The impact of biological control on the distribution and abundance of Chondrilla juncea in south-eastern Australia.
Changes in the distribution, frequency and abundance of 3 forms of the weed Chondrilla juncea in south-eastern Australia are documented for the period 1968-80. The abundance of the most widespread form has declined as a result of the impact of a number of host-specific natural enemies (including Cystiphora schmidti (Rubs.) and Phytoptus chondrillae G. Can. (Aceria chondrillae), but not Bradyrrhoa gilveolella (Treitschke) (which has recently been established but has had no influence as yet)) deliberately introduced as biological control agents. The distribution of the other 2 forms has extended. The effect of the most aggressive of these agents, the rust fungus Puccinia chondrillina, on the competitive interaction between the 2 morphologically more dissimilar forms of Chondrilla 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 the fungus, dry weight of the non-susceptible Form C was increased by at least 10%, whilst that of Form A declined substantially.