1. We hypothesised that, for weed biological control, using species that feed as both adults and larvae would be advantageous. Here, we test the impacts of adult and larval feeding of Larinus minutus (Col.: Curculionidae), a successful biological control agent of diffuse knapweed, Centaurea diffusa (Asteraceae), in British Columbia, Canada.
2. At one site, the observations of the present study showed that the intensity of adult weevil feeding did not predict the level of larval attack. Experiments found no evidence for plant-mediated competition between the life-history stages.
3. At two sites and in two years, experimental manipulations of adult and/or larval feeding damage were conducted and plant performance measured. Neither antagonistic nor synergistic interactions occurred, but at each of the two sites a different life-history stage was responsible for reduction in the number of seeds produced by the plants.
4. Although one of the two different feeding modes was redundant at each site, the ability of adults and larvae to reduce plant performance in different areas makes the species effective in a wider range of environments.