PREDISPERSAL SEED PREDATION ON CARDUUS-NUTANS (ASTERACEAE) IN SOUTHERN EUROPE

Predispersal seed predation of the ''biennial'' Carduus nutans L., by insects in the southern part of its native European distribution was s tudied as part of a biological control program against this weed for A ustralia. Three populations of the weed were monitored in detail over a 2-4 year period in southern France to quantify levels of seed loss, attack patterns and the impact of each capitulum feeding species. Capi tulum damage and seed predation reduced seed production by between 81- 99%. Four insects were largely responsible for this; the receptacle we evils Rhinocyllus conicus, Larinus jaceae and L. sturnus and the tephr itid gall-fly Urophora solstitialis. Seed predation caused a shift of peak seed production from the large early capitula to the later more n umerous small capitula from the mid to late flowering period. Attack b y R. conicus and U. solstitialis was spatially dependent on capitulum density of C. nutans among sites. Sites with greater densities of capi tula were attacked by a greater number of these insects per capitulum. No such pattern was found at any other spatial scale. The implication s of the seed loss on C. nutans populations in southern Europe are dis cussed in relation to other factors in the dynamics of this weed. Comp arisons with existing studies from new environments demonstrate that a ) predispersal seed predation is much greater in native populations th an has been described in infestations of this weed in other countries where biological control has been successful and b) the main agent of this control, R. conicus, is causing similar amounts of seed predation in native populations as in countries where it has been released with out natural enemies.