastern Australia are documented for the period 1968-1980. (2) The abundance of the most widespread form has declined as a result of the impact of a number of host-specific natural enemies deliberately introduced as biological agents. The distribution of the other two forms has extended. (3) The effect of the most aggressive of these agents, Puccinia chondrillina, on the competitive interaction between the two morphologically most dissimilar forms of C. juncea is demonstrated in a replacement series experiment. When grown in the absence of P. chondrillina both forms competed equally; when form A plants were regularly infected with P. chondrillina, dry weight of the non-susceptible form C was increased by at least 10%, whilst that of form A declined substantially.