Exploitation of Canada Thistle by the Weevil Rhinocyllus conicus (Coleoptera: Curculionidae) in Northern Utah

Abstract

A census was taken of a stand of Canada thistle, Cirsium arvense , in northern Utah throughout the growing seasons of 1991 and 1992 to determine the degree to which this weed was attacked by the weevil Rhinocyllus conicus , originally introduced to the region to attack musk thistle, Carduus nutans. Overall, the weevil oviposited on 34–37% of Canada thistle flowering stems (both sexes combined); male and female flower heads appeared equally suitable for oviposition and larval maturation. However, relatively few large flower heads (6–10%) with potential to produce seed were oviposited on by the weevil. The seasonal timing of weevil oviposition versus host maturation suggested that low rates of attack derived primarily from few overwintered weevil females persisting long enough into the growing season to oviposit on large flower heads (preferred oviposition sites) as these became available. Field experiments in which the rate of plant maturation was hastened, or in which comparisons were made among plants differing naturally in seasonal timing of development, confirmed that overwintered weevil females oviposited much more heavily on early versus later maturing flower heads. Thus, as has been found for other thistle species, the impact of the weevil on Canada thistle at the study site appears limited by poor synchronization between insect and host life cycles. Nevertheless, field results indicate that considerable numbers of the weevil develop in Canada thistle flower heads at the study site each year, and thus the weed may serve as an important alternate host in maintaining naturalized populations of the insect in northern Utah.