Oviposition response of the biocontrol agent Rhinocyllus conicus to resource distribution in its invasive host, Carduus nutans

Musk thistle, Carduus nutans, is a major noxious weed throughout its non-native range. The flower head weevil, Rhinocyllus conicus, deemed likely to be a strong candidate for biocontrol based on laboratory tests, has proven variable in its efficacy, suggesting a possible influence of ecological context. To improve our understanding of the dynamics of this system, we examined how R. conicus responds to the spatial distribution of musk thistle individuals. We manipulated the size and density of host plant patches to determine their effect on weevil oviposition. Neither patch size nor plant density significantly affected the number of eggs laid by R. conicus. However, plant characteristics such as flower head size, as well as plant height and number of flower heads per plant, significantly influenced oviposition. Although larger thistle flower heads provide more sites for oviposition, there is also more seed production in these heads. Thus, while R. conicus oviposition is highest in larger flower heads, the surplus of seed offsets the effectiveness of biological control. This observation may in part explain the variable levels of control by this biological control agent and ascertains circumstances when augmentative management procedures may be needed.