Below-ground Herbivory in Carduus nutans (Asteraceae) and the Potential for Biological Control

The impact of root-feeding insects attacking the 'biennial' Carduus nutans L. in the southern part of its native European distribution was studied as part of a biological control programme against this weed for Australia. Surveys were conducted from Spain to Greece to parallel previous studies in central Europe and three populations of the weed were monitored in detail over a 2-4-year period in southern France. The damage levels and attack patterns of each insect species were recorded. The most common species damaging the roots and meristems were the weevils Hadroplontus (=Ceutorhynchus) trimaculatus F. and Trichosirocalus horridus Panzer and the syrphid fly Cheilosia corydon Harris (=grossa Fallen). In manipulative experiments, made to assess their impact on the host plant, C. corydon reduced seed production by up to 45%, while the two weevils mainly altered plant architecture. The results are discussed in relation to previous experience of using root feeders for the biological control of C. nutans.