The impact of a root-crown weevil and pasture competition on the winter annual Echium plantagineum.
Root-feeding insect herbivores may suppress their host populations by altering the competitive balances within the plant community of the host-plant. In this study, repeated field experiments conducted at Canberra, Australian Capital Territory, south-eastern Australia during 1994-96 using caged and natural populations of a root-crown weevil, Mogulones larvatus, on the annual weed Echium plantagineum (Boraginaceae) were conducted with and without pasture competition in its exotic range. The experiment used a split-plot design, excluding weevils with insecticide and plant competition through weeding. The greatest effect of the weevils was observed when natural densities of weevils occurred on naturally growing E. plantagineum rosettes in the field. Weevils reduced plant survival by 43%, and the size and seed weight of survivors by 58 and 74%, respectively. Pasture competition had a similar impact on E. plantagineum as the weevils for plant size and seed weight, except that there was no impact on rosette survival. There were also no significant interactions between the effects of weevils and competition. Most impact occurred in the second half of the growing season. The number of weevil eggs and larvae and all plant reproductive parameters measured per unit biomass were independent of plant size. Such impact experiments help develop simplified impact assessment methodologies for assessing the success of biological control projects in the field.