Host Utilization of Field-Caged Native and Introduced Thistle Species by Rhinocyllus conicus

Rhinocyllus conicus Fröelich was introduced from Europe into North America as a biological control agent of the exotic weed Carduus nutans L. Concern exists over the feeding of this weevil on at least 25 species of native Cirsium thistles. Beginning in 2008, cage studies isolating adults of R. conicus on buds and flower heads of all eight thistle species (native and introduced) recorded from Tennessee were conducted to test if R. conicus could use these species for reproduction and what impacts larval feeding of R. conicus may have on seed production. Larvae of R. conicus completed development in heads of the native species C. carolinianum (Walter) Fernald and Schubert, and C. horridulum Michaux, and significant reductions in seed numbers of both species occurred during 2008. Rhinocyllus conicus oviposited on both C. carolinianum and C. horridulum at significantly greater levels than the introduced species C. arvense (L.) Scopoli and C. vulgare (Savi) Tenore. Infested heads of C. carolinianum contained numbers of R. conicus per centimeter of plant head width similar to Ca. nutans in 2008, and both native species contained numbers of R. conicus per centimeter of plant head width similar to C. arvense and C. vulgare in 2009. Body length was similar between R. conicus reared on native thistles and its target host Ca. nutans. This report is the first documentation of R. conicus feeding and reproducing on C. carolinianum and C. horridulum. Although R. conicus has been observed only on introduced thistles in naturally occurring populations in this region, the utilization of C. carolinianum and C. horridulum as host species in controlled conditions warrants continued monitoring of field populations and further investigation into factors that may influence nontarget feeding in the future.