Interspecific competition between Rhinocyllus conicus and Urophora soistitialis L on nodding thistle in Canterbury

Nodding thistle receptacle weevil Rhinocyllus conicus and gallfly Urophora solstitialis attack the capitula of nodding thistle Carduus nutans L. Between 31 October and 15 December 2003, the phenology of both R. conicus and U. solstitialis was studied at a dryland site in Canterbury. Adult R. conicus were more numerous than U. solstitialis on capitula throughout the experiment. Larvae of R. conicus were fi rst found on 11 November (15% of capitula infested) and peaked on 2 December with 53% of capitula infested. Only 3% of capitula were infested by U. solstitialis. Adult R. conicus or U. solstitialis emerged from 79% of the selected primary and secondary capitula. The majority of infested capitula (81%) contained only R. conicus, 2% contained only U. solstitialis while 17% contained both insect species. Parasitism of R. conicus by the braconid parasitoid Microctonus aethiopoides was low and occurred when most weevil eggs had been laid.