EXPLOITATION OF CANADA THISTLE BY THE WEEVIL RHINOCYLLUS-CONICUS (COLEOPTERA, CURCULIONIDAE) IN NORTHERN UTAH

A census was taken of a stand of Canada thistle, Cirsium arvense, in n orthern Utah throughout the growing seasons of 1991 and 1992 to determ ine the degree to which this weed was attacked by the weevil Rhinocyll us conicus, originally introduced to the region to attack musk thistle , Carduus nutans. Overall, the weevil oviposited on 34-37% of Canada t histle flowering stems (both sexes combined); male and female flower h eads appeared equally suitable for oviposition and larval maturation. However, relatively few large flower heads (6-10%) with potential to p roduce seed were oviposited on by the weevil. The seasonal timing of w eevil oviposition versus host maturation suggested that low rates of a ttack derived primarily from few overwintered weevil females persistin g long enough into the growing season to oviposit on large flower head s (preferred oviposition sites) as these became available. Field exper iments in which the rate of plant maturation was hastened, or in which comparisons were made among plants differing naturally in seasonal ti ming of development, confirmed that overwintered weevil females ovipos ited much more heavily on early versus later maturing flower heads. Th us, as has been found for other thistle species, the impact of the wee vil on Canada thistle at the study site appears limited by poor synchr onization between insect and host life cycles. Nevertheless, field res ults indicate that considerable numbers of the weevil develop in Canad a thistle flower heads at the study site each year, and thus the weed may serve as an important alternate host in maintaining naturalized po pulations of the insect in northern Utah.