The impact of a root-crown weevil and pasture competition on the winter annual Echium plantagineum

1. Root-feeding insect herbivores may suppress their host populations by al tering the competitive balances within the plant community of the host-plan t. In this study, repeated field experiments using caged and natural popula tions of a root-crown weevil, Mogulones larvatus, on the annual weed Echium plantagineum (Boraginaceae) were conducted with and without pasture compet ition in its exotic range. 2. The experiment used a split-plot design, excluding weevils with insectic ide and plant competition through weeding. 3. The greatest effect of the weevils was observed when natural densities o f weevils occurred on naturally growing E. plantagineum rosettes in the fie ld. Weevils reduced plant survival by 43%, and the size and seed weight of survivors by 58% and 74%, respectively. 4. Pasture competition had a similar impact on E. plantagineum as the weevi ls for plant size and seed weight, except that there was no impact on roset te survival. There were also no significant interactions between the effect s of weevils and competition. Most impact occurred in the second half of th e growing season. 5. The number of weevil eggs and larvae and all plant reproductive paramete rs measured per unit biomass were independent of plant size. 6. Such impact experiments help develop simplified impact assessment method ologies for assessing the success of biological control projects in the fie ld.